Gravity for Marla

CINDY BLOBAUM

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ISBN

Educational Consultant, Marla Conn

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Printed in the United States.

CONTENTS

Chapter

Chapter

Chapter

Chapter

Interested in Primary Sources?

Look for this icon.

Some of the QR codes in this book link to primary sources that offer first-hand information about the topic. Many photos are often considered to be primary sources as well, because a photograph takes a picture at the moment something happens. Use a smartphone or tablet app to scan the QR code and explore more! You can find a list of the URLs on the Resources page. You can also use the suggested keywords to find other helpful sources.

Circa 340 BCE: In Greece, Aristotle describes a model of gravity pulling everything to round layers surrounding the earth.

Circa 150 CE: In Egypt, the mathematician and astronomer Claudius Ptolemy publishes the Almagest, a set of 13 books showing the math behind the movements of the sun, moon, and planets. This book was used as the main astronomy textbook for over 1,300 years.

1798: Henry Cavendish measures the force of gravity between two masses.

1845: John Couch Adams and Urbain Le Verrier use Newton’s laws of gravity and observations of the planets to predict the existence of a planet past Uranus. The next year, Neptune is discovered.

1487: The astronomer Nicolaus Copernicus creates a model of the solar system showing the earth and the other planets orbiting the sun in circular paths. Earlier models showed the earth at the center of the solar system.

1589: Galileo Galilei proposes that all objects fall at the same rate, no matter their size.

1609: Johannes Kepler combines math, Copernicus’s model of the solar system, and observations of the planets and stars to create three new laws about the elliptical orbits of planets.

1678: Robert Hooke invents the spring scale and introduces Hooke’s Law about how much force it takes to stretch a spring a certain distance.

1687: Sir Isaac Newton publishes his Universal Laws of Gravity.

1916: Albert Einstein publishes his Geometric Theory of Gravitation.

1942: Germany’s V2 rocket overcomes the earth’s gravity and reaches outer space.

1957: The Soviet Union places the first satellite, named Sputnik, into space.

1969: American astronaut Neil Armstrong becomes the first human on the moon.

1986: The Soviet Union begins construction of Mir, the first space station to continuously orbit the earth.

1998: Construction of the International Space Station begins.

2004: NASA launches the Gravity Probe B to measure differences in gravity around the earth.

2012: NASA maps the gravity of the moon using GRAIL Probes.

2022: Scientists discover a new way to detect gravitational waves. They can track the ripples in spacetime caused by the explosion of massive objects in space.

Need a few more recent entries

WHICH WAY IS DOWN?

ink about a cup falling o the counter. Does the cup fall up or down? Why? is amy seem like a silly question, but the answer involves a very important concept in the universe gravity !

On Earth, the invisible force of gravity pulls everything toward Earth’s center. Th is is something we learn at as we’re fi rst learning to walk. It’s not easy! And gravity continues to be a huge part of our lives, and the world around us, for as long as we live. You can’t turn it on or off. It pulls you down whether you want it to or not. Without it, you would float away!

ESSENTIAL QUESTION

How does gravity keep you safe? What forces can you use to work against gravity and why do we need to do this sometimes? Why does the position of your belly button matter? How do astronauts work in zero gravity? Can gravity make life easier? You’ll learn the answer to these and more in this book!

What force keeps us on the earth?

GRAVITY

WORDS TO KNOW

universe: everything that exists everywhere.

gravity: a natural force that pulls objects on and near Earth to the earth.

force: a strength or energy that can change the motion of an object.

astronaut: a person trained for spaceflight.

matter: anything that has mass and takes up space.

magma: melted lava.

attraction: an invisible power that pulls things together.

IT’S ALL A MATTER OF MATTER

Gravity works on all matter. Matter is anything that takes up space and can be measured. Chocolate cake is matter. It takes up space. You can stick a fork in it, and describe how it looks and tastes. You can measure how much it weighs, how large it is, and if it is hot or cold.

A feather is matter. So are dogs and cats, a car, this book, your body, and even a speck of dust. What about air and other gases? Are those matter? Can invisible things be matter? Yes! Even though we can look right through the air around us and it’s a little trickier to weigh, air and other gases are matter.

Ups and Downs

You might think of the earth as being a big ball, but it really isn’t. There are tall mountains and deep valleys. If you took all the water away, it would look bumpy and lopsided. Since the earth is not even, there are some places with more mass and some places with less. This means the pull of gravity is a bit stronger in some places and a bit weaker in others. Sri Lanka, an island off the tip of India, has the lowest pull of gravity on the planet. Scientists think it might be because of the way melted lava, or magma, is moving under the crust. If you visit there, you might be able to jump a bit higher than usual. But you might not feel like jumping at all if you climb to the top of a mountain in the Himalayas, such as Mount Everest. Gravity is stronger than average there. Plus, all that climbing gear you were carrying would pull you down!

What’s it like to live without gravity? Astronauts who have been to space have the answer to that question! Take a look at this video showing how people move and function in a zero-gravity environment. What are some of the challenges these astronauts face? How can they solve some of these challenges? What things might be easier in zero gravity?

Aeon zero gravity

What isn’t matter? Your thoughts and feelings. They are real, but they don’t take up space. You can’t touch or measure them. How about light? Not matter! Gravity is another thing that isn’t matter. It doesn’t take up space and you can’t touch it.

Gravity’s force is a natural attraction that affects all matter. Gravity pulls all matter closer together. There is gravity between an apple and a banana sitting on a table. There is gravity between a bird and a tree. Most of the time, the pull of gravity between objects is so small that we don’t notice it. But the more matter an object has, and the closer it is, the stronger its pull.

The Scientific Method

A scientific method worksheet is a useful tool for keeping your ideas and observations organized. The scientific method is the way scientists ask questions and then find answers. Use a notebook as a science journal to make a scientific method worksheet for each experiment.

Question: What are we trying to find out? What problem are we trying to solve?

Research: What is already known about this topic?

Hypothesis: What do we think the answer will be?

Equipment: What supplies are we using?

Method: What procedure are we following?

Results: What happened and why?

WORDS TO KNOW

gravitational pull: the force of gravity acting on an object.

Gravity

The sun’s gravity is keeping all our planets in place. The sun even pulls on you! But it is so far away you don’t notice it. Earth is by far the biggest, closest object to humans, so it has the strongest pull on us.

The earth is very, very big. Its gravitational pull is stronger than anything for millions of miles around it. Earth’s gravity pulls you, the air, plants, trees, and even the moon, closer to its center. It keeps our feet on the ground!

In this book, we’ll take a closer look at the phenomenon of gravity and discover how it works, why it works, and the history of the science that brought us to our current understanding of gravity. The deceptively simple concept of gravity provides the basis for our entire existence on Earth, and will play a role in determining which planets we might someday explore! Let’s get started!

Gravity—and balance—keeps these stones standing.

Essential Questions

Each chapter of this book begins with an essential question to help guide your exploration of gravity. Keep the question in your mind as you read the chapter. At the end of each chapter, use your science journal to record your thoughts and answers.

ESSENTIAL QUESTION

What force keeps us on the earth?

TEXT TO WORLD

Have you ever watched a rocketship take off? How is the rocketship able to break away from the force of the earth’s gravity?

GROWING DOWN

Plant roots grow down and stems grow up. Roots bring plants the water they need. Stems hold leaves up toward the sun so they can make food. Why do roots grow down? Is it because water is stored in the soil or because gravity pulls them down? Make a hypothesis, do an experiment, and record your thoughts on a new scientific method worksheet in your science journal. Then track the results to see if you are right!

TOOL KIT

science journal

16 dried beans (soaked in water overnight)

4 paper towels

4 zippered sandwich bags

tablespoon

stapler

sunny window with sill or flat surface

tape

›

Fold each paper towel in half, then fold it again. Put one in each bag. It should fill the bottom half of the bag.

› Pour about 2 tablespoons of water into each bag. You want the paper towels to be damp, but not dripping wet.

› Staple each bag in three evenly spaced places about 2 inches from the top of the bag. This makes four spots for seeds.

› In one bag, place a bean in each space between the staples. Change the direction of the bean each time. Start with the first bean facing up, then down, then right, then left. Repeat this in the same order for each bag.

› Zip the bags shut. Find a window that gets a lot of sunlight. Tape one bag to the window with the zipper on top. Tape one bag to the window with the zipper at the bottom. Tape one bag to the window with the zipper facing right or left. Lay one bag flat on the window sill or a flat surface with the same sunlight.

› Check on your seeds every day. In what direction are the roots growing? Is gravity at work? Record your observations. Is it what you predicted ? It is possible that some beans might not sprout.

Try This!

What happens if you change the direction of the bag after the roots have started to grow? Do they continue to grow in the same direction or do they change?

WORDS TO KNOW

vertical: straight up and down. predict: to estimate what might happen before it happens.

FINDING DOWN WITH A PLUMB BOB

TOOL KIT

° science journal

° 4 pieces of string, each 3 feet long

° 4 pencils

° 4 different items of different weight (craft stick, magnet, small plastic toy, nail)

In ancient times, such as when ancient Egyptians were building their pyramids, builders used a plumb bob. This tool uses gravity to show a vertical line. People still use them today! A simple plumb bob is a small, heavy object on the end of a string. Plumb bobs always point straight down. Make your own to find out what materials work best for this tool.

›

Tie one end of each string to the center of a pencil. Tie one item to the other end of each string. Predict which item will make the best plumb bob.

› Hold the pencil at each end and lift up. What happens to the object at the end of the string? What happens if you tilt the pencil so one end is higher?

› Repeat this with each pencil and item. Record your observations in your science journal.

WHAT’S HAPPENING?

The plumb bob with the most mass is the most affected by gravity and will show the most direct route to the ground.

Try This!

What happens when you try to use each plumb bob near a refrigerator? In a tub of water? In front of a blowing fan? Which item makes the best plumb bob? How might this be a helpful tool for you?

GETTING TO KNOW GRAVITY

Before you were born, you oated around in amniotic fl uid inside your mother’s womb. You didn’t feel gravity’s pull because the uid held you up. Also, your muscles didn’t have to work very hard. ink about oating in a pool of water. Do you feel like you weigh less than when you’re on dry land? at’s a very similar sensation.

ESSENTIAL QUESTION

Which of your senses are most affected by gravity?

As soon as you were born, your body felt the full effect of gravity. Your muscles were so weak, you couldn’t even lift your head! Adults had to hold it up for you. They also had to pick you up and move you around because your muscles weren’t developed enough to control. As you grew, you practiced pushing against gravity. Your muscles grew stronger. You learned to lift your head. You did your fi rst push-ups. Then you learned to crawl, walk, and even jump.

GRAVITY

WORDS TO KNOW

amniotic fluid: the liquid in a womb that surrounds a developing infant.

womb: the female organ in mammals that carries an infant during its development before birth.

proprioception: the awareness of the position of your body.

nervous system: the communication system of the body, made of nerve cells that connect the brain and extend through the body.

eardrum: the part of the ear that separates the inside of the ear from the outside.

One thing your body—and brain—learn as you get used to gravity is how your body is positioned.

Have you ever been knocked over by a wave and been briefly confused about which way is down and which way is up? Or maybe you’re a sleepwalker who’s woken up while standing instead of lying in your bed and felt disoriented.

The sense that tells you where your body is in the space around you is proprioception. Let’s take a closer look at the connections between the force of gravity and proprioception.

This newborn foal isn’t quite ready to test the effects of gravity yet!

Walking Wonder!

Watch this horse stand minutes after being born. Why can some mammals stand shortly after birth, but not humans? Scientists have discovered that all mammals stand at about the same time in the development of their brains. Those that can have better developed muscles, bones, and brains! They also need to be able to walk to get away from potential predators.

Can you think of some other animals that can and can’t walk immediately after their birth?

UP OR DOWN?

Back in babyhood, while your muscles were getting stronger, your brain was learning to use information sent by other parts of your body. When you are held upright or are standing, gravity is pulling all your matter toward the bottom of your feet. Your nervous system sends information to your brain which learns to recognize this feel of gravity’s pull as down. Your eyes learn what you usually see when you look up, down or side to side.

Your body continues to send signals to your brain when you are lying down, standing on your head, sitting on a stool, or doing a cartwheel. Your brain uses these signals to figure out which way is down or up. That’s how you can control your muscles and make them do what you want, whether you’re running up the stairs, doing a backflip on a trampoline, or simply sitting down to eat dinner.

Another way your body figures out which way is down is through your ears. If you could look inside an ear past the eardrum , you would see three tiny tubes bent in the shape of horseshoes and two tiny pockets.

WORDS TO KNOW

calcium: a mineral found in shells and bones.

nerve: a bundle of thread-like structures that sends messages between different parts of the body and the brain.

sense of balance: your eyes, ears, and body senses all working together to help you stay upright and not fall over.

geometric: straight lines or simple shapes such as circles or squares. level: not tilted, horizontal. horizontal: straight across from side to side.

visual: relating to sight or seeing.

