1. Chapter 8
Rotational Motion

2. To impress his friends while riding on a carnival Ferris wheel, Prankster Paul gets out of his chair and climbs along the spoke of the wheel toward its center while the wheel continues in uniform motion. While climbing toward the center, Paul’s linear speed
a. increases while his angular speed remains unchanged.
b. decreases while his angular speed remains unchanged.
c. and his angular speed increase.
d. and his angular speed decrease.
Answer: B

3. To impress his friends while riding on a carnival Ferris wheel, Prankster Paul gets out of his chair and climbs along the spoke of the wheel toward its center while the wheel continues in uniform motion. While climbing toward the center, Paul’s linear speed
a. increases while his angular speed remains unchanged.
b. decreases while his angular speed remains unchanged.
c. and his angular speed increase.
d. and his angular speed decrease.
Answer: B

4. When you roll a tapered cup across a table, the path of the cup curves because the wider end rolls
a. slower.
b. at the same speed as the narrow part.
c. faster.
d. in an unexplained way.
Answer: C

5. When you roll a tapered cup across a table, the path of the cup curves because the wider end rolls
a. slower.
b. at the same speed as the narrow part.
c. faster.
d. in an unexplained way.
Answer: C

6. The rotational inertia of an object is greater when most of the mass is located
a. near the center.
b. away from the rotational axis.
c. on the rotational axis.
d. off-center.
Answer: B

7. The rotational inertia of an object is greater when most of the mass is located
a. near the center.
b. away from the rotational axis.
c. on the rotational axis.
d. off-center.
Answer: B

8. For round objects rolling down an incline, the fastest objects are generally those with the
a. greatest rotational inertia compared with mass.
b. lowest rotational inertia compared with mass.
c. most streamlining for reduced air drag.
d. highest center of gravity.
Answer: B

9. For round objects rolling down an incline, the fastest objects are generally those with the
a. greatest rotational inertia compared with mass.
b. lowest rotational inertia compared with mass.
c. most streamlining for reduced air drag.
d. highest center of gravity.
Answer: B

10. Which of these will roll down an incline fastest?
a. Disk
b. Hoop
c. Solid ball
d. The one with the greatest mass will roll fastest.
Answer: C

11. Which of these will roll down an incline fastest?
a. Disk
b. Hoop
c. Solid ball
d. The one with the greatest mass will roll fastest.
Answer: C

12. A torque is a force a. like any other force.
b. multiplied by a lever arm.
c. that is fictitious.
d. that accelerates things.
Answer: B

13. A torque is a force a. like any other force.
b. multiplied by a lever arm.
c. that is fictitious.
d. that accelerates things.
Answer: B

14. When two children of different weights balance on a seesaw, they each produce
a. equal torques.
b. unequal torques.
c. equal-magnitude torques in opposite directions.
d. equal forces.
Answer: C

15. When two children of different weights balance on a seesaw, they each produce
a. equal torques.
b. unequal torques.
c. equal-magnitude torques in opposite directions.
d. equal forces.
Answer: C

16. The center of mass of a donut is located
a. in the hole.
b. in material making up the donut.
c. near the center of gravity.
d. over a point of support.
Answer: A

17. The center of mass of a donut is located
a. in the hole.
b. in material making up the donut.
c. near the center of gravity.
d. over a point of support.
Answer: A

18. The center of gravity of an object
a. lies inside the object.
b. lies outside the object.
c. may or may not lie inside the object.
d. is near the center of mass.
Answer: C

19. The center of gravity of an object
a. lies inside the object.
b. lies outside the object.
c. may or may not lie inside the object.
d. is near the center of mass.
Answer: C

20. An object resting on a surface will topple over if its center of gravity
a. lies outside the object.
b. extends beyond the support base.
c. is displaced from its center of mass.
d. lowers at the point of tipping.
Answer: B

21. An object resting on a surface will topple over if its center of gravity
a. lies outside the object.
b. extends beyond the support base.
c. is displaced from its center of mass.
d. lowers at the point of tipping.
Answer: B

22. The center of gravity of your best friend is located
a. near the belly button.
b. at different places depending on body orientation.
c. near the center of mass.
d. at a fulcrum when rotation occurs.
Answer: B

23. The center of gravity of your best friend is located
a. near the belly button.
b. at different places depending on body orientation.
c. near the center of mass.
d. at a fulcrum when rotation occurs.
Answer: B

24. When an stable object is made to topple over, its center of gravity
a. is at first raised.
b. is at first lowered.
c. plays a minor role.
d. plays no role.
Answer: A

