REVIEW QUESTIONS
1. What do we mean when we say that motion is relative? What is everyday motion
relative to? Measured in a reference frame, usually the earth’s surface. But if a train is
moving and you are in it, the reference frame is in motion. A person is standing on the
ground and a train goes by. A person on the train throws up an apple. To the person
on the ground in front of the train, the apple seems to draw out a curved path. To the
person throwing up the apple, the apple just seems to go straight up and down.
2. Speed is the rate at which what happens? The rate at which distance covered changes.
3. You walk across the room at 2 kilometers per hour. Express this speed using abbreviated
units. 2 km/hr.
4. What is the difference between instantaneous speed and average speed? Instantaneous
speed is speed at an instant (a photographic moment). Average speed is distance
covered over a period of time.
5. Does the speedometer of a car read instantaneous speed or average speed? It reads
instantaneous speed at every moment.
6. What is the difference between speed and velocity? Speed is distance over time.
Velocity is distance over time with a direction.
7. If the speedometer of a car reads a constant speed of 40 km/hr, can you s ay that the car
has a constant velocity? Why or why not? No, because if the car goes around a curve
and changes direction, even if the speed doesn´t change, its velocity changes.
8. What two controls on a car cause a change in speed? What control causes a change in
velocity? Brakes and accelerator pedal cause a change in speed. Steering wheel can
cause a change in velocity because it changes the direction of the car.
9. What quantity describes how quickly you change how fast you’re traveling, or how
quickly you change your direction? Acceleration.
10. Acceleration is the rate at which what happens? Rate at which velocity changes.
11. What is the acceleration of a car that travels in a straight line at a constant speed of 100
km/hr? Zero, since there is no change in velocity. Constant speed means no change.
12. What is the acceleration of a car moving along a straight-line path that increases its
speed from zero to 100 km/hr in 10 sec? a = (vf – vi)/(tf – ti) = (100 km/hr – 0)/(10 s – 0)
= 100 km/hr/10 s = 10 km/hr s
13. By how much does the speed of a vehicle moving in a straight line change each second
when it is accelerating at 2 km/ hrs? 2 km/ hr. At 4 km/ hrs? At 4 km/ hr. At 10 km/
hrs? 10 km/ hr
14. Why does the unit of time enter twice in the unit of acceleration? Once for speed
change, and again for time change. Acceleration is the change of velocity (which is
m/second) over time (which is seconds). There are two time units.
15. What is the meaning of free fall? The fall is free of air drag and force other than
gravity.
16. For a freely falling object dropped from rest, what is the instantaneous speed at the end
of the fifth second of fall? v = gt = (10 m/s/s)(5 s) = 50 m/s. The sixth second of fall?
v = gt = (10 m/s/s)(6 s) = 60 m/s.
17. For a freely falling object dropped from rest, what is the acceleration at the end of the
fifth second of fall? The sixth second? At the end of any elapsed time? The acceleration
will always be Gravity (10 m/s per second) after any time t.
18. Toss a ball upward. What is the change in speed each second on the way up? After one
second the ball will have lost 10 m/s due to gravity. Gravity (10 m/s per second). On
the way down? After one second the ball will have gained 10 m/s due to gravity.
Gravity (10 m/s per second).
19. How far will a freely falling object fall from rest in five seconds? The distance fallen
from rest is given by y = ½gt2= ½(10)(5)2 = 125 m. Six seconds? The distance fallen from
rest is given by y = ½gt2= ½(10)(6)2 = 180 m
20. How far will an object move in one second if its average speed is 5 m/s? It will move 5
meters.
21. How far will a freely falling object have fallen from a position of rest when its
instantaneous speed is 10 m/s? It will have fallen 5 m. Gravity (10 m/s per second)
makes objects reach a velocity of 10 m/s after one second. After one second, the
distance fallen from rest is given by y = ½gt2= ½(10)(1)2 = 5 m
22. What does the slope of the curve on a distance versus time graph represent? The
velocity. The formula for slope is m = (y2 – y1)/(x2 – x1) and the formula for velocity is
v = (vf – vi)/(tf – ti)
23. What does the slope of the curve on a velocity versus time graph represent? The
formula for slope is m = (y2 – y1)/(x2 – x1) and the formula for acceleration is
a = (vf – vi)/(tf – ti)
24. Does air resistance increase or decrease the acceleration of a falling object? It
decreases the acceleration because the only acceleration downwards is due to gravity
and air resistance is a force that acts upwards, against gravity.
25. What is the appropriate equation for how fast an object freely falls from a position of
rest? v = gt For how far that object falls? y = ½gt2