GRAVITY

These tubes and pockets have hair and fluid in them. The tiny pockets have small, hard crystals made of calcium. All the hairs are connected to nerves.

If your head is upright, the fluid and crystals settle around the hairs at the bottom of each structure. When you move your head, gravity pulls the fluid and crystals to a new, lower place. The nerves send a message to your brain telling you which way is down. This is how you maintain your sense of balance. It’s how you know where your body is in space.

If the crystals get out of their pocket and into a tube, they can make you feel dizzy, even if you are just lying flat in bed!

Spinning makes you dizzy because the fluid in your ears adjusts to your motion so that the hairs straighten and inform your brain all is as usual.

When you stop, the fluid is still moving and the hairs bend, making the brain assume you’re spinning, even though you’re standing still.

Get Lost in Escher

The eyes are easy to trick! Have you heard of an artist called M.C. Escher (1898–1972)? He was a Dutch artist who is famous for his geometric drawings. In one of his pieces of art, people are walking up walls and going upside down on stairs. Doors and trees are sideways. It’s hard to tell which side of the picture is up and which is down! Escher was interested in math and the relationships between figures and the space around them.

Look at some of his work at this website. What do his drawings make you think of? Can you figure out in what direction the people are heading?

Shrinking!

The daily pull of gravity is strong enough to make you shorter! Measure your height as soon as you get up one morning. Spend the day running, jumping, and playing, then measure your height again right before you get into bed. Are the numbers the same, or did you shrink? Most people are a little bit shorter at night, after gravity has been pulling them down all day. Would the same thing happen to someone who spent the day in bed? Sitting in a chair?

Tilt your head to one side. After 25 to 30 seconds of tilting your head, the fluid and crystals in your ears settle in place and you feel you are level.

This is okay if you are lying in bed, but it might mean trouble for a pilot flying a plane in the clouds. Pilots need to know when a plane is turning or moving up or down. If a pilot was making a long, tilted turn but their ears told them they were flying level and horizontal to the ground, they could get confused—and in trouble. That’s one reason planes have instruments and computers to help pilots.

Your brain also uses your eyes to help you know the difference between up and down. Look up! What do you see? If you’re outside, you’ll see the sky. Your brain has learned that the sky is up. The ground is down.

Although both your eyes and ears help you maintain a sense of balance, gravity on Earth affects your ears more than any other sense organ.

What happens if it’s dark or you’ve closed your eyes? Then you don’t have any visual clues to help you know which way is down and you need to rely on the rest of your senses to maintain your balance and get where you need to be.

That’s when your sense of proprioception is very important!

WORDS TO KNOW

data: information in the form of facts and numbers.

G-force (g): a measure of the force of gravity.

GRAVITY

Do you know people who have a very highly developed sense of balance? Gymnasts and tightrope walkers might fall into this category. What makes them different from the rest of us? How are they able to defy gravity and stay standing on a balance beam or tightrope for so long?

What’s it like to balance on a tightrop?

Take a look at some of the ways gymnasts and tightrope walkers use their bodies to balance. How does the balance pole help with balance?

Seeker balance tightrope

Experiencing gravity is one of the first things you do as a human, and that experience continues the rest of your life. Gravity is a key part of living on this planet!

Tightrope walkers must have a finely tuned sense of balance. Credit:

MEASURING GRAVITY

Lots of practice! Just like a muscle, your sense of balance gets better the more you practice. Gymnasts and tightrope walkers learn to lower their centers of gravity so they can perform amazing stunts without falling.

Measuring is a big part of scientific inquiry. Without having specific data about the topic you are studying, you won’t be able to come to any conclusions or gain new knowledge. But how do you measure gravity?

One unit that measures how gravitational pull feels on your body is called G-force, or Gs. Here on Earth, you usually feel a G-force of 1. Higher Gs feel like gravity is pulling you down extra hard. Your body seems heavier, and it takes more energy to move.

Imagine that fi rst you lift something that weighs 10 pounds at a G-force of 1. At a G-force of 2, it would feel as if you were trying to lift something that weighed double, or 20 pounds.

How many Gs are in the biggest roller coaster in the world?

Two—in the word BIGGEST!

G-forces also measure how fast something changes speed. When you jump off a step or even sit down quickly, the G-force you feel goes up a little. You also feel higher Gs when you go up in an elevator or airplane, speed up in a car, or zoom off on a roller coaster ride.

Gravity hasn’t changed, so why does it feel like it’s pulling you harder? Your body is used to feeling a G-force of 1. When you suddenly speed up, all your skin, blood, muscles, and bones are pulled a little harder than you’re used to. Th is makes it feel like the gravitational pull has increased.

GRAVITY

WORDS TO KNOW

rate: the speed of something measured in an amount of time, such as miles per hour or feet per second.

Sometimes you feel fewer Gs. Try jumping up in an elevator that is going down. For just a second, you will feel lighter. When you are headed downhill on a roller coaster, you are falling at the same rate as your seat.

But you don’t feel gravity pulling on you. You feel like you would fly away if the seat belt wasn’t holding you in! The feeling that no gravity is pulling you down is called zero-G.

Gravity Fun

To test your sense of gravity, go to a fun house that has rooms with slanted floors. These floors confuse your sense of balance. To trick your eyes, the fun house might have fake windows with pictures of the sky at the bottom and the ground at the top. Tables might be bolted to the ceiling. These tricks can confuse your brain so much that you find it hard to walk! What’s the best way to get it straight? Trust your body sense. Let gravity pull you down to your hands and knees, then crawl through the fun house.

In 1954, U.S. Air Force pilot John Stapp set a land speed record of 632 mph in five seconds, meaning he experienced 20 Gs during acceleration. When the sled stopped, Stepp experienced a force of 46.2 Gs, the highest level voluntarily withstood by a human.

Many different sports require people to move very quickly. For example, running, jumping, skiing, skateboarding, and riding bobsleds or luges are all activities that make participants feel changes in Gs for a short time. Ski jumpers and skateboarders might feel lighter than air when they leave the ground. Lugers, lying on their backs as they zip downhill on small sleds, feel much higher Gs. Pilots, astronauts, and racecar drivers can also feel wide ranges of G-force.

Most people can’t handle anything higher than 5 Gs for longer than a few seconds. The muscles in your heart and lungs aren’t strong enough to pump blood to your brain under those conditions.

Ride the Vomit Comet

There’s a special plane called Zero-G. People, including astronauts, use it to feel what it’s like to be almost weightless. First, the pilot flies the plane up at a steep angle. During this time, everyone feels about 1.8 Gs. Then, the pilot zooms down at a steep angle. Suddenly, everyone feels almost zero Gs for 10 to 20 seconds.

The pilot flies up and down like this 12 to 15 times! Most people who ride in Zero-G get sick from so many changes in the G-force. That’s how this plane got its nickname of the Vomit Comet!

Astronauts, fighter pilots, and racecar drivers go through special training and sometimes wear special suits so they can handle up to 9 Gs for short periods of time. Otherwise, they could pass out when they really need to be paying attention!

People have long been using gravity and working against gravity. Early hunters used gravity by sitting in trees, waiting for their prey to it under them. They could then throw rock spears down to kill them.

GRAVITY

Later, warriors built gravityassisted machines called trebuchets to help them win wars. Almost 3,000 years ago, Chinese people were working against gravity by using kites to measure distances, send messages and more.

What does it look like to be exposed to higher Gs? Check out this video and see a pilot’s face as they do an extreme take-off in a jet! What might be some of the long-term effects of being exposed to a higher G-force?

We still use air-power to lift things today, as well as fuel-powered engines to send things skyward. To use gravity or work against it, it helps to have a solid background understanding what really matters. And getting to the heart of matter is exactly what you’ll do in the next chapter.

Angry Planet G-force Earth and the moon have gravitational attraction. It’s the moon’s pull on Earth that is responsible for the tides we see in the oceans!

ESSENTIAL QUESTION

Which of your senses are most affected by gravity?

PUSHING UP!

You can do exercises to make your muscles strong enough to work against gravity. For loads that are too big or heavy for your muscles, you need to use another force. Wind, water, and machines can provide extra force to move things. But you can also use another simple force—magnetic force! How strong is magnetic force?

TOOL KIT

3 very strong donut magnets

3 small stickers

pencil or other straight wooden rod

clump of clay

science journal

3 extra large plastic paper clips

2 pieces of string 6 inches long

›

Strong magnets have a positive side and a negative side. If you try to put two positive (or two negative) sides together, the magnets will push each other apart. Test your magnets to see which sides pull together or push apart. Use the stickers to mark the sides.

small non-metal objects such as plastic toys, wooden sticks

› Stand the pencil up in the clump of clay. Stack the magnets on the pencil so they are pushing each other apart. Record your observations in your science journal.

› Clip the paper clips together to make a chain. Tie one end of each piece of string around the free end of one paper clip on each end of the chain. Place the paper clip that’s in the middle of the chain across the top of the top magnet.

› Is the magnetic force still strong enough to lift the three paper clips? Tie small objects to the strings on the side paper clips. How much can the magnetic force lift? Be sure to record your actions and observations!

Try This!

What happens if you tie different objects to each side of the paper clip chain? If you stack the magnets so two stick together and one floats, can you lift a bigger load?

<ed note: having trouble picturing this…> <needs some illustrations to clarify what is supposed to happen>

TO WORLD

FPO FPO

GET FIT WITH PHYSICS

Did you know that you have your very own anti-gravity machine—your muscles!

The muscles going from your lower back to your feet do most of the pushing against gravity. If you don’t like gravity making you shorter, you need to make these muscles stronger. Get ready to give your anti-gravity machine a workout with these exercises!

› Squats (10–25, each day):

» Stand with your feet under your shoulders and your arms out in front.

» Keeping your back straight, bend your knees as if you’re sitting down.

» Stop when your thighs are level with the ground. Keep your back straight and stand.

› Toe Raises (30–50, three days a week):

» Stand with your feet apart and slowly push your heels off the floor until you are balanced on your toes.

» Slowly lower your body until your heels are on the floor.

› Bottom Busters (5, five days a week):

» Lay on your belly and bend your knees so your heels face up.

» Squeeze the muscles in your rear end as you lift your knees off the floor. Your heels should push toward the ceiling.

Gravity Can Save Lives!

WORDS TO KNOW

anti-gravity: free from the force of gravity. avalanche: a large amount of snow that slides down a mountain very quickly.

Knowing about gravity can help you if you are buried in snow from an avalanche. When snow surrounds your body, you’re lifted up so it doesn’t feel like gravity is pulling you down. Your head can be tilted for so long that you don’t know which way is up. Your senses might not be working right, but gravity is! If you cry or spit, gravity will pull the water down. Then all you need to do is dig the other way—up—and out of the snow!

ON THE STRAIGHT AND LEVEL!

In the Introduction, you made a plumb bob that used gravity to make sure things are vertical. Now, you will make a water level, a tool that uses gravity to make sure things are straight across! A water level works because when water is in a confined space, gravity makes sure the top of it is level.

Caution: This activity is best done outside.

TOOL KIT

2 clear cups

science journal

food coloring

10–15 feet of clear plastic tubing

clothespins or other clamps

bucket

funnel

8–10 books

a friend to help

› Fill both cups about half-full of water. Stand them side by side. Notice how the water goes straight across. Tilt one cup slightly, but not enough to spill it. Look at the water—is the top of it tilted or straight across? Compare the top edges of water in both cups. Draw what you see in your science journal.

› Add several drops of food coloring to the water and stir to mix.

› Bend one end of the clear plastic tubing. Use one or more clothespins to clamp the bent end closed. Place this end into a bucket on the ground.

› Insert the funnel into the open end of the tubing. Slowly pour the colored water into the tube. Do not fill it all the way. You should have a few inches of empty space at the end.

› Hold both ends of the tube up at the same height. Remove the clothespins from the clamped end. The water line should move until it is at the same level on both sides. If it isn’t, you probably have air bubbles in the tube. Gently tap the sides to get the air bubbles to move out.

Try This!

What happens if you put the stacks of books on stairs or a small hill? Can you use the level to find things that are at the same height in other parts of the room or yard? Can you check whether the top of a table, deck, or fence top is level?

› Find a flat place. Make two stacks of books about 10 feet apart. Have a friend hold one end of the open tube so the water level is lined up with the top edge of one pile of books. Hold the other open end of the tube near the top edge of the second pile of books. It is okay if some of the tube is just lying on the ground. You know the stacks are even—the exact same height—if the water lines up with the top of both stacks. If the stacks are not even, add or remove books to each stack until they are even.

<note author: more primary sources if possible>

MATTER AND MASS

When scientists talk about gravity, they use the words “matter” and “mass.” As we discussed in the Introduction, matter is anything that takes up space and can be measured. One way that scientists measure matter is through its mass.

