A 10 kg cart is moving to the left at 2 m/s. Define positive as “towards the right”, so its initial velocity is –2 m/s.

The cart suddenly stops. What is the change in momentum of the cart?

1. –20 kg m/s
2. –10 kg m/s
3. 0 kg m/s
4. 10 kg m/s
5. 20 kg m/s

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Question 8.1

• A ball is thrown directly upward.
• At time t₁ it passes a specific height y₁ on the way upward.
• At time t₂ it passes the same height y₁ on the way downward.
• How does the momentum of the ball at time t₁ compare to t₂?
• ​[Neglect air resistance in this question.]

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1. The momentum is the same.
2. The momentum is different.

Question 8.2

• Two ice skaters, Paula and Ricardo, push off from each other. They were both initially at rest. Ricardo has a greater mass than Paula. Which skater has the greater magnitude of momentum after the push-off?
• Ricardo
• Paula
• neither

Question 8.3

• Two ice skaters, Paula and Ricardo, push off from each other. They were both initially at rest. Ricardo has a greater mass than Paula. Which skater has the greater magnitude of velocity after the push-off?
• Ricardo
• Paula
• neither

Question 8.4

Imagine you are trapped in a canoe in the middle of a still lake with no paddles. There is a large pile of heavy rocks in the canoe.

If you start throwing rocks, can you propel the canoe this way? And, if so, and you want to get to shore, which way should you throw the rocks?

1. No, you can’t move the canoe significantly
2. Yes, throw the rocks toward the shore
3. Yes, throw the rocks away from the shore.

Question 8.5

During a head-on collision, how does an air bag protect a passenger in the car?

1. It decreases the impulse exerted the passenger.
2. It decreases the initial momentum of the passenger.
3. It lengthens the stopping time interval for the passenger, and decreases the force exerted on them during the collision.
4. It shortens the stopping time interval for the passenger, and increases the force exerted on them during the collision.

Question 8.6

A basketball with mass 0.1 kg is traveling down and to the right with v_x = +5 m/s, and v_y = –5 m/s.

It hits the horizontal ground, and then is traveling up and to the right with v_x ′= +5 m/s, and v_y ′ = +4 m/s.

During the collision with the ground, what is the change in the x-component of the ball’s momentum?

1. +0.5 kg m/s
2. –0.5 kg m/s
3. +1.0 kg m/s
4. +0.9 kg m/s
5. zero

x

y

Question 8.7

A basketball with mass 0.1 kg is traveling down and to the right with v_x = +5 m/s, and v_y = –5 m/s.

It hits the horizontal ground, and then is traveling up and to the right with v_x ′= +5 m/s, and v_y ′ = +4 m/s.

During the collision with the ground, what is the change in the y-component of the ball’s momentum?

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1. +0.5 kg m/s
2. –0.5 kg m/s
3. +1.0 kg m/s
4. +0.9 kg m/s
5. zero

Question 8.8

x

y

1. They exert equal impulses.
2. The putty ball exerts a larger impulse.
3. The rubber ball exerts a larger  impulse.

• Two balls of the same mass and the same speed collide with a block of wood.
• One ball is made of putty, and stops when it hits the block of wood.
• One ball is made of rubber, and bounces backward when it hits the block of wood.�Which ball delivers the larger impulse to the block of wood?

Demonstration Prediction

Question 8.9