Raymond A. Serway Chris Vuille
Chapter Two
Motion in inOne Dimension Motion One Dimension
Dynamics • The branch of physics involving the motion of an object and the relationship between that motion and other physics concepts – Force and mass
• Kinematics is a part of dynamics – In kinematics, you are interested in the description of motion – Not concerned with the cause of the motion
Quantities in Motion • Any motion involves three concepts – Displacement – Velocity – Acceleration
• These concepts can be used to study objects in motion
Displacement • Position : Defined in terms of a frame of reference – A choice of coordinate axes – Defines a starting point for measuring the motion – One dimensional, so generally the x- or y-axis
Displacement • Defined as the change in position – ∆x ≡ xf − xi • f stands for final and i stands for initial
– Units are meters (m) in SI
• Distance is the total length of travel. SI unit is meter.
Displacement Examples From A to B • • • •
xi = 30 m xf = 52 m ∆x = 22 m The displacement is positive, indicating the motion was in the positive x direction
From C to F • • • •
xi = 38 m xf = -53 m ∆x = -91 m The displacement is negative, indicating the motion was in the negative x direction
Displacement, Graphical
A car moves back and forth along x-axis
Graph of position vs. time
Vector and Scalar Quantities • Displacement is a vector quantity. • Vector quantities need both magnitude (size) and direction to completely describe them • Scalar quantities are completely described by magnitude only
Displacement Isn’t Distance • The displacement of an object is not the same as the distance it travels – Example: Throw a ball straight up and then catch it at the same point you released it • The distance is twice the height • The displacement is zero
Velocity • The average speed of an object is defined as the total distance traveled divided by the total time elapsed path length Average speed = elapsed time d v = t – Speed is a scalar quantity
Speed, cont • Average speed totally ignores any variations in the object’s actual motion during the trip • The path length and the total time are all that is important – Both will be positive, so speed will be positive
• SI units are m/s
Path Length vs. Distance • Distance depends only on the endpoints
– The distance does not depend on what happens between the endpoints – Is the magnitude of the displacement
• Path length will depend on the actual route taken
Velocity • It takes time for an object to undergo a displacement • The average velocity is rate at which the displacement occurs
• Velocity can be positive or negative – ∆t is always positive
• Average speed is not the same as the average velocity
Velocity continued • Direction will be the same as the direction of the displacement, + or - is sufficient in one-dimensional motion • Units of velocity are m/s (SI) – Other units may be given in a problem, but generally will need to be converted to these – In other systems: • US Customary: ft/s • cgs: cm/s
Speed vs. Velocity
• Cars on both paths have the same average velocity since they had the same displacement in the same time interval • The car on the blue path will have a greater average speed since the path length it traveled is larger
Graphical Interpretation of Velocity • Velocity can be determined from a position-time graph • Average velocity equals the slope of the line joining the initial and final points on the graph • An object moving with a constant velocity will have a graph that is a straight line
Average Velocity, Non Constant
• The motion is nonconstant velocity • The average velocity is the slope of the straight line joining the initial and final points
Instantaneous Velocity • The limit of the average velocity as the time interval approaches zero
• The instantaneous velocity indicates what is happening at every point of time – The magnitude of the instantaneous velocity is what you read on a car’s speedometer
Instantaneous Velocity on a Graph • The slope of the line tangent to the position vs. time graph is defined to be the instantaneous velocity at that time – The instantaneous speed is defined as the magnitude of the instantaneous velocity
Convert to km/hour!
Graphical Instantaneous Velocity • Average velocities are the blue lines • The green line (tangent) is the instantaneous velocity Section 2.2
Acceleration • Changing velocity means an acceleration is present • Acceleration is the rate of change of the velocity
• Units are m/s² (SI), cm/s² (cgs), and ft/s² (US Cust)
Average Acceleration • Vector quantity • When the object’s velocity and acceleration are in the same direction (either positive or negative), then the speed of the object increases with time – Speeding up
• When the object’s velocity and acceleration are in the opposite directions, the speed of the object decreases with time – Slowing down / decelerate
Same direction −1
−1
∆v −20ms − (−10ms ) a= = = −5ms −2 ∆t 2s • Minus sign of velocities indicate negative x direction. • The car is not slowing down. • Minus sign of acceleration also indicate negative x direction.
• IF v and a are in same directions; speed increase!
Instantaneous and Uniform Acceleration • Definition: The limit of the average acceleration as the time interval goes to zero
• When number
gets to zero, ratio gets to a fixed Instantaneous acceleration!
Graphical Interpretation of Acceleration • Average acceleration is the slope of the line connecting the initial and final velocities on a velocity vs. time graph • Instantaneous acceleration is the slope of the tangent to the curve of the velocity-time graph
Average Acceleration – Graphical Example
Motion Diagrams
1-D Motion with Constant Acceleration A particle moving along the x-axis with constant acceleration, a a) The acceleration vs time graph b) The velocity vs time graph c) The position vs time graph
• These equations are used in situations with uniform acceleration
Notes on the equations
• Gives displacement as a function of velocity and time • Use when you don’t know and aren’t asked for the acceleration
Notes on the equations • Shows velocity as a function of acceleration and time • Use when you don’t know and aren’t asked to find the displacement
Graphical Interpretation of the Equation
Notes on the equations
• Gives displacement as a function of time, velocity and acceleration • Use when you don’t know and aren’t asked to find the final velocity • The area under the graph of v vs. t for any object is equal to the displacement of the object
Notes on the equations
• Gives velocity as a function of acceleration and displacement • Use when you don’t know and aren’t asked for the time
Freely Falling Objects • 1564 - 1642 • Galileo Galilei formulated the laws that govern the motion of objects in free fall
Free Fall • A freely falling object is any object moving freely under the influence of gravity alone – Free fall does not depend on the object’s original motion
• All objects falling near the earth’s surface fall with a constant acceleration • The acceleration is called the acceleration due to gravity, and indicated by g = 9.80 m/s²
Free Fall – an object dropped • Initial velocity is zero • Let up be positive
+y
– Conventional
• Use the kinematic equations – Generally use y instead of x since vertical
• Acceleration is g = -9.80
m/s2
-y
Free Fall – an object thrown downward • a = g = -9.80 m/s2 • Initial velocity ≠ 0 – With upward being positive, initial velocity will be negative
Free Fall – object thrown upward • Initial velocity is upward, so positive • The instantaneous velocity at the maximum height is zero • a = g = -9.80 m/s2 everywhere in the motion
v=0
Actually straight back down
Thrown upward, cont. • The motion may be symmetrical – Then tup = tdown – Then v = -vo
• The motion may not be symmetrical – Break the motion into various parts • Generally up and down