ESSENTIAL QUESTION

Mass is the amount of matter in an object. The more matter an object has, the more mass it has. What happens when you blow up a balloon until it’s the same size as your head? Compare the balloon and your head. The balloon is full of air and your head is full of your skull, brain, blood, and some other parts. Both the balloon and your head are matter. They are real and you can measure them. But which one has more mass? Your head! It has more matter than the balloon.

What is the relationship between mass and gravity?

GRAVITY

WORDS TO KNOW

balance: a tool that shows if the mass of objects is even. scale: a measuring device. center of balance: the point on an object where its mass is even all the way around. center of gravity: the point on an object where it can be supported and stay in balance.

If an object doesn’t change, then its mass doesn’t change. You can throw the balloon into the air, drop it in a pool of water, or take it to the store. The balloon will still have the same mass. Of course, if you let air out of the balloon, you change it. The balloon will have less matter and so less mass.

Scientists use a balance, or a scale, to compare the mass of different objects. A balance is a tool that can be as simple as a long bar with a hook on each end. To make a balance work, you try to get the same amount of mass on each end. When you do this, you can hold the balance in the middle and it stays level. If one side has more mass than the other, that side goes down. You can use a balance to compare the masses of very different things. For example, how many marshmallows do you think you’d need to equal the mass of a marshmallowsized rock?

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Have you ever played on a seesaw? That’s another kind of balance! If both kids are pretty equal in mass, they will rise up and sink down pretty evenly. But if one person has more mass than the other, what happens? The heavier person sinks faster and the lighter person rises more quickly.

Scientists usually use an object with a known mass on one side of a balance to figure out the mass of the object on the other side.

Balance is an interesting word. The tool called a balance lets you compare the mass of two things. When you use a balance, you try to make its two sides level.

And what are you doing when you stand on one foot? Trying

to keep your balance! If you get distracted, you might lose your balance and fall over. Why? Because you have more mass on one side than the other. The side with more mass is pulled down. The same thing can happen when you stack rocks or hold a ruler straight up on your finger or walk on a balance beam.

This type of balance would show you the same results on the moon, Mars, and even Jupiter

To keep matter from falling over, you need to find its center of balance. Th is is also called the center of gravity. All matter has a center of gravity. Earth has a center of gravity. So do you. So do rocks, hammers, pencils, and shoes.

Center of Gravity

Put one finger under each end of a ruler. Move both fingers toward each other at the same time. When your fingers are together, the ruler is balanced. That is its center of gravity. Measure the distance from the center of gravity to each end. Are they equal? Now, do the same thing with a broom. Do you get the same results? Why or why not?

If the mass in an object is spread evenly, then the center of balance is easy to find—it’s right in the middle. You could balance the object on that center point and the object would stay level. The center of balance of this book is near the middle. A round ball has its center of gravity right in the middle of it.

GRAVITY

WORDS TO KNOW

stable: steady and firm.

resistance: the force of air pushing against an object.

If the mass of an object is uneven, then the center of balance will not be in the middle. A hammer has a lot more mass at one end than the other. If you wanted to hold it at one spot and have it balance and stay level, you would need to hold it very close to the end with more mass. That spot would be the hammer’s center of gravity, even though it is not in the middle.

Objects with more mass on their upper part have a high center of gravity. Objects with a high center of gravity fall over more easily. This is good if you are a gymnast, diver, or high jumper. A high center of gravity makes it easier to jump or do flips.

Try balancing a spoon on one finger. How close does your finger need to be to the head of the spoon to keep it from toppling to the floor?

However, a high center of gravity can be a problem for objects that aren’t supposed to flip. Have you ever seen a warning label in a tall car that explains the vehicle can tip easily? That’s because of a high center of gravity.

Some objects have a low center of gravity. More of their mass is on their lower part. A low center of gravity makes objects more stable

How does this dancer keep their balance?

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Football players and race cars perform better with a low center of gravity. Football players are less likely to tumble to the ground when they’re tackled by members of the opposing team, and racecars encounter less air resistance since they travel so low to the ground. Brooms have a low center of gravity. Can you balance a broom on the bristle end? What about on the handle end?

Which do you think has more mass—a liter of water or a liter of mini-marshmallows?

Watch rock-balancing artist Kokei Mikuni use his knowledge of center of gravity to create some amazing towers! Can you see how the center of gravity tracks through the columns of rocks?

You can see people trying to lower their center of gravity in many sports. It is very easy to see in judo and wrestling matches, when athletes assume a low squatting position. You can also see it watch winter skiing events.

Slalom skiers crouch low, shifting their center of gravity to make tight turns around race gates. Ski jumpers go low as they push off the jump, then stretch out far, maximizing their lift, working against gravity to glide as far as they can.

GRAVITY

Plus, take a look at how differently the players start on a football line of play. The blockers squat down and put one or two hands on the ground, lowering their center of gravity, making it harder to knock them over. Meanwhile, the pass receivers may do a semi-crouch to start, with a sprinting start down field where their bodies are upright and stretched.

Skateboarders are often low to their boards as they use gravity to increase their speed going down, giving them the momentum to ride up and above the other side of the bowl where they can do flips and turns in mid air.

Take a look at Mike Osterman’s freestyle skateboarding! How does he use gravity and balance to do his tricks?

Mike Osterman skateboarder

Now that you know what to look for, watch the top athletes of your favorite sport. Look for when and how they use or fight gravity and the tricks they use.

Scientists have studied athletes to discover if they can predict who will be the faster runners and swimmers. One thing they compared was the height of athletes and the height of their belly buttons. Your belly button is often your center of gravity. They discovered that if you have two athletes of the same height, the one with the higher belly button will be the faster runner.

This athlete has a higher center of gravity, which gives them the advantage when they are leaning forward as they race. The one with the lower belly button will be the faster swimmer. This athlete has a lower center of gravity.

Even when you are floating in water, gravity is pulling you down. Having the lower center of gravity gives the swimmer a slight tilt, helping to keep a swimmers’ arms above the surface of the water. Th is makes it easier for them to do their strokes, the powerhouse of their sport. Of course, a swimmer with a center of gravity perfectly centered is ideal, as both their arms and legs are balanced—for the strokes and the kicks!

HOW IS MASS MEASURED?

Fosbury Flop

A college engineering student completely changed the way athletes compete in the high jump by using his understanding of gravity. Until Dick Fosbury won the Olympic gold medal in 1968 all the high jumpers competed against each other AND gravity by trying to get their entire body over the bar at the same time. Fosbury did some calculations and practiced a new jump style—one where his center of gravity never had to go over the bar! Instead of going over the crossbar feet first, Fosbury ran sideways towards the bar, took off and turned around as he arched his back. He went head first and then lifted up his feet at the end. He set a record, and a new namesake style.

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Instead of comparing marshmallows and rocks, scientists use the kilogram as their measure of mass. A kilogram is the mass of 1 liter of water. If you have a liter bottle in your house, fi ll it with water. Lift it to see what 1 kilogram of mass feels like. Scientists have calculated that the mass of planet Earth is 5,972,200,000,000,000,000,000,000 kilograms. That’s a lot of mass!

The names of some common measurements come from the things people have used to balance masses of different objects. In the past, precious gems such as diamonds were balanced against carob seeds.

WORDS TO KNOW

carat: the unit of weight for gems and pearls. One carat equals 200 milligrams.

Imperial system: a system of measuring units that was first officially approved by King Henry VII. It includes inch, foot, ounce, pound and others. metric system: a system of weights and measures based on the meter and the kilogram.

GRAVITY

Today the mass of diamonds is measured in carats. Meat and wool were measured against stones. Some people in Great Britain still use a stone (14 pounds) as a unit of measure for body weight. And bullets were once balanced against grains of barley. Today the mass of a bullet is listed in grains.

In the United States, most people use the Imperial system of measurement instead of the metric system. We measure mass in ounces and pounds rather than grams and kilograms. But scientists, even those in the United States, use the metric system because almost all countries in the world use the metric system. It would be confusing if some scientists provided information in ounces and pounds and others in grams and kilograms! Look at the metric conversion chart on page XX to see how the different units compare.

ESSENTIAL QUESTION

Just as people created different systems of measurements, the deep thinkers of the past had some very different ideas about what gravity was and how it worked. As they experimented, theorized, made mistakes and even argued with one another, they carefully weighed and tested each idea. What were some of their early ideas and how have they changed? Dive into the next chapter to weigh some yourself!

What is the relationship between mass and gravity?

Giving Gravity a Boost

Race cars are low to the ground, but they can move so fast that the air flow works against gravity and pushes them up like a kite. To solve that problem, fast cars often have a long bar across the top of the back end. This is called a spoiler, because it “spoils” the flow of air across the top of the car. These physical devices give gravity a boost, making it harder for the fast-moving air to lift the car wheels off the track.

WHERE’S YOUR CENTER OF GRAVITY?

When you are standing, your center of gravity is over your feet. Your mass is spread around equally. This is true for everyone. To see if you have a high or low center of gravity, you are going to have to get off your feet.

› Kneel on the floor. Set the box down in front of you.

TOOL KIT

° small box, such as a matchbox, individual-size cereal box, or rectangular block

° science journal

› Put your elbows in front of your knees with your fingers pointing forward. Bend forward until your forearms are flat on the ground. Move the box so it is at the tips of your fingers.

›

Kneel back up. Put your hands behind your back. Lean forward and try to knock the box over with your nose without falling or putting your hands down.

WHAT’S HAPPENING?

If you fall over, you have a high center of gravity. If you don’t fall over, you have a low center of gravity. Can you change your center of gravity? What if you add mass to the lower part of your body?

Try This!

Ask your friends and people in your family to try this experiment. In your science journal, keep track of the age and gender of each person who tries the experiment. You can make a chart to organize your data. Use your knowledge of belly buttons to make predictions. Do you notice any patterns?

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TOOL KIT

FINGERTIP

BALANCE

Make this paper balance to compare the mass of some small objects.

› In each cup, use the hole punch to make three evenly spaced holes just under the top rim.

› Cut three 12-inch pieces of string for each cup.

› Thread one end of a piece of string through each hole. Tie it near the hole leaving one long piece.

› Gather the loose ends of the strings from one cup together. Tie these ends together. You will use these cups in several activities in this book.

› Copy this pattern on the cardboard and cut it out.

› Try to balance the middle tip of the card on the tip of one finger.!

› Put a paper clip on the bottom tip of each side. Try to balance it again.

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› Hang a cup from each paper clip. Put a penny in one cup and a nickel in the other. Is the balance level?

› Add more pennies or nickels to each side until they are balanced. How hard is it to make the two sides balance?

THINK ABOUT IT!

A seesaw is also a type of balance!

Try This!

Use your balance to compare the mass of other small objects. Also try to make balances of different sizes. Does the size of the balance make any difference to how it works? What happens if you make a balance that is not even on both sides?

BUILD THE BEST SWIMMER

When you swim, water is pushing you up while gravity is pulling you down! If your weight is distributed evenly, you can float flat with ease. If you have more mass at one end of your body or the other, that end will sink a bit.

›

Fill the bowl about three-quarters full of water.

TOOL KIT

° large clear plastic bowl/tub

° water

° rectangular Styrofoam block (small enough to fit in bowl)

° pencil

° science Journal

° ruler

° metal washers

° large rubber bands

›

Use the rubber bands to secure a metal washer near each corner of the block. These represent a swimmer’s muscular arms and legs. Use your pencil to mark one end as the head.

›

Put the block in the water. This represents a perfectly balanced swimmer. Record your observations (including measurements above and below water) in your science journal.

› Put one finger on the leg end edge and gently push the swimmer forward in the water. How much resistance do you feel?

› Remove the block from the water and move the washers from the head end closer to the middle of the block. This represents a low center of gravity (more mass near the bottom).

› Return the block to the water and record your observations.

› Remove the block from the water. Move the washers to create and test a swimmer with a high center of gravity (more mass near the head end). What happens?

WHAT’S HAPPENING?

As you add matter and mass to the foam block, you increase the gravitational pull. When you move the washers forward, backward, and side to side, you change its center of gravity and how it floats in the water. Which model is built best for doing the side stroke? On which side?

Try This!

Add more washers to the front, to the back, to one side, to opposite sides. Make careful observations of what happens each time. Can you think of any real-life instances where swimmers have bodies with similar balances to those you construct?

BALANCE + SCULPTURE = MOBILE

TOOL KIT

° box of paper clips

° 3 drinking straws

° crayons

° scissors

° paper

In 1931, artist Alexander Calder (1898–1976) invented a new form of art. He hung a small wooden ball at one end of a rod. At the other end of the rod, he hung a heavy cast-iron ball. He attached a wire at the center of balance of the rod and hung the rod from the ceiling. When the heavy ball was tapped, it made the lighter ball spin around. Calder had made the first mobile —art that uses gravity to work! You can use your knowledge of gravity and balance to make your own piece of gravity art for your room.

›

Use the end of a paper clip to poke a hole near the end of one of the straws. Thread the paper clip through the hole so it hangs down from the straw. Repeat this so you have a paper clip hanging down from both ends of all 3 straws.

› Use the end of a paper clip to poke a hole all the way through the center of each straw. Thread the paper clips through so they point up from the straw.

› Hook paper clips together to make six chains of different lengths.

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WORDS TO KNOW

mobile: a construction or sculpture made of balanced wire and shapes that can be set in motion by the movement of air.

›

Thread one chain through the paper clip at the end of each straw.

›

Pick one of the straws with two paper clip chains to be the top straw. Attach its chains to the middle paper clips of the other two straws.

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›

Color and cut out four paper letters, shapes, or pictures. You can have a theme, such as a trip to the beach, or pick several different things to color and cut out.

›

Thread your papers through each open end of your paper clip chains.

›

Make one more paper clip chain and thread it through the middle of the top straw of your mobile. Hold the mobile by this paper clip chain. Is your mobile balanced? If not, what can you do to balance it?

See Alexander Calder’s art in action! Why do you think he appreciated having his grandchildren in his studio while he worked?

MoMA Calder modern

GRAVITY

Even before anyone ever used the term “gravity,” it was clear something was acting to pull things down. A rock thrown into the air falls to the ground. Water runs down a hill, not up. Is it harder to lift a log than to let it fall? Everyone knows the answer! Humans didn’t have a word for the phenomenon, but they sure knew about it.

Aristotle (384 BCE –322 BCE) was a famous Greek philosopher. He explained gravity by saying that all matter had a place it was supposed to be. He called this an object’s “natural place.” The natural place for most things was to be near the center of the universe.

ESSENTIAL QUESTION

Aristotle imagined the whole universe to be like huge transparent balls wrapped around each other. He thought that the natural place for Earth was in the very center. Then, he imagined there were layers for each planet, the moon, and the sun. He believed everything moved to its natural place based on whether it was heavy or light.

What forces push against gravity?

WORDS TO KNOW

BCE: put after a date, BCE stands for Before the Common Era and counts down to zero. CE stands for Common Era and counts up from zero. The year this book was published is 2013 CE. philosopher: a person who thinks about and questions the way things are in the world and in the universe.

An object with mass would fall toward the center of Earth, and the heavier it was, the faster it would fall. Lighter things such as gases would drift upward, toward the layers of the moon and the planets.

Many of Aristotle’s ideas tried to explain the way things are in the world and in the universe. But not everyone agreed with everything Aristotle thought. Philosophers and scientists are always asking questions and disagreeing with each other. It’s this scientific curiosity and debate that drives scientific discovery—even today!

In 1543, a scientist from Poland named Nicolaus Copernicus (1473–1543) wrote that Earth was not the center of the universe. He believed the sun was at the center with Earth and other planets orbiting it. Then, around 1589, an Italian scientist named Galileo Galilei (1564–1642) argued that a heavy ball and a light ball fall at the same rate. Th is directly opposed Aristotle’s view that heavy things fall faster than lighter things. If Galileo was correct, scientists needed a new explanation for why and how objects fell toward the ground.

Aristotle’s vision of the universe, 1519

Aristotle’s view that the universe was made of layers of balls was not something he could test. People just had to believe him. But could people test whether light and heavy things fell at different rates? When Galileo started thinking that a heavy ball and a light ball would fall at the same rate, people tested his idea. How? By dropping two objects with different masses at the same time and watching what happened. Legend has it that Galileo tested his own theory by dropping weights off the top of the Leaning Tower of Pisa and measuring which one reached the ground quickest. However, there is no record of this actually happening!

But other tests were recorded, and after many tests, people agreed that objects fall at the same speed. If you drop a hammer and a marshmallow at the same time, they land at the same time. However, if you try this yourself in your backyard, the hammer will hit the ground fi rst. So, does this mean that scientists were wrong? Keep reading to find out!

Watch this video and learn more about spacetime. How are space and time connected and what does that have to do with gravity?

ScienceABC Spacetime

Next, scientists wanted to measure just how fast these objects were falling.

WORDS TO KNOW

free fall: to be pulled through the air by only the force of gravity.

MEASURING SPEED

When you drop an object, it is in free fall.

The object is falling without anything to speed it up or slow it down. If you drop a stick off a bridge, the stick is in free fall. But if you tie a parachute to the stick, the parachute slows down the stick so it is not in free fall. And if you could fasten a jet engine to the stick, the stick would move faster and would also not be in free fall.

Scientists timed how long it took for the same object to fall from different heights. They saw that as things fell, they sped up. Eventually, scientists figured out that an object in free fall on Earth falls 32.2 feet per second per second.

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What does that mean? In the fi rst second that an object is falling, it travels 32.2 feet. But because an object speeds up as it falls, it goes faster in the next second of free fall. In this next second, the object goes 32.2 + 32.2 = 64.4 feet. In the third second, it speeds up even more. It travels 32.2 + 32.2 + 32.2 = 98.6 feet!

After a while though, a falling object stops speeding up. It can’t go any faster. It keeps falling at the same speed. That speed is called an object’s terminal velocity.

Sproing!

Take a spring out of a pen. Stand the spring upright on a piece of tape or small bit of clay. Stand a ruler behind it. Record the height of the spring. Put a stamp on top of the spring. Record its height. Put a paper clip on top of the spring. Record the height. Which item is gravity pulling on more?

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NEWTON’S LAWS OF GRAVITATIONAL PULL

English scientist Isaac Newton (1643–1727) read about what others were doing. He agreed with the ideas of Copernicus that the moon, sun, and stars were not stuck in transparent layers as Aristotle described. But he still wanted to know, “Why do things fall toward the ground?”

He thought about this question for many years. He wrote letters to other scientists. He told them he wondered why apples falling from trees hit the ground. Why didn’t they fly up? Why didn’t they go sideways? And why did heavy and light things fall at the same rate?

Free fall is whenever an object is traveling on its own with nothing pushing or pulling it. That means free fall can happen when an object is moving up, not just down!

Newton turned his attention to objects in the sky, too. Why did the moon stay in a circle around Earth? The moon never fell onto Earth. Why didn’t the moon float away? Was planet Earth going around the sun in a circle? Everything on Earth and in the sky seemed to be falling toward something else. Was the same force at work?

WORDS TO KNOW

terminal velocity: the fastest an object will travel in free fall.

calculus: a branch of mathematics that deals with calculating things such as the slopes of curves. orbit: the path of an object circling another object.

scientific law: a description of something that happens in nature, but not why. Scientific laws are based on observations.

weight: a measure of the gravitational pull on an object.

GRAVITY

In 1687, Newton wrote about a force that makes all objects attracted to each other. He invented a new math called calculus to explain how this force worked all over the universe. This force works to keep us on the ground. It also works to keep Earth orbiting the sun. From the Latin word gravitas, which means “heaviness,” he wrote, “It is now established that this force is gravity, and therefore we shall call it gravity from now on.”

Newton did not know why gravity existed, but he made observations and used those observations to come up with some basic rules about gravity. His work became known as Newton’s scientific law of universal gravity.

Newton’s rules for gravity included the idea that gravity is a force of attraction that exists between all objects, and that gravity pulls things toward the center of each object. The more mass an object has, the stronger the pull of its gravity. What else affects that pull?

How fast do you think this roller coaster is falling?

The closer two objects are together, the stronger the pull of gravity. Scientists use this equation to describe this phenomenon.

Many scientists used Newton’s laws to make new discoveries. Some scientists wanted to measure how hard gravity was pulling things down. They knew a feather landed softly and a hammer with a hard thud. How could they measure how hard gravity was pulling on these different masses?

At the time Newton was doing his scientific inquiry, some people were beginning to think that things fell toward the ground because the earth attracted them.

Between 1658 and 1678, English scientist Robert Hooke (1635–1703) conducted scientific tests with a spring. He knew a spring could be pushed together or pulled apart. He found that the distance a spring was pulled out or pushed in depended on the amount of force used on it. When gravity is the force pulling down on a spring, you can measure how far the spring moves. So, they called this new measurement weight.

Intergalactic Gravity

American Astronomer Vera Rubin (1928–2016) was fascinated with the sky as a kid. Her father helped her build her own telescope and brought her to amateur astronomers’ meetings. As a professional astronomer, she did studies on the rotation of galaxies. She noticed that some of her calculations and observations did NOT agree. This led her to theorize that the galaxies were full of invisible, unseeable, very heavy dark matter. Data, images and observations from later studies provided additional proof to her theories. Astronomers are still studying her ideas.

“In a spiral galaxy, the ratio of dark-to-light matter is about a factor of 10. That’s probably a good number for the ratio of our ignorance-toknowledge. We’re out of kindergarten, but only in about third grade.” ~Vera Rubin

WORDS TO KNOW

compressed: pressed together very tightly, so it takes up less space.

A bathroom scale with a dial shows how this works. The dial on the scale is connected to a spring. When you place an object on the scale, the spring is compressed, or pushed together. The more the spring is compressed, the higher the number you see on the dial.

The higher the number, the more the object on the scale weighs. The greater the weight, the stronger the force of gravity pulling on the object.

A scale at the grocery store for weighing fruits and vegetables also works this way. Have you ever weighed your bananas to find out how much they are going to cost? You put your food in a tray hanging off a spring that’s attached to a dial. As the spring stretches, the hand on the dial moves up and shows the weight of the food.

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Weight is a force caused by gravity. All around the world, people measure weight in pounds or kilograms. But guess what? The real way to measure weight is in . . . newtons! Anywhere on Earth, an object with a mass of 1 kilogram weighs 2.2 pounds, which is 9.8 newtons. If you could take that object to the moon where there is less gravity, it will weigh less.

FALLING FACTS

Have you ever seen a leaf, feather, or snowflake float slowly to the ground?

If Galileo’s idea was correct, shouldn’t they fall the same as everything else? Shouldn’t they reach the ground the same time as a bowling ball dropped from the same height? They would if the air pushing against them, making them slow waaaaay down!

Air resistance can push against gravity. Have you ever watched water drops on a car window as the car speeds down a street? They go up, around, and over as the air resistance pushes against gravity. For objects, air doesn’t make much difference. But for light, flat objects, such as leaves and feathers, the force of air pushing up can really slow them down!

Watch a scientist demonstrate gravity and explore some of Newton’s errors! How does scientific study allow us to correct our ways of thinking?

BBC biggest vacuum

The same thing happens if you drop things in water. A leaf, feather, piece of paper, or even a stick will float on top of the water’s surface. But almost all rocks and balls of metal sink and drop to the bottom.

Take a spring out of a pen. Stand the spring upright on a piece of tape or small bit of clay. Stand a ruler behind it. Record the height of the spring. Put a stamp on top of the spring. Record its height. Put a paper clip on top of the spring. Record the height. Which item is gravity pulling on more?

So where can you see a light and heavy object fall at the same rate? You need to drop them in a vacuum! A vacuum has no matter, not even air.

Scientists have made special vacuum tubes to run experiments. And in these vacuums, a flat, light feather falls at the same rate as a round, heavy rock.

WORDS TO KNOW

vacuum: a space that is empty of matter

GRAVITY

NEWTON WASN’T ALWAYS RIGHT

As scientists continued to make and use better tools, they discovered that some facts Newton used to develop his laws of gravity were wrong!

In the 1950s, Albert Einstein (1879-1955) theorized that instead of large objects in the universe such as the sun and planets attracting each other, they curve the fabric of space and time around themselves, creating a well that other objects fall into. Even light!

Picture a bowling ball resting on a trampoline. If you set a marble on the edge of the trampoline, what would it do?

Roll toward the bowling ball. Einstein believed that was how objects in the universe behaved, and scientists have been testing his theories ever since. It’s called the spacetime theory.

An artist’s concept of the spacetime theory

Add picture black hole or the space station?

But even though Newton did have some errors in his work, his law of gravity still helped to explain how we expect most things in the universe to work and provided a foundation on which future scientists have built up their knowledge.

Watch what happens when a bowling ball and a feather are dropped at the same time—in a vacuum! Does seeing the experiment in action change your perspective on how gravity works?

You may have heard that nothing exists in space—but that is not true! Stars, planets, comets, meteors, asteroids, satellites, space stations and more are types of matter with mass found in space and they all are affected by gravity. Understanding gravity is the key to studying orbiting objects, space travel, and even researching black holes! Read on to learn more about gravity in space, including why it seems like astronauts have no gravity in the international space station and how that affects their lives!

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BBC biggest vacuum

ESSENTIAL QUESTION

What forces push against gravity?

Which of the armed services is the strongest? THE AIR FORCE!

THE PULL OF GRAVITY

Aristotle believed heavy things fell faster than lighter ones. Galileo believed all things fell at the same rate. Which one do you believe? Start a scientific method worksheet to organize an experiment to investigate this idea. When you drop several different items, which one do you think will fall the fastest? Which one will fall the slowest?

› Hold the shoe in one hand and a pencil in the other.

