Unit 4 Review Problems: Momentum; Work, Power, and Energy
？ Momentum Conceptual Problems
1. Can a bullet have the same momentum as a truck? Explain.
2. Two identical objects have the exact same momentum. Do the velocities of these objects necessarily have
a. the same directions?
b. the same magnitudes (size)? Give your reasoning in each case.
3. Thinking of the impulse-momentum theorem, explain why cars are made with
a. bumpers that can be pushed in during a crash?
b. Air bags?
4. Thinking of the impulse-momentum theorem, if you are sitting at a baseball game and a foul ball comes in your
direction, which of the following scenarios will enable you to catch it barehanded so the pain (force) to your hand is
the smallest: Should you move your hands toward the ball thereby decreasing the time of the ball’s momentum
change, hold them still, or move them in the same direction as the ball is moving to increase the time the of the ball’s
momentum change? Explain.
5. Is it possible for an object to obtain a larger impulse from a smaller force than it does from a larger force? Explain.
6. A white cue ball moves across a pool table at 3m/s and collides head-on with a stationary 8-ball. The two balls have
equal mass. After the collision the white cue ball is at rest. What must be true of the linear speed of the 8-ball?
7. During a "space walk", the tether connecting an astronaut to the space capsule breaks. Using a pistol that ejects gas
particles (matter moving very fast), the astronaut manages to get back to the capsule. Using the conservation of
momentum, explain how she does this.
8. In movies, Superman hovers in midair, holding a villain by the neck, and throws him forward. Superman, however,
remains stationary. Using the conservation of momentum, explain what is wrong with this scene. 9. A satellite explodes in outer space, far from any other body, sending thousands of pieces in all directions. How does
the total momentum of the satellite before the explosion compare with the total momentum of all the pieces together
after the explosion? Account for your answer using the conservation of momentum.
10. An iceboat is coasting along on a frozen lake. Friction between the ice and the boat is negligible, and so is air
resistance. Nothing is propelling the boat. From a bridge someone jumps straight down (the person has no horizontal
component to his velocity) and lands on the boat which continues to coast straight ahead.
a. Does the forward momentum of the boat change? Explain.
b. Does the speed of the boat increase, decrease, or remain the same? Explain your answers using the
conservation of momentum.
11. On a distant asteroid, a large catapult is used to “throw” chunks of stone into space. Could such a device be used as a
propulsion system to move the asteroid further from the earth? Explain using the conservation of momentum.
12. When driving a golf ball, a good “follow-through” helps to increase the distance of the drive. A good follow-through
means that the club head is kept in contact with the ball as long as possible. Using the impulse-momentum theorem,
explain why this technique allows you to hit the ball farther.
？ Work, Power, and Energy Conceptual Problems
13. Two people of the same mass climb the same flight of stairs. The first person climbs the stairs in 25 s; the second
person takes 35 s.
a. Which person does more work? Explain your answer.
b. Which person produces more power? Explain your answer.
14. A sailboat is moving at a constant velocity.
a. Is there a net amount of work being done by a net force acting on the boat? Explain.
b. If the only forces that are acting on the boat are the wind propelling the boat forward and the friction
provided by the water, how does the work done by the wind’s force compare to the work done by the water’s
15. A ball is moving forward with a speed of 15 m/s. Only one force acts on the ball. After this force acts, the speed of
the ball is only 7 m/s. Has the force done positive or negative work on the ball? Explain.
16. Is it correct to conclude that someone is doing twice the work of another just because they are generating twice the
power? Explain, taking into account the time the work is being completed in.
17. Chris, walking forward with a constant velocity, carries a 10N bag of groceries along a horizontal level hallway to the
kitchen (a distance of 3.50 m). He carries the bag by applying a force straight up on the handle. How much work did
that particular force that Chris applied to the handle do on the bag?
18. How much work does a 10N centripetal force do in one revolution on an object that is in uniform circular motion? 19. Explain how energy and work are related.
20. Can a baseball have potential energy and kinetic energy at the same time? Explain.
21. Can a single object have kinetic energy but no momentum? Explain.
Unit 4 AP Review Problems: Momentum, Work, Power, and Energy 1
22. A net force acts on a particle. Is this enough information to conclude that
a. the velocity of the particle changes?
b. the kinetic energy of the particle changes?
c. The momentum of the particle changes? Give your reasoning for each case.
23. Is it possible to exert a net force and yet not cause a change in kinetic energy? Explain.
24. If two identical bowling balls are raised to the same height, one on Earth and the other on the moon, which has the
larger potential energy relative to the surface of the planets (I’m classifying the moon as a planet here…)?
25. Suppose the total energy “KE + PE” of an object is conserved (the sum is constant). g
a. If the kinetic energy decreases, what must be true about the potential energy?
b. If the potential energy decreases, what must be true about the kinetic energy?
c. If the kinetic energy does not change, what must be true about the potential energy?
Solve all Problems using the ideas of Momentum, Work, Power, and Energy
？ Momentum Problems
26. Two men pushing a stalled car apply a net force of +680 N for 7.2 s. What is the final momentum of the car? (4.90
27. A 62-kg person, standing on a diving board, dives straight down into the water. Just before striking the water, her
speed is 5.5 m/s. At a time of 1.65 s after entering the water, her speed is reduced to 1.1 m/s. What is the average net
force (magnitude and direction) that acts on her when she is in the water? (165.33N, up)
28. Hannah is standing on the ice. Another player fires a puck (m= 0.17 kg) at her with a velocity of +65 m/s. -3a. If she catches the puck with her glove and brings it to rest in a time of 5 x 10s, what is the average force
(magnitude and direction) exerted on the puck by her glove? (2.21 E3 N, opp the dxn of motion for puck)
b. Instead of catching the puck, she slaps it with her stick and returns the puck straight back to the player with a -3velocity of –65 m/s. The puck and stick are in contact for a time of 5 x 10s. Now, what is the average force
exerted on her by the puck? (4.42 E3N)
29. A basketball (m= 0.6 kg) is dropped from rest. Just before striking the floor, the magnitude of the basketball’s
momentum is 3.1 kg？m/s. At what height was the basketball dropped? (1.36m above the ground)
30. A 0.15 kg projectile is fired with a velocity of 715 m/s at a 2 kg wooden block that rests on a frictionless table. The
velocity of the block, immediately after the projectile passes through it, is 40 m/s. Find the velocity with which the
projectile exits the block. (181.67 m/s)
31. Kevin has a mass of 87 kg and is skating with in-line skates. He sees his 22 kg younger brother up ahead standing on
the sidewalk with his back turned. Coming up from behind, he grabs his brother and rolls off with him at a speed of
2.4 m/s. Ignoring friction, find Kevin’s speed just before he grabbed his brother. (3.01 m/s)
32. Batman (mass = 91 kg) jumps straight down from a bridge into a boat (mass = 510 kg) in which a criminal is fleeing.
The forward velocity of the boat is initially +11 m/s. What is the velocity of the boat after Batman lands in it?
33. The lead female character in a movie is standing on the edge of an offshore oil rig. As she fires a gun, she is driven
back over the edge and into the sea. Suppose the mass of a bullet is .01 kg and its velocity is +720 m/s. Her mass
(including the gun) is 51 kg.