Q: A 0.25 kg ball is rolling to the right at 1.3 m/s and strikes a wall, rebounding to the left at 1.1 m/s. What is the change in the ball’s momentum?

$$\Delta p = 0.25 \times ( -1.1 - 1.3 ) = -0.6\ \mathrm{kg\cdot m/s}$$

Q: A 2.0 kg object moving to the right at 0.50 m/s experiences a force that brings it to rest. What is the impulse delivered to the object?

$$J = 2.0 \times (0 - 0.50) = -1.0\ \mathrm{kg\cdot m/s}$$

Q: A 150 g baseball is thrown at 20 m/s and hit straight back at 40 m/s. What is the change in momentum of the ball?

$$\Delta p = 0.150 \times ( 40 - ( -20 )) = 9.0\ \mathrm{kg\cdot m/s}$$

Q: A 1-kg box slides along a frictionless surface and collides with a 2-kg box. They stick together after the collision. If the 1-kg box was moving at 2 m/s and the 2-kg box was at rest, what is their final speed?

$$v_f = \frac{1 \times 2 + 2 \times 0}{1 + 2} = \frac{2}{3}\ \mathrm{m/s}$$

Q: A 30 g ball is fired from a 1.2 kg toy rifle at 15 m/s. What is the recoil speed of the rifle?

$$v_{\text{rifle}} = -\frac{0.030 \times 15}{1.2} = -0.38\ \mathrm{m/s}$$

Q: A peregrine falcon (0.80 kg) swoops at 18 m/s at a 45° angle and grabs a 0.36 kg pigeon flying horizontally at 9.0 m/s. What is the speed of the combined mass immediately after the collision?

$$v = \frac{\sqrt{ (0.80 \times 18 \cos 45^\circ + 0.36 \times 9)^2$ + (0.80 \times 18 \sin 45^\$$circ)^2$$ }}{0.80 + 0.36} \approx 15\ \mathrm{m/s}$$

Question 1

A 0.25 kg ball is rolling to the right at 1.3 m/s and strikes a wall, rebounding to the left at 1.1 m/s. What is the change in the ball’s momentum?

Accepted Answer

$$\Delta p = 0.25 	imes ( -1.1 - 1.3 ) = -0.6\ \mathrm{kg\cdot m/s}$$

Question 2

Which of the following best describes impulse?

Accepted Answer

Impulse is the area under the force-versus-time curve: $$J = \int F\,dt$$

Question 3

A 2.0 kg object moving to the right at 0.50 m/s experiences a force that brings it to rest. What is the impulse delivered to the object?

Accepted Answer

$$J = 2.0 	imes (0 - 0.50) = -1.0\ \mathrm{kg\cdot m/s}$$

Question 4

A 150 g baseball is thrown at 20 m/s and hit straight back at 40 m/s. What is the change in momentum of the ball?

Accepted Answer

$$\Delta p = 0.150 	imes ( 40 - ( -20 )) = 9.0\ \mathrm{kg\cdot m/s}$$

Question 5

A light plastic cart and a heavy steel cart are both pushed with the same force for 1.0 s, starting from rest. After the force is removed, the momentum of the light plastic cart is ______ that of the heavy steel cart.

Accepted Answer

Equal to

Question 6

A 1-kg box slides along a frictionless surface and collides with a 2-kg box. They stick together after the collision. If the 1-kg box was moving at 2 m/s and the 2-kg box was at rest, what is their final speed?

Accepted Answer

$$v_f = \frac{1 	imes 2 + 2 	imes 0}{1 + 2} = \frac{2}{3}\ \mathrm{m/s}$$

Question 7

A 30 g ball is fired from a 1.2 kg toy rifle at 15 m/s. What is the recoil speed of the rifle?

Accepted Answer

$$v_{	ext{rifle}} = -\frac{0.030 	imes 15}{1.2} = -0.38\ \mathrm{m/s}$$

Question 8

A peregrine falcon (0.80 kg) swoops at 18 m/s at a 45° angle and grabs a 0.36 kg pigeon flying horizontally at 9.0 m/s. What is the speed of the combined mass immediately after the collision?

Accepted Answer

$$v = \frac{\sqrt{ (0.80 	imes 18 \cos 45^\circ + 0.36 	imes 9)^2$ + (0.80 	imes 18 \sin 45^\$$circ)^2$$ }}{0.80 + 0.36} \approx 15\ \mathrm{m/s}$$

Question 9

Which of the following statements about the law of conservation of momentum is correct?

Accepted Answer

The total momentum of an isolated system remains constant if no net external force acts on it.

Question 10

A force acts on a cart for 1 s, starting from rest. To achieve the same final speed with a force half as big, the force would need to push for:

Accepted Answer

2 s

Question 11

A mosquito and a truck have a head-on collision. Which has the larger change in momentum?

Accepted Answer

They have the same change of momentum.

Question 12

A 36 kg child stands at the center of a 200 kg merry-go-round rotating once every 2.5 s. The child walks to the edge (2.0 m from center). What happens to the period of rotation?

Accepted Answer

The period increases because the moment of inertia increases.

Question 13

Which of the following is true for a perfectly inelastic collision?

Accepted Answer

The colliding objects stick together and move with a common final velocity.

Question 14

A system of two objects is isolated. If the objects explode apart, what can be said about the total momentum of the system?

Accepted Answer

The total momentum remains constant.

Question 15

A hammer hits a nail. According to Newton’s third law, the force of the nail on the hammer is:

Accepted Answer

Equal to the force of the hammer on the nail.

Question 16

A 1.0 kg cue ball is struck by a cue stick with a force of 100 N for 2 ms. What is the impulse delivered to the ball?

Accepted Answer

$$J = 100 	imes 0.002 = 0.2\ \mathrm{N\cdot s}$$

Question 17

A system has a net external force acting on it. What happens to the total momentum of the system?

Accepted Answer

The total momentum changes.

Question 18

A figure skater pulls in her arms during a spin, reducing her moment of inertia. What happens to her angular velocity?

Accepted Answer

Her angular velocity increases to conserve angular momentum.

Chapter 9: Momentum – Impulse, Conservation, and Collisions

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