TOOL KIT

° science journal

° several items to drop, such as a paper clip, shoe, rock, marble, pencil, coin, and ball

° a friend to help you observe

› Raise your hands until they are both shoulder high. Drop both items at the same time. Does one fall faster? Does one hit the ground before the other? Make a chart to record your observations in your science journal.

› Take turns dropping any two of your objects at the same time. Does one fall faster? Record your observations.

› What can you conclude from your data? Does your conclusion support your prediction?

Try This!

What happens when you drop the same pairs of items from a second-story window? Be sure you can’t damage anything outside when you do this.

Add illo?

FREE FALL VS FOOT FALL RACE

TOOL KIT

° tape measure

° large open space

° 4 craft sticks

stopwatch

Can you run 322 feet as fast as a ball can fall? Try it and see! The key to setting up the race is to remember a falling object goes faster each second. So the distance you need to run each second will get bigger!

› Measure 322 feet in a large open space. Remember, the rate of free fall is 32.2 feet per square second.

› Label the craft sticks.

» 1st second 32.2 feet per second

» 2nd second 64.4 feet per second

3rd second 96.6 feet per second

» 4th second 128.8 feet per second

› Place a craft stick at each of these distances:

» 32.2 feet, or the distance an object free falls in the first second

» 96.6 feet, or the total distance an object free falls in two seconds (32.2 + 64.4)

» 193.2 feet, or the total distance an object free falls in three seconds (32.2 + 64.4 + 96.6)

» 322 feet, or the total distance an object would free fall in four seconds (32.2 + 64.4 + 96.6 + 128.8)

› Try the race two ways. Time how long it takes you to run 322 feet and record the time in your science journal. Then measure how far you can run in four seconds. What did you learn?

Try This!

What happens if you set up the racecourse going downhill? Or run on a windy day with the wind at your back? Are you running your fastest, at terminal velocity, between the first two sticks or the last two sticks? How can you find out?

ELEVATOR EXPERIMENTS

An elevator is a great place to test some ideas about gravity. Try to pick a time when no one else needs it.

› Copy the chart below into your science journal so you can record your experiment results.

TOOL KIT

science journal and pencil

portable bathroom scale with a dial

8 pennies

2 cups with strings from Chapter 2

fingertip balance from Chapter 2

2 paper clips

elevator

›

Stand on the bathroom scale. Write down the number. This is your weight and a measure of how much gravity is pulling you down.

› Watch the dial as you bend your knees, stand on tiptoes, and then jump. Did you notice the number change? Did the number go up or down?

› Put four pennies in each cup. Use a paper clip to attach one cup to each side of the balance. Put the balance on the tip of one finger. Watch as you bend your knees, stand on tiptoes, and then jump. Does either side of the balance change?

Scale when bending knees, standing on tiptoes, and jumping: Did the numbers on the scale change?

Finger balance when bending knees, standing on tiptoes, and jumping: Did the finger balance change position?

Scale in the elevator: What happened to the scale numbers when you went up? What happened when you went down?

Finger balance in the elevator: What happened to the finger balance when you went up? What happened when you went down?

WORDS TO KNOW

›

With an adult’s permission, take the scale and balance with you into the tallest, fastest elevator you can find. Start at the lowest level of the building. Stand on the scale and record the number. Press the button for the highest floor. Watch what happens to the number on the dial as you rise. Does the number go up or down? Repeat the test going down. Does the scale act the same way?

helium: a light gas often used to fill balloons.

oxygen: a gas in the air that people and animals need to breathe to stay alive.

atmosphere: the blanket of air surrounding the earth.

› Start at the lowest level again. Put the balance and cups on a finger. Press the button for the highest floor. Watch the balance as you rise. Does either side rise or fall? Repeat the test going down.

WHAT’S HAPPENING?

The spring scale measures weight, which is the pull of gravity. Your weight will change in the elevator as the G-force changes. It will go up when you feel more G-force. It will go down when you feel less G-force. The balance is comparing mass. It will stay in balance as long as there is a G-force of any strength.

Jeronimo!

People today are still testing ideas about free fall and terminal velocity. In October 2012, Felix Baumgartner rode a helium balloon up and up, high above the earth. He had to wear a special suit to protect him from the cold. He also had to carry oxygen because the atmosphere miles above the earth does not contain enough oxygen to breathe. When he was 24-miles high, he jumped out, but he did not open his parachute for more than 4 minutes. As Baumgartner was in free fall, he reached a speed of 834 miles per hour. This is the highest terminal velocity ever experienced by a human.

Watch Felix Baumgartner’s record-breaking free fall! What might this event show about the relationship between science and entertainment?

TOOL

GOING UP?

Sometimes we see things that don’t make sense. Have you heard of an anti-gravity game called Shoot the Moon? To play the game, you try to get a marble to roll up a track made of two rods. The higher the marble climbs, the more points you get. Make your own Shoot the Moon game using funnels instead of a marble to see how it works.

› Tape the wide mouths of the two funnels together.

› Place the ends of the two dowel rods close to each other, but not touching. Wrap a loop of tape around these ends. There needs to be a space between the dowels that can get a little bit wider when you want it to.

› Put a 1-inch-thick book at one end of a table. Put the taped end of the dowel rods on this book.

› Make a stack of books about 3 inches high at the other end of the table. Rest the other end of the dowel rods on top of this stack.

stack.

›

Place the wide part of the taped funnels between the dowel rods on the single book.

›

Slowly open the gap by moving the top ends of the dowel rods apart. Watch what happens to the funnels.

› How far up can you get the funnels to move? What can you change to make the funnels move farther up?

WHAT’S HAPPENING?

Although the funnels seem to be going against gravity, they really aren’t. Try to balance the taped funnels on your two index fingers held together. Where is the center of gravity for the taped funnels? It is at the widest part.

When you put the taped funnels at the lower end of the rods, their center of gravity is on top of the rods. When you move the rods apart, gravity pulls the center of the funnels down between the rods. This makes the funnel tips go uphill while the middle part moves closer to the ground. Gravity is still working the way it always does.

Try This!

What happens if you change the angle? How can you do this? At what point will the funnel not move up at all? What happens if you add weight to the middle of the funnels?

MEASURING AIR

Isaac Newton said that gravity exists between all things that have mass. Does air really have mass? Is it really pulled down by gravity? Make a balance and see!

› Cut two pieces of string about 6 inches long .

› Tie one end of one string to an empty balloon.

› Tie the other end to the 1-inch mark on the ruler.

TOOL KIT

string

scissors

2 balloons

stiff 12-inch ruler

› Blow air into the other balloon and tie the end shut. Tie one end of the other string to the blown-up balloon. Tie the other end to the 11-inch mark on the ruler.

Try This!

› Put a finger under the 6-inch mark on the ruler and lift the ruler. Which end of your balance is heavier?

Can you think of a way to make the ruler balance?

Add illo?

IN SPACE

<note author: (1) More primary sources would be helpful. (2)

We’ve seen how gravity behaves on Earth—how it makes di erent objects weigh di erent amounts and how it a ects the speed at which objects fall. We know that gravity depends on mass. And we learned that weight is a way to measure gravity.

ESSENTIAL QUESTION

How is gravity in space different from gravity on Earth, and why is it important to understand the difference?

How does gravity behave in places other than Earth? What does it mean for planets to be gravitationally pulled toward each other? What might happen if there was no gravitational attraction between Earth and the sun? Let’s look at what we know about gravity in space.

WORDS TO KNOW

new moon: the time when the moon is directly between Earth and the sun so it is not clearly visible or is seen as just a tiny sliver.

A TURNING TIDE

Isaac Newton’s scientific law of gravity theorized that Earth’s gravity pulls on the moon and the moon’s gravity pulls on Earth. While, according to Einstein, this may not be the entire explanation, these ideas are part of a long line of theories stretching back to ancient times.

Scientists and thinkers keep records of what they observe happening around them. Then, they can look back at the records to see if they can spot any patterns. One pattern that people noticed a long time ago is that the tides in the ocean are higher when there is a full moon or a new moon.

Why does this happen?

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Isaac Newton believed his ideas on gravity explained how the tides and the moon are connected. He wrote that because Earth and the moon both have mass and are so close to each other, the gravity of each pulls on the other. The moon’s gravity causes the tides by pulling on the water of the oceans.

All the ocean water in the world is connected. At its most basic level, when the moon is pulling the water on one side of Earth towards it, water gets pulled from everywhere else too, putting more water on one side and less in other places. That’s what creates the high and low tides.

The sun, planets, and moons stay in their orbits because of gravity.

NOTE -- add a short PS sidebar?

Tides

The sun is pulling on the water too, but it is much farther away. When the sun and moon are on opposite sides of Earth, there is a high tide on each side, and low tides in between. When the sun and moon are on the same side and are pulling the water in the same direction, the high tide gets even higher!

The same forces that cause tides in the oceans cause the solid parts of Earth to change shape very slightly.

Since Earth is bigger than the moon, its gravity pulls harder on the moon than the moon’s gravity pulls on Earth. Earth’s gravity pulls on the moon so hard that the moon

move any farther away from Earth.

attaching a magic pen to a baseball. The pen draws a line in the air that marks where the ball goes. If you hit the ball, the force of gravity pulls the ball closer to the ground a little at a time. This motion makes the line a curve. The harder you hit, the larger the curve.

WORDS TO KNOW

centripetal force: the inward force that keeps an object moving at a steady speed in a circular path around another object.

to make a best guess using facts you know.

Newton thought of the moon and Earth as giant balls. He knew that balls are curved and that they fall in a curve when they are hit or thrown.

To explain his theory, Newton drew a picture of a mountain and a cannonball. If you could shoot a cannonball high enough and fast enough from the top of a very tall mountain, it would fall in a circle all the way around Earth. But why would the curve of its fall match the curve of planet Earth? Because gravity pulls with similar amounts of force everywhere on Earth.

The matching curves of the falling ball and Earth means that gravity keeps pulling the ball down in the same curved line. The ball never lands. Newton named this phenomenon centripetal force. He wrote that gravity, working as a centripetal force, is what keeps moons moving around planets. It is also what keeps planets moving around the sun.

Centripetal force

this fairground ride.

THE MOON

People have studied the moon for a long time. Ancient peoples wondered about the moon. As we developed language and writing, scholars told stories and wrote about the moon. They asked questions such as, “How big is the moon? How far away is it? Why does it travel in a circle around Earth? Is the moon bare because everything floated away?”

The moon might look round when it’s full, but it’s actually more of a lemon shape!

To estimate the size of the moon, people measured things on Earth. They compared these measurements to the moon in the sky. They then used math to estimate the size of the moon and the distance between Earth and the moon.

Isaac Newton made his own estimates of the moon’s size and its distance from Earth to explain his law of gravity. He also estimated the mass of the moon. Newton used these numbers to estimate the pull of gravity on the moon. He thought it would be less than the pull of Earth. How could anyone check Newton’s ideas? Go to the moon! Simple, right?

GRAVITY

WORDS TO KNOW

NASA: National Aeronautics and Space Administration, the U.S. organization in charge of space exploration.

It took many years and a lot of work before scientists were ready to test Newton’s ideas in space. On July 20, 1969, NASA’s Lunar Module Eagle landed on the moon. Was Newton’s scientific law correct? Would the gravity on the moon be less than the gravity on Earth? With cameras rolling, astronaut Neil Armstrong (1930–2012) stepped down a ladder wearing a 180-pound space suit. Even with all that extra weight, he had no trouble walking! Newton had been correct. The gravitational pull of the moon was weaker than that of Earth.

Who First Measured the Moon?

Aristarchus (c. 310 230 BC) was a Greek astronomer who was the first to put forth the idea that the sun is at the center of our Universe. He used his observations and knowledge of astronomy to measure the length of time Earth’s shadow covered the moon during a lunar eclipse. He used this number, his estimate of the size of Earth, and geometry to come up with the size of the moon and its distance from Earth. How close was he? He estimated the moon’s diameter to be about one-third the size of Earth’s, when it is closer to one quarter. He calculated the distance to the moon to be 248,548 miles when it is actually 238 855 miles away on average.

Modern Moon Map

In 2012, two space probes the size of washing machines followed each other in orbit around the moon for several months. They stayed close to the moon’s surface. When the front probe passed over something with more mass, gravity pulled it a bit harder. It sped up and the distance between the two probes increased. Special instruments on each probe measured that distance. And scientists used those numbers to figure out the mass of the area below the probes. They made a map showing how mass was different on the surface of the moon.

On a later trip to the moon, astronaut David Scott (1932– ) tested Galileo’s findings that objects fall at the same speed. The moon was a perfect place for a test because with no air resistance, objects are almost in a vacuum. When Scott dropped a feather and a hammer at the same time, they landed at the same time. Galileo’s law was right!

So far, the record for continuous time in space is held by Cosmonaut Valeri Polyakov. He spent 437.7 straight days in space.

Going to the moon was a big step in space exploration. But with dreams of exploring other planets, it seemed more important to learn about space travel. So trips to the moon were put on hold, and NASA concentrated on the creation of the space shuttle--a reusable, giant glider that would circle Earth to allow research to happen in space.

Watch U.S. Astronaut David Scott drop a hammer and feather on the moon! Why is it important to repeat experiments more than once?

In 1981, NASA launched its first space shuttle, which is like a giant glider. A rocket provided the energy to lift it 200 miles above Earth. Once the shuttle was beyond Earth’s atmosphere, it glided around the world in a circular orbit. It fell at 17,500 miles per hour.

Gravity

Th is speed kept the shuttle falling around the curve of Earth, just like Newton described. Earth’s gravity was pulling on the shuttle, but everything on the shuttle was in constant free fall. People floated because no gravity was pulling them down.

The space shuttle offered scientists the fi rst opportunity to experience weightlessness and begin to do experiments in that unusual environment.

Plant Pillows

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SPACE SCIENCE

After many experiments with trying to grow plants in space, the NASA team has developed space gardens with “pillows” for each plant. The pillows hold the growth media, fertilizer, water, and air. Astronauts take seeds up in space with them, then put them in the plant pillows. So far, astronauts have enjoyed fresh salad with three types of lettuce, Chinese cabbage, mizuna mustard, red Russian kale and zinnia flowers. Scientists must solve lots of challenges—such as growing food in space—to make longterm space travel possible

Scientists all around the world are interested in doing experiments in places with low gravity. They want to know how plants, animals, crystals, and science tools act in space. Why? One reason is that someday humans might travel to the planet Mars, or beyond. A trip to Mars would take between 150 and 300 days. It would take just as long to get back to Earth. So, Mars astronauts would be in space for almost two years!

That’s a long time to live without gravity. What effect might weightlessness have on the human body for that length of time?

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How would people keep their muscles strong and bodies healthy in zero gravity?

Bodies change in space. Muscles don’t have to push against gravity, so they get weaker. Even hearts are affected. Scientists want to find ways to keep people healthy in space.

The lack of gravity is hard on a body . . . at least when it returns to Earth. An astronaut can feel and work fine while in space, but need to have help walking away from the return capsule when they land back on Earth. The effects are more noticeable the longer an astronaut is in space.

Astronauts left tools, equipment, food, and human waste on the moon because it was too expensive to bring back to Earth.

Astronauts have lost up to 1 percent of their bone mass for each month they are in space and a total of 20 percent of their muscle mass! They also lose blood volume—but not by bleeding—which causes the heart to become weaker because it doesn’t have to work so hard.

to purchase image

WORDS TO KNOW

microgravity: very weak gravity. International Space Station (ISS): a massive space station orbiting Earth where astronauts live, conduct experiments, and study space.

GRAVITY

The gravity on the moon is too weak to even hold most gases around it! Gravity is what holds the atmosphere around Earth. Without gravity we wouldn’t have the air we breathe.

While exercises and diet help astronauts regain their blood volume and muscles in a relatively short time, doctors worry that their bones might never fully recover. To help slow down the losses, astronauts can now hook themselves up to a giant vacuum cleaner as they exercise in space. This increases the pressure their bones and muscles feel and helps to mimic the effects of gravity.

Scientists must solve lots of challenges to make longterm space travel possible.

THE INTERNATIONAL SPACE STATION

The shuttles were a huge step forward, but they could only be used for 17 days at a time. Scientists wanted to study how things worked in microgravity during long periods of time. Several countries worked together to create the International Space Station (ISS). what year

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The ISS is a permanent research lab that orbits 200 miles above Earth. Astronauts from around the world live on the station for months at a time doing experiments. When new astronauts come to the ISS, the other astronauts travel back to Earth. Sometimes they bring the results of their experiments with them.

The main construction was completed between 1998 and 2011, but they keep adding new parts and changing others as technology and funding allows. When the ISS opened, the shuttles were grounded.

Birds Need Gravity to Drink

Most birds do not have the same throat muscles as humans have. To get a drink, they dip their beaks into water, get a mouthful, then tip their heads back to swallow. Without gravity, they can’t drink! So, birds such as robins, blue jays, and sparrows can never live on a space station where there is no gravity. But pigeons and hummingbirds can! Those species have muscles that move liquid and solids down their throats to their digestive

WORDS TO KNOW

atom: a very small piece of matter. Atoms are the tiny building blocks of everything in the universe.

black hole: a place in space where gravity is so strong even light gets pulled in. light-year: the distance light travels in one year, about 5.9 trillion miles.

galaxy: a collection of star systems held together by gravity.

The ISS not only provides a way for scientists to perform experiments that are impossible to do on Earth, it also shows what humans from different countries can accomplish when they work together. Astronauts have worked on more than 3,000 experiments during their time on the space shuttle and International Space Station. They study how plants, animals, fire, and even their own bodies react to different conditions in the weightless environment. They hope to use a lot of the information they gain to make future long term space adventures safer and better for all involved.

Black Holes

Some space scientists spend their time studying stars. Stars are heavy balls of energy and are the most massive objects in space. Sooner or later, the energy burns out, but all the atoms that created that energy still exist. When stars burn out, no energy, heat, or light is being produced. The only force at work is gravity. The pull of gravity is very, very strong and pulls everything nearby toward it. Scientists call that area a black hole. The nearest black hole is 3,000 light-years away from Earth. Scientist Andrea Ghez and her team recently confirmed the existence of a super-massive black hole at the center of the galaxy

NASA Space Place

WHAT IS MARS LIKE?

Scientists have studied Mars for many years. They have sent equipment to take pictures and measurements, and to test small samples of its atmosphere and land. Scientists estimate that the gravity on Mars is about 38 percent of the gravity on Earth. That means that something that weighs 100 pounds on Earth would weigh about 38 pounds on Mars.

Because Mars has less gravitational pull than Earth, it has bigger mountains! Mount Everest is the highest mountain on Earth. When measured from the bottom of the sea to its peak, it is 8 miles tall. Only about 3 miles of that is above sea level. Compare that to Olympus Mons on Mars, which is about 14 miles tall.

Much of what we see of Mars comes from instruments called rovers that sent to the planet to collect samples, images, and other data. Six rovers have been sent to Mars, including five from NASA named Sojourner, Spirit, Opportunity, Curiosity, and Perseverance.

Because the moon has a thinner atmosphere than Earth, it also has no wind or weather. Footprints from astronauts who walked on the moon more than 40 years ago are still there, and it will probably take another 10 to 100 million years before they are covered by dust.

WORDS TO KNOW

centrifugal force: the outward force on an object moving in a curved path.

GRAVITY

GETTING TO MARS, AND BEYOND!

To make it possible to travel in space, scientists and engineers solve problems, including how to move a spacecraft most efficiently. After all, it takes a lot of fuel to combat the effects of gravity on a rocket. Also, fuel is heavy, so it’s difficult to carry it into space for longer space travel.

Do you wonder how astronauts, eat, sleep, or even go to the bathroom when there is no gravity? Check out this link to see the answers from astronaut who had been on the space station! What other questions do you want to ask about living in space?

NASA potty system

One way NASA solves this problem is by using a method called the gravity assist. To send unmanned spacecraft, telescopes, and probes into deep space, engineers developed a way to aim the spacecraft so it comes close to a moon or another planet in the direction it is spinning. As the spacecraft gets close to the planet, it starts to get pulled in by that planet’s gravity. That speeds up the spacecraft.

With the right timing and a boost of power, the spacecraft then shoots out of orbit with a burst of extra speed, just like a rock leaving a slingshot. The force that causes this is called centrifugal force.

Gravity in space

Mariner 10 was the first spacecraft to make use of a gravitational slingshot maneuver, using the planet Venus to bend its flight path.

Space travel has given us a lot more information about gravity, how it works and how to work without it! From the many experiments that astronauts have done in space, and the studies done on the astronauts that have returned from space, it is clear we have many more challenges to overcome before we look at long-term space exploration or living on the moon or another planet. This means that if you are interested, there is plenty that you can do! To get started, take a look into the next chapter and see how gravity has been put to work on Earth.

WORLD

ESSENTIAL QUESTION

How is gravity in space different from gravity on Earth, and why is it important to understand the difference?

FLOATING OR FALLING?

TOOL KIT

° hole punch

° paper cup

° water

When something is in free fall, it is falling at the speed of gravity. It acts like it has no weight and looks like it is floating in air.

Caution: This activity is messy, so you may want to do it outside.

› Punch two holes near the bottom of the cup.

› Fill the cup with water. What happens to the water?

› Hold your fingers over the holes. Refill the cup.

› Remove your fingers at the same time you drop the cup. Watch carefully as the cup falls. Does any water come out?

WHAT’S HAPPENING?

The water and the cup are in free fall. The water is travelling at the same speed as the cup. When does the water come out?

What season is the cheapest? FREE FALL.

Try This!

What happens if you punch holes at different levels on opposite sides—one near the bottom of the cup and one near the top? Or a higher and lower hole on the same side of the cup? Can you explain your observations?

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TEST CENTRIPETAL FORCE

Centripetal force is an inward force that holds objects in place. Try swinging a ball in a cup to see centripetal force for yourself.

TOOL KIT

° cup from “Fingertip Balance” activity in Chapter 2

›

Put a small ball in one cup. Hold the strings. Your hand represents Earth and the strings are gravity. The cup holding your ball is the moon.

Try This!

›

Rock the cup back and forth a few times to get up speed. Swing the cup in a circle in front of you. Does the ball fall out, even when it is upside down?

What happens if you spin the cup very slowly? What happens if you spin the cup superfast? What happens if you use longer or shorter strings?

° small ball Add illo?

The Slingshot Method

Find a park that has a pole or skinny tree in the middle of a field. Run toward the pole as fast as you can. When you get close, reach out one arm. Grab the pole, swing halfway around, and let go. Keep running in the new direction. Do you feel the extra burst of speed? You are using centrifugal force. That’s the slingshot method at work!

SPIN A BASKETBALL AND TEST CENTRIFUGAL FORCE!

You’ve learned about centripetal and centrifugal forces. See these forces in action as you learn a fun sports skill!

› Bend your arm at the elbow. Hold the basketball on the tips of all the fingers on one hand.

TOOL KIT

° basketball

What’s an astronaut’s favorite drink?

› Twist your wrist and, at the same time, toss the basketball up in the air a few inches. Does the basketball spin? The ball should land right back in your hand if you are not throwing it too hard.

› Practice this twist and toss motion a few times.

›

Now as you twist and toss, point your index finger up. Quickly move this finger under the center of the ball. You want the basketball to gently land on this finger. You might need to do this many times before you get it right.

› Once the ball is spinning on your finger, slap the side of it a few times with your other hand to keep it going. This is applying the inward, centripetal force. But if you slap it too hard and it goes flying off, you are seeing the outward, centrifugal force at w

Check out the Harlem Globetrotters to see basketball spinning masters! What forces are at work on the court?

Harlem Globetrotters spinning

Try This!

Once you get comfortable spinning a basketball, try moving the spinning ball from one hand to the other. Can you get good enough to do the transfer behind your back?

<note author: below it would help to clarify where centripetal force is at work; only centrifugal is mentioned…>

TOOL KIT

MOCK EARTH ORBIT

SuppNow that you know how things orbit, create a model and see how well you can control the forces at work!

› Trace the bottom of the bowl on the paper.

› Draw a picture of Earth on the circle. Cut it out.

› Tape the picture inside the bottom of the bowl.

› The sides of the bowl represent space near Earth. The marble is a shuttle you want to orbit the earth. Put the marble in the bowl.

› Start moving the bowl in a circle. The marble will start moving up the sides of the bowl as it spins.

WHAT’S HAPPENING?

If the marble travels too fast, it overcomes the centripetal force which keeps the marble circling around the bowl—orbiting Earth. The marble leaves its orbit around Earth in your bowl and goes sailing into space. Centrifugal force moves the ball away. The same thing could happen with space shuttles. If they go too fast, they could leave Earth’s orbit!

Try This!

Test your estimating skills! Fill a big jar and a small jar with the same type of small items, such as marbles, pasta, or beads. Empty the small jar and count the items. Use that number to estimate how many are in the big jar. Write down your estimate. Empty the big jar and count. Don’t feel bad if your estimates are not close. Newton’s estimate of the mass of the moon was wrong!

What happens when you spin the bowl faster? Slower? Can you turn the bowl sideways and keep the marble in? Can you turn it upside down? What if you use other kinds of balls?

TIMES OF TIDES

TOOL KIT

° wide and large rubber band

baseball

string

tennis ball

scissors

basketball

2 friends

Since the molecules in liquids are not as firmly bound together as the molecules in solids are, it is easier to see gravity’s effect on liquids. The oceans on Earth are pulled by the gravity from the moon and sun. Since the moon is closer to Earth than the sun, the moon’s pull is twice as strong as that of the sun. When the moon and sun are lined up, the gravitational pull of the sun and moon work together. The water in the oceans moves more and the tide is higher. These higher tides occur during a full moon and a new moon.

›

Imagine the rubber band is water. The baseball is Earth. Put the rubber band around the middle of the baseball. Make sure the rubber band is large enough so you can pull it several inches away from the ball.

Private Ride Into Space

›

Tie a piece of string around the tennis ball. Cut off any extra string. The tennis ball is the moon. Tie string around the basketball. Cut off any extra string. The basketball is the sun. Cut a 6-inch piece of string. Tie one end to the string around the tennis ball. Tie the other end to the rubber band around the baseball.

NASA is not the only organization building and testing space launching systems. In 2020, NASA astronauts took a new ride to the International Space Station aboard the Falcon rocket built by a private company SpaceX! The rocket was smaller, as were the space suits. The ride was mostly smoother and quieter. Between May 2020 and April 2022, SpaceX launched 22 people into orbit with its Dragon spacecraft.

You can read more about it here!

› Cut a 46-inch piece of string. Tie one end to the string around the basketball. Tie the other end directly opposite the other string tied to the rubber band on the baseball.

› Have one friend hold the basketball and one friend hold the tennis ball. Stand between them and hold the baseball.

An object that weighs 100 pounds on Earth will weigh about 16 pounds on the moon. The pull of gravity on the moon is one-sixth as strong as the pull of gravity on Earth. Neil Armstrong’s 180-pound suit felt like only 29 pounds on the moon.

Try This!

What might our tides be like if Earth had more than one moon? Add more tennis balls, strings and friends to experiment and see!

› Have your friends slowly pull their balls away from you. Notice how your rubber band—which is the water—gets pulled away from the surface of the baseball. This is like gravity pulling water away from Earth and toward the sun or moon. This is high tide. Try This: Sometimes, the sun and moon are lined up on the same side of Earth. What happens to the tide when they are pulling in the same direction? Other times the sun and moon are at right angles, and the sun, moon, and Earth make an L-shape in the sky. What happens to the tide then?

GRAVITY TO WORK—AND PLAY!

We’ve learned that the sun, planets, and other giant objects in space are constantly tugging on each other through the force of gravity. It might seem surprising, but even small objects here on Earth have gravitational attraction to each other. So why isn’t your chair moving closer to your table because of gravity?

ESSENTIAL QUESTION

How can gravity help us accomplish tasks?

Because neither your chair nor your table have enough mass to really affect each other. The more mass an object has, the stronger its pull. We feel the pull of gravity on our bodies from the earth because the earth is so big. And we need to use force to get past that pull of gravity. Jumping works, but you don’t stay up for very long!

<note author: (1) More primary source such as videos would be good here. (2) Very little about play; needs more info.>

WORDS TO KNOW

waterwheel: a wheel with paddles attached that spin when water flows over them. The wheel can lift water, power machines, or create electricity. trebuchet: a machine that uses weight and gravity to hurl large stones and other objects at enemies.

What would the world be like without gravity? Wouldn’t it be amazing if someone invented an anti-gravity machine? Well, not exactly. Scientists agree that a world without any gravity at all wouldn’t support life. The water in oceans, lakes, and rivers would float away. Even Earth’s atmosphere would float away. All the air would head out into space, leaving us with nothing to breathe! Sounds like the moon, right?

That doesn’t keep some engineers from trying to invent an anti-gravity machine. Without gravity, builders wouldn’t need huge cranes to lift heavy materials. Rockets wouldn’t need so much fuel to get into outer space. An anti-gravity machine that we could turn off and on as we needed could help make tough jobs easier.

Need to purchase image

while-jumping-against-the-skygm171361973-21143412

If you watch movies about space, you may think that someone has already invented an anti-gravity machine! You see people on spaceships floating in the air. Or they jump super high and must grab onto something before they float away. These scenes are done with special effects. Filmmakers sometimes film parts of these movies on the Vomit Comet, the Zero-G airplane. Or, they attach clear wires to people and objects and lift them up so it looks like they’re floating. They might put items on clear glass plates and hold cameras in different ways to fool you!

How can you increase your force? Jumping on

a trampoline is one way. You jump higher and stay in the air longer. But Earth’s gravitational pull still brings you back down.

No matter what they show in movies, anti-gravity doesn’t exist. But . . . you can make your own special effects using a digital camera. Tape a piece of colored poster board on a wall. Hold your camera sideways and take a picture of someone pouring juice into a glass in front of the poster board. Try not to get the glass in the picture. When you show others the picture, turn it so it looks like the juice is going sideways by itself! What else can you do to create the illusion of zero gravity?

GRAVITY AT WORK

You can push against gravity, pull with gravity, or even use gravity to play tricks, but you can’t make it go away. You can’t go anywhere in the world that isn’t affected by gravity. Even the moon has a weak gravitational pull.

People—and other animals—have been using gravity to do work since ancient times. Even before people knew how or why gravity worked, they used it to make their lives easier. They used it to move water where they wanted it before electric pumps were invented. They used falling water to power waterwheels . They used gravity to fire weapons called trebuchets. Gravity even helped in cooking. Let’s take a closer look.

WORDS TO KNOW

society: a group of people with shared laws, traditions, and values.

aqueduct: a pipe or bridge that moves water using gravity.

GRAVITY

WATER POWER

Imagine that you live somewhere without a faucet in your house or even a place to get water in your town. If you want a drink of water, you must go looking for water. You might lean down and take a sip from a river or lake. With pots and water bags, you can carry water home, but you must still go to find water. A long time ago people wondered, “How can we get water to come to us?” Gravity was a huge part of the answer.

If there is a lot of water up high in the mountains, you can use gravity to move the water downhill to where you want it. That’s exactly what many ancient societies did. They dug ditches and tunnels to lead the water to farms and people. Brilliant!

But what if people wanted to move the water across a valley? Gravity would pull the water down the hill on one side, but how would the water get up the hill on the other side?

Natural Water Tower

Some people get their water from artesian wells. These wells are like natural water towers. Water soaks into the soil on top of a hill. It seeps straight down through the soil until it hits a layer of rock. The layer of rock acts like a slide, moving the water down the hill. The water flows downhill underground between layers of rock that don’t let it escape. When someone near the bottom of the hill makes a hole in the rock layer above the water, water comes out of the hole. No pump is needed. Gravity does all the work.

The ancient engineers figured out that if they dug a dip in the channel right before the hill, gravity would pull the water faster down the dip, and then the momentum, along with the pressure of all the rushing water behind it, would push it up the hill on the other side. You may have experienced something similar if you have ever ridden a bike or skateboard down a hill and coasted up the other side.

ROMAN AQUEDUCTS

The ancient Romans built some of the world’s most famous aqueducts . They built hundreds of miles of aqueducts to bring clean water to Roman cities for drinking, cooking, and cleaning. The tallest bridge aqueduct was over 200 feet tall. The longest pipeline aqueduct was almost 60 miles long. You can still see many ancient stone arch aqueducts today!

this explanation of how water wheels work. What innovations have people made to improve on water wheels from past centuries?

People built special bridges with a channel carved into the top to move water from one side of a valley to the other. These aqueducts moved millions of gallons of water every year. Aqueducts are still used today, but most of them are underground pipes since underground pipes lose less water to spillage and evaporation.

Boeree waterwheels

GRAVITY

WORDS TO KNOW

counterweight: a weight that balances another weight. missile: an object or weapon that is propelled toward a target.

An aqueduct is only one way people have used gravity and water. Have you ever been near a waterfall or a dam? Then you know that moving water has a lot of power. A waterwheel can use that power. Each wheel has many blades sticking out from the center. As water hits each blade, it makes the wheel spin. At first people just hooked up these spinning waterwheels to saws, grinding stones, and other objects that needed power to move. Waterwheels were also used in getting water to different crops that needed it.

Today, water pouring out of the giant Hoover Dam on the border between Arizona and Nevada turns waterwheels that make electricity. You can find many other dams like this around the country and the world.

trebuchet in

COUNTERWEIGHT POWER

Have you ever been on a seesaw opposite someone much heavier than you? If you don’t hang on, when they go down you can find yourself flying off ! Keep this idea in mind as you learn how gravity is used to fi re a weapon called a counterweight trebuchet.

Long before guns and tanks were invented, armies at war used trebuchets to shoot rocks and other missiles over walls and into enemy camps. They also used trebuchets to destroy walls.

A trebuchet had a wooden frame that looked like the letter

Learn more about the Aqua Virgo—the only ancient Roman aqueduct still in use today! It drops only 12.7 feet in height over its nearly 13-mile run! How has our use of gravity for water delivery changed since ancient times?

Cultural Heritage Aqua Virgo

A. On top of the frame was a long pole. A very heavy weight hung from one end of the pole. Soldiers pulled down the end of the pole opposite the weight and held it down. They attached a long sling with a heavy rock in a pouch to this end. Then, they placed the pouch under the middle of the A-frame.

GRAVITY

To fire the trebuchet, the soldiers let go of the lower end. Gravity quickly pulled down the heavier end. The pouch shot out from under the frame so fast that the rock stayed inside the pouch until the pole stopped moving. Then centrifugal force sent the rock sailing off, hitting the enemy town or camp.

Test out the power of a counterweight! Place a pencil flat on a table. Put a stiff ruler over the center of the pencil so that it looks like a seesaw. Place something very light, such as a marshmallow, on one end of the ruler. Quickly push your hand down on the opposite end. What happens?

old house window counterweight

Counterweights are also used for the good. If you live in a building with older windows, when you push up the lower windowpane, it is held up with a counterweight on the side. Giant construction cranes can lift thousands of pounds of materials hundreds of feet into the air. Although they are very heavy machines with a wide base, as they lift the materials, they are changing their center of gravity. The crane would be in danger of toppling over if it didn’t have a counterweight to balance it out.

Moving Water Against Gravity

Using the effect of gravity on water to create power is a super way to harness a natural force. Another natural force works against gravity to move water and other water-based liquids UP! This force is called capillary action. It is used by paper towels, super skinny glass tubes (used when they poke your finger to get a sample of blood) and plants (including giant sequoia trees!). Water molecules are attracted to each other, that’s why they form drop shapes. This attraction is called cohesions. Water molecules are also attracted to other molecules, including glass and paper. This attraction is called adhesion. If the space in an object is small enough, once one water molecule fills the space, another one attached to it will push it up. This can continue until the liquid is gone or the water molecules have reached the top of the space. You can see this by putting a cut celery stalk in a glass of colored water and watching the color climb!

Take a look at an old window counterweight. Do you something like that in your home?

GRAVITY FOR GOOD!

Have you ever looked at a drop of your blood? It looks like a slightly thick red liquid. Blood has four main parts—red and white blood cells, plasma, and platelets. Each of these parts has its own job and its own mass.

When blood is taken from your body, each of these parts can be separated—using gravity! To help gravity work faster, they put a tube of your blood in a machine called a centrifuge. (This should remind you of the centrifugal force you learned about in Chapter 4!) The centrifuge spins the blood, separating the parts into layers for easier study or use.

If you put fresh whole milk in a tall jar, cream rises to the top. You can take the cream off to make butter or whipped cream and drink the milk!

GRAVITY

WORDS TO KNOW

anatomy: the study of the structure of living things including naming all the parts and their functions.

Scientists have even studied how gravity can help get some medicines working in your body faster. Looking at human anatomy and gravity, researchers discovered a trick to help make a pill work the fastest. After you swallow a pill, lie down on your right side. This position sends the pill to the deepest part of the stomach, closest to your intestines (where the medicine actually gets absorbed into your bloodstream. This simple trick can put the medicine to work over 10 times faster than if you lie on your left side!

We still use gravity today in many of the same ways. Turn on a water tap. Gravity is probably helping to bring water to your sink. Turn on a light. Gravity may be helping to make the electricity that powers the light. Play a game of catch and gravity affects how the ball travels. Race down a hill on your bike and gravity plays a part on how fast you can go. Plan a new stunt on your skateboard and think about shifting your center of gravity to give you greater air. Listen to a news story about people traveling in space and plan your adventure there.

Finding Downhill

Cut a paper cup down opposite sides and across the bottom. Cut off the bottom of each piece so that you have two curved pieces. Tape the two pieces together to make a channel. Go outside and lay them down. Pour a bit of water into the middle. Watch to see if the water moves to one end or the other. If it does, you know which way is downhill. If the water doesn’t move, you know the ground is level.

ESSENTIAL QUESTION

How can gravity help us accomplish tasks?

As we work and play and go on with our day, gravity is affecting us on all sides, everything we do. Use your knowledge to figure out how to make gravity work for you!

TEXT TO WORLD

STAND THE BOTTLE!

Some carnival games use balance and gravity to make it harder to win. Practice before you go and you might come home a winner! This game seems easy enough. All you must do is pull a bottle that is lying on its side to make it stand up. At the fair, you can’t see through the bottles, but you will make see-through bottles and test their tricks.

TOOL KIT

3 clear plastic drink bottles

water

scissors

string

stick

curtain ring that fits around the necks of the bottles

chalk

sidewalk

›

Fill three bottles about ¼ full with water. Place one in the freezer on its side, one upside down and one upright. Let them freeze overnight.

