# Review Questions: Linear Motion (Answers)

## Multiple Choice Questions

1. Galileo's use of inclined planes allowed him to effectively

a. Slow down the acceleration of free fall
b. Increase the acceleration beyond that of free fall.
c. Eliminate the acceleration of free fall
d. All of these
e. None of these

Reasoning: Rolling objects down the inclined plane allow him to study their motion at a lower acceleration than the free fall. See page 24 of the text.

2. An airplane that flies at 100 km/h with a 10 km/h tailwind travels at 110 km/h relative to the ground. If it instead flies into a 10 km/h headwind, its ground speed is

a. 90 km/h
b. 100 km/h
c. 110 km/h
d. 120 km/h

3. The two measurements necessary for calculating average speed are

a. Acceleration and time
b. Velocity and time
c. Distance and time
d. Distance and acceleration
e. Velocity and distance

Reasoning: The definition of the average speed is (covered distance)/(elapsed time).

4. The average speed of a horse that gallops a distance of 10 kilometers in a time of 30 minutes is

a. 10 km/h
b. 20 km/h
c. 30 km/h
d. More than 30 km/h

Reasoning: V(aveage) = (10 km) / (.5 hr) = 20 km/h

5. What is the acceleration of a car that maintains a constant velocity of 100 km/hr for 10 seconds?

a. 0
b. 10 km/hr/s
c. Both of these
d. None of these

Reasoning: Acceleration is related to change in velocity. Since the velocity remains constant, it means the acceleration is zero.

6. As an object freely falls downward, its

a. Velocity increases
b. Acceleration increases
c. Both of these
d. None of these

Reasoning: In a free fall velocity keeps increasing. However, the acceleration remains constant and is = 9.8 m/s/s.

7. If an object falling freely downward were somehow equipped with a speedometer on a planet where the acceleration due to gravity is 20 meters per second per second, then its speed reading would increase each second by

a. 10 m/s
b. 20 m/s
c. 30 m/s
d. 40 m/s
e. Depends on its initial speed.

8. If an object falling freely downward were somehow equipped with an odometer to measure the distance it travels, then the amount of distance it travels each succeeding second would be:

a. Constant
b. Less and less
c. Grater than the second before.

Reasoning: the distance traveled each second increases due to the fact that the velocity is increasing. See page 46.

9. Ten seconds after starting from rest, an object falling freely downward will have a speed of about:

a. 10 m/s
b. 50 m/s
c. 100 m/s
d. More than 100 m/s

Reasoning: Speed increases at a rate of 10 m/s (actually 9.8 m/s) every second. Thus after 10 seconds, the speed is 10 x 10 = 100 m/s.

10. If an object falls with constant acceleration, the velocity of the object must:

a. Be constant also
b. Continuously change by the same amount each second
c. Continuously change by varying amounts depending on its speed
d. Continuously decrease
e. None of these.

11. A heavy object and a light object are dropped at the same time from rest in a vacuum. The heavier object reaches the ground:

a. Sooner than the lighter object
b. At the same time as the lighter object
c. Later than the lighter object.

Reasoning: In a vacuum, there is no air resistance. The acceleration of the light and heavy object is the same.

12. If a car increases its velocity from zero to 60 km/h in 10 seconds, its acceleration is:

a. 3 km/h s
b. 6 km/h s
c. 10 km/h s
d. 60 km/h s
e. 600 km/h s.

Reasoning: acceleration = (change in V) / (time) = (60 - 0 km/h) / (10 s) = 6 km/h/s

13. If a rocket initially at rest accelerates at a rate of 50 m/s2 for one minute, its speed will be

a. 50 m/s
b. 500 m/s
c. 3000 m/s
d. 3600 mm/s
e. None of these.

Reasoning: Speed is increasing at the rate of 50 m/s every second. Thus in one minute (i.e. in 60 seconds) speed = (5 m/s/s) x (60 s) = 3000 m/s.

14. Drop a rock from a 5-m height and it accelerates at 10 m/s2 and strikes the ground 1 s later. Drop the same rock from a height of 2.5 m and its acceleration of fall is:

b. The same
c. More.

15. While a car travels around a circular track at constant speed its:

a. Acceleration is zero
b. Velocity is zero
c. Both of these
d. None of these.

Reasoning: Around a circular track velocity is changing because its direction is changing. The accelartion is changing as well. Thus (d) is the answer.

16. A car accelerates from rest at 2 meters per second per second. What is its speed 3 seconds after the car starts moving?

a. 2 m/s
b. 3 m/s
c. 4 m/s
d. 6 m/s
e. None of these.

17. A ball is thrown upwards and caught when it comes back down. Neglecting air resistance, its speed when caught is:

a. More than the speed it had when thrown upwards.
b. Less than the speed it had when thrown upwards.
c. The same as the speed it had when thrown upwards.

18. An object travels 8 meters in the first second of travel, 8 meters again during the third second. Its acceleration in meters per second is:

a. 0
b. 5
c. 8
d. 10
e. More than 10.

19. At one instant an object in free fall is moving upward at 50 meters per second. One second later its speed is about:

a. 100 m/s
b. 60 m/s
c. 55 m/s
d. 50 m/s
e. 40 m/s.

Reasoning: In a free fall, the acceleration is about 10 m/s/s. Thus the objects's speed will decrease by 10 m/s every second.

20. Disregarding air resistance, objects fall with constant:

a. Velocity
b. Speed
c. Acceleration
d. Distances each successive second.

21. A ball is thrown upwards and caught when it comes back down. In the presence of air resistance, the speed with which it is caught is:

a. More than the speed it had when thrown upwards.
b. Less than the speed it had when thrown upwards.
c. The same as the speed it had when thrown upwards.

Reasoning: In the presence of air resistance, the ball is going to reach its highest point which is shorter than the case when there is no air resistance. As the ball falls, its acceleration is going to be less than 9.8 m/s2. Thus at the end its final velocity will be less than the no air restsnce case.

22. An apple falls from a tree and hits the ground 5 meters below. It hits the ground with a speed of about:

a. 5 m/s
b. 10 m/s
c. 15 m/s
d. 20 m/s
e. Not enough information given to estimate.

Reasoning: For the apple to fall 5 m in its fall, it must be falling for about 1 second, (read pages 46-47, and chack Table 3.3). Thus in one second, its speed increases by about 10 m/s. It hits the ground with a speed of 10 m/s.

23. It takes 6 seconds for a stone to fall to the bottom of a mine shaft. How deep is the shaft?

d. More than 200 m

Reasoning: Using d = (1/2) g t2 = (1/2)(9.8)(36) = about 180 m. (see page 46).

24. In each second of fall, the distance a free falling object will fall is:

c. The same, but no 5 m or 10 m
d. Increasing
e. None of these.

Reasoning: In the first second (after the object is dropped) the distance is = 5 m. But in each second afterwards, the distance keeps increasing. See Table 3.3 on page 47.

25. If a projectile is fired straight up at a speed of 10 m/s, the time it takes to reach the top of its path is about:

a. 1 second
b. 2 seconds
c. 10 seconds
d. Not enough information to estimate.

Reasoning: At the top, its speed must go to zero. But acceleration (i.e.change of speed in every second) is 10 m/s/s. Thus its speed decreases by 10 m/s every second. Since it started with 10 m/s, it will take 1 second for its speed to go to zero and reach the top.