Q: A 1500 kg car is moving at 20 m/s. What is its momentum?

$$1500 \times 20 = 30{,}000\ \mathrm{kg\cdot m/s}$$

Q: A 0.2 kg baseball is pitched at 40 m/s and is hit back at 60 m/s. What is the change in momentum of the ball?

$$\Delta p = m(v_f - v_i) = 0.2 \times (60 - (-40)) = 0.2 \times 100 = 20\ \mathrm{kg\cdot m/s}$$

Q: A 5 kg fish swims at 1 m/s and swallows a 1 kg fish at rest. What is