›

science journal

Cut a piece of string about 18 inches long. Tie one end to one end of the stick.

› Tie the other end to the curtain ring.

› When the water in the bottles is frozen, take them outside. Use the chalk to draw a circle on the sidewalk that is about 1 inch bigger than the bottom of the bottle.

›

Lay one bottle on its side with its bottom edge in the middle of the circle. Put the ring over the neck of the bottle.

› Copy the chart on the next page into your science journal and predict which bottle will be the easiest to stand up.

› Pulling on the stick, try to lift the bottle to an upright position in the circle. Do the same thing with each bottle.

Try It!

Fill each bottle with a different amount of water, but do not freeze them. Predict which amount of water will make the bottle the easiest to stand up straight. What amount of water makes a bottle the hardest to stand up straight? Record your predictions then test them to see if you were correct.

WHAT’S HAPPENING?

The bottle that has ice on the top has a high center of gravity. The one with ice on the bottom has a low center of gravity. The one with ice along the side has a center of gravity somewhere along that side. Which bottle was the easiest to stand up? Which type do you think they use in the game at the fair to make it harder to win?

LIQUID

Video Game Gravity

The next time you play a video game, keep your eye on the ball, and everything else. Are the objects and characters in the game realistically affected by gravity? Would you want to play a game in which you couldn’t predict what would happen if you threw a ball or jumped off a ledge? When game creators are programming the computers, they have to put a lot of effort into thinking about how things should mov, float and fall! They are putting their knowledge of gravity to work!

›

Use the marker to label the cups A, B, C, and D.

Food Power

›

Pour ¾ cup of water into cup A. Add a few drops of food coloring and stir with the spoon.

› Pour ¼ cup of the colored water into cups B and C. You should have 3 plastic cups with colored water and one empty cup (D).

Do you like turkey with gravy? One of the main ingredients in gravy is meat broth. Broth is mostly water, but it also includes some animal fat. A little bit of fat in the broth adds flavor to the gravy. Too much fat makes gravy taste greasy.

›

Pour ¼ cup of cooking oil into cup A. Let it sit for 1 minute. Record your observations on the chart in your journal.

›

Pour ¼ cup of syrup into cup B. Let it sit for 1 minute. Record your observations.

›

Pour ¼ cup of baby oil into cup C. Let it sit for 1 minute. Record your observations.

›

Pour ¼ cup of cooking oil and ¼ cup of baby oil into cup D. Let it sit for 1 minute. Record your observations.

› Make some predictions. When all four liquids are poured into one bottle, which one will be on the bottom? Which one will float on top? Test your predictions by pouring ¼ cup of each liquid into the bottle. Were your predictions correct?

To get most of the fat out of the broth, cooks put gravity to work. They pour all the broth into a tall cup or jar and simply let it sit. Gravity pulls the water down because it’s heavier than the fat. The fat floats to the top because it’s lighter. All the cook has to do is skim the grease off the top. You can also use this trick to separate cream from fresh milk, or create your own gravity drink using colored juices that have different amounts of sugar!

Try This!

What happens if you add an ice cube to the bottle with the four liquids? Will the layers re-form if you shake the bottle? Do all liquids fall at the same speed? How can you use a dropper to find out?

MAKE A WATER TOWER

TOOL KIT

° scissors

°

plastic bottle

° bendable straw

clay

° water

Instead of using aqueducts to move water, some cities store water in tall towers. They build the towers on the highest place around and use pumps to push water through pipes to the top of the tower. When you turn on a water tap, gravity pulls water down through other pipes and to the tap. This uses less energy than a pump pushing water through miles of pipes. Try making your own water tower.

Caution: This activity is messy so be sure to test it outside or over a sink.

›

Cut off the bottom of the plastic bottle. Hold the bottle upside down.

› Put the short part of the straw up into the neck of the bottle. Push clay around it so that no water will leak out. Make sure that you don’t block the opening of the straw with any clay.

› Bend the straw so the long free end outside the bottle is pointing up as high as possible. Cover the end of the straw with a finger. Fill the bottle with water.

› Remove your finger from the end of the straw as you slowly bend it down. When does the water shoot the farthest? When does it run the slowest? Does it ever stop when there is still water in the bottle?

Try This!

Most water towers send water through more than one pipe. Measure the farthest distance water can shoot out of your bottle with just one straw pipe. Then add a second straw to the neck of the bottle. How far can the water shoot with both straws open? Predict how far the water would shoot if you added a third straw pipe. Then try it!

TOOL KIT

COUNTERWEIGHT

Some houses have windows that you push up to open. Why doesn’t gravity pull the window down as soon as you let go? A counterweight may be hidden on the side of the window. Many counterweights work with a pulley.

› Poke the pencil through the middle of the spool. This will be your handle.

› Tie a binder clip to each end of the string.

› Find the middle of the string and wrap the string once around the spool. Try to leave even lengths of string on each side of the spool.

› Attach one cup to each binder clip.

› Put three marbles in one cup and six marbles in the other cup. Lift the spool and cups off the table using the pencil handle. What happens to each cup?

› Add one marble to a cup, pull it down, and let go. Does the other cup stay up or fall back down? Do you need to have the same number in each cup for the spool to be balanced?

See a counterweight in action! How does this gravity goods ropeway work? Why does the top load need to be heavier than the bottom load?

Practical Action gravity goods

Try This!

Where else are counterweights used? Here are some places to look for them. Can you think of more?

•

On the scale at a doctor’s office.

• Inside a grandfather clock.

• Holding the bar of lights up over a stage.

• On a construction crane.

• In an elevator shaft.

Add illo?

CANTILEVER

Most benches that you sit on have legs right under you. A picnic table with attached benches is different. The benches don’t have anything right under them. They have a beam called a cantilever that is attached to the table.

TOOL KIT

° science journal

° flat stiff ruler

° table

° 30 pennies

How much mass can a cantilever support? Get ready to use what you know about gravity and center of balance to find out! Use a chart like this one to record your data.

› The ruler is your cantilever. Lay it on a table. Move one end over the edge until it is just in balance.

› Place four pennies on the part of the ruler that is on the table, 3 inches from the table’s edge. These pennies are the support for your cantilever.

See how to use simple wooden planks to construct a model cantilevered bridge!

Then look for real life examples the next time you travel. What are some similarities among cantilevers? What are some differences?

Keva Cam cantilever

› How many pennies do you think you can place on the unsupported part of the ruler? Try it and see. Start with one penny 3 inches from the edge of the table. Add one penny to the stack at a time. When does it tip over? In the chart, record the number of pennies in the stack before it falls (row 1, column 3).

Simple Machines

A pulley is a simple machine. A simple machine is called simple because it has few parts. A simple machine cannot do anything by itself. It needs you to provide energy to make it do its job. A pulley does not lift or lower the window unless you push up or down. But it makes it easier for the window to move and then stay in place after you let go. The inclined plane is another simple machine that helps you beat gravity. Inclined planes include ramps and slides. It takes less work to push an object up an inclined plane than to lift it up, especially when moving an object a long way.

› Set up the ruler again. This time place four support pennies (on the table side) 2 inches from the edge. Repeat the experiment by stacking one penny at a time 2 inches off the edge of the table. Record your results in the chart (row 2, column 3).

› Set up the ruler one more time with four support pennies 4 inches from the edge and stack pennies 4 inches from the edge. Record your results in the chart (row 3, column 3).

› Complete column 4 by multiplying column 2 by column 3. Compare column 4 to the number in column 1. What do you notice?

This!

What happens with different distances and numbers of pennies? Make stacks of pennies in many places at the same time and record your results in the chart. What do you notice? Can a cantilever ever fall? Can you find a balance point where you can put no pennies on the table side and one penny on the free side without having it fall?

Supported, on table

off table

Pennies

EXPLORE GRAVITY

MAD LIB

° glossary

° pencil

Use glossary words to fill in the blanks. You will end up with a silly story that may make you fall down laughing! It won’t be gravity’s fault this time!

noun: a person, place, or thing plural noun: more than one noun adjective: a word that modifies a noun (a red balloon)

verb: an action word adverb: a word that modifies a verb (I walked slowly)

Down to Earth your name

It was a adjective day on place your name was verb through the noun . All of a sudden, a noun past-tense verb and disappeared into place . That’s adjective , thought your name . I wonder if person one has been verb the gravity controls again. your name verb to the noun and verb for person two to come adverb . plural noun are adjective ! If you don’t verb , they will verb to place ! “Can you verb ?” asked person two . “I will go verb person one . your name past-tense verb to the place .” Once there, your name past-tense verb the plural noun . By the time the noun was done, your name was very adjective . But it was a adjective feeling to know that gravity was verb again.

air resistance: the force of air pushing against an object.

amniotic fluid: the liquid in a womb that surrounds a developing infant.

anatomy: the study of the structure of living things including naming all the parts and their functions.

anti-gravity: free from the force of gravity.

aqueduct: a pipe or bridge that moves water using gravity.

astronaut: a person trained for spaceflight.

atmosphere: the blanket of air surrounding the earth.

atom: a very small piece of matter. Atoms are the tiny building blocks of everything in the universe.

attraction: an invisible power that pulls things together.

avalanche: a large amount of snow that slides down a mountain very quickly.

balance: a tool that shows if the mass of objects is even.

GLOSSARY

BCE: put after a date, BCE stands for Before the Common Era and counts down to zero. CE stands for Common Era and counts up from zero. The year this book was published is 2013 CE.

black hole: a place in space where gravity is so strong even light gets pulled in.

calcium: a mineral found in shells and bones.

calculus: a branch of mathematics that deals with calculating things such as the slopes of curves. carat: the unit of weight for gems and pearls. One carat equals 200 milligrams. center of balance: the point on an object where its mass is even all the way around. center of gravity: the point on an object where it can be supported and stay in balance.

centrifugal force: the outward force on an object moving in a curved path.

centripetal force: the inward force that keeps an object moving at a steady speed in a circular path around another object.

compressed: pressed together very tightly, so it takes up less space.

counterweight: a weight that balances another weight.

data: information in the form of facts and numbers.

eardrum: the part of the ear that separates the inside of the ear from the outside. estimate: to make a best guess using facts you know. force: a strength or energy that can change the motion of an object.

free fall: to be pulled through the air by only the force of gravity.

galaxy: a collection of star systems held together by gravity. geometric: straight lines or simple shapes such as circles or squares.

G-force (g): a measure of the force of gravity.

gravitational pull: the force of gravity acting on an object. gravity: a natural force that pulls objects on and near Earth to the earth.

helium: a light gas often used to fill balloons.

horizontal: straight across from side to side.

Imperial system: a system of measuring units that was first officially approved by King Henry VII. It includes inch, foot, ounce, pound and others.

International Space Station (ISS): a massive space station orbiting Earth where astronauts live, conduct experiments, and study space.

level: not tilted, horizontal.

light-year: the distance light travels in one year, about 5.9 trillion miles.

magma: melted lava. matter: anything that has mass and takes up space.

metric system: a system of weights and measures based on the meter and the kilogram.

microgravity: very weak gravity.

missile: an object or weapon that is propelled toward a target.

mobile: a construction or sculpture made of balanced wire and shapes that can be set in motion by the movement of air.

NASA: National Aeronautics and Space Administration, the U.S. organization in charge of space exploration.

nerve: a bundle of thread-like structures that sends messages between different parts of the body and the brain.

nervous system: the communication system of the body, made of nerve cells that connect the brain and extend through the body.

new moon: the time when the moon is directly between Earth and the sun so it is not clearly visible or is seen as just a tiny sliver.

orbit: the path of an object circling another object.

oxygen: a gas in the air that people and animals need to breathe to stay alive.

philosopher: a person who thinks about and questions the way things are in the world and in the universe.

predict: to estimate what might happen before it happens.

proprioception: the awareness of the position of your body.

rate: the speed of something measured in an amount of time, such as miles per hour or feet per second. scale: a measuring device.

scientific law: a description of something that happens in nature, but not why. Scientific laws are based on observations.

sense of balance: your eyes, ears, and body senses all working together to help you stay upright and not fall over.

society: a group of people with shared laws, traditions, and values.

stable: steady and firm. terminal velocity: the fastest an object will travel in free fall.

trebuchet: a machine that uses weight and gravity to hurl large stones and other objects at enemies.

universe: everything that exists everywhere.

vacuum: a space that is empty of matter vertical: straight up and down. visual: relating to sight or seeing.

waterwheel: a wheel with paddles attached that spin when water flows over them. The wheel can lift water, power machines, or create electricity.

weight: a measure of the gravitational pull on an object. womb: the female organ in mammals that carries an infant during its development before birth.

Metric Conversions

Use this chart to find the metric equivalents to the English measurements in this book.

If you need to know a half measurement, divide by two. If you need to know twice the measurement, multiply by two. How do you find a quarter measurement? How do you find three times the measurement?

English Metric

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