25. When an stable object is made to topple over, its center of gravity
a. is at first raised.
b. is at first lowered.
c. plays a minor role.
d. plays no role.
Answer: A

26. Which more accurately describes what happens in the circular spin cycle of a washing machine?
a. Water is forced away from the clothes.
b. Clothes are forced into a circular path while water is not.
c. Centripetal force acts on the water but not the clothes.
d. Centrifugal force acts on the clothes but not the water.
Answer: B

27. Which more accurately describes what happens in the circular spin cycle of a washing machine?
a. Water is forced away from the clothes.
b. Clothes are forced into a circular path while water is not.
c. Centripetal force acts on the water but not the clothes.
d. Centrifugal force acts on the clothes but not the water.
Answer: B

28. When you whirl a tin can in a horizontal circle overhead, the force that holds the can in the circular path acts in a direction
a. inward.
b. outward.
c. either inward or outward.
d. parallel to the force of gravity.
Answer: A

29. When you whirl a tin can in a horizontal circle overhead, the force that holds the can in the circular path acts in a direction
a. inward.
b. outward.
c. either inward or outward.
d. parallel to the force of gravity.
Answer: A

30. When you whirl a tin can in a horizontal circle overhead, the force that the can exerts on the string acts in a direction
a. inward.
b. outward.
c. either inward or outward.
d. parallel to the force of gravity.
Answer: B

31. When you whirl a tin can in a horizontal circle overhead, the force that the can exerts on the string acts in a direction
a. inward.
b. outward.
c. either inward or outward.
d. parallel to the force of gravity.
Explanation: The string pulls on the can, action; in reaction, the can pulls on the string in the opposite direction.
Answer: B

32. A bug inside a can whirled in a circle feels a force of the can on its feet. This force acts in a direction
a. toward the center of the circular path.
b. away from the center of the circular path.
c. either toward or away from the center of the circular path.
d. parallel to the force of gravity.
Answer: A

33. A bug inside a can whirled in a circle feels a force of the can on its feet. This force acts in a direction
a. toward the center of the circular path.
b. away from the center of the circular path.
c. either toward or away from the center of the circular path.
d. parallel to the force of gravity.
Explanation: This is the support force of the floor on the bug’s feet, equal and opposite to the fictitious centrifugal force on the bug. If this is confusing, be kind to yourself. It also confuses the best of students.
Answer: A

34. Gravity can be simulated for astronauts in outer space if their habitat
a. is very close to Earth.
b. is in free fall about Earth.
c. rotates.
d. revolves about Earth.
Answer: C

35. Gravity can be simulated for astronauts in outer space if their habitat
a. is very close to Earth.
b. is in free fall about Earth.
c. rotates.
d. revolves about Earth.
Answer: C

36. For an object traveling in a circular path, its angular momentum doubles when its
a. linear speed doubles and radius remains the same.
b. radius doubles and its linear speed remains the same.
c. mass doubles and linear speed and radius remain the same.
d. All of these.
Answer: D

37. For an object traveling in a circular path, its angular momentum doubles when its
a. linear speed doubles and radius remains the same.
b. radius doubles and its linear speed remains the same.
c. mass doubles and linear speed and radius remain the same.
d. All of these.
Explanation: The equation mvr guides the answer. Double either m, v, or r and angular momentum is doubled.
Answer: D

38. The angular momentum of a system is conserved
a. never.
b. under some conditions.
c. under all conditions.
d. when angular velocity remains unchanged.
Answer: B

39. The angular momentum of a system is conserved
a. never.
b. under some conditions.
c. under all conditions.
d. when angular velocity remains unchanged.
Explanation: A condition for angular momentum being conserved is that no net torque acts on the system. Steady angular velocity and changing radial distance produce changing angular momentum, so choice d is incorrect.
Answer: B

40. As Earth’s daily rotation decreases due to tidal friction, the Moon undergoes
a. a decrease in angular momentum.
b. an increase in angular momentum.
c. wobbling in its orbit about Earth.
None of these.
Answer: B

41. As Earth’s daily rotation decreases due to tidal friction, the Moon undergoes
a. a decrease in angular momentum.
b. an increase in angular momentum.
c. wobbling in its orbit about Earth.
None of these.
Answer: B
Explanation: Angular momentum of the Earth–Moon system is conserved. So a decrease in angular momentum of Earth due to tidal friction (an internal force or torque) is accompanied by an increase in angular momentum of the Moon (an increase in distance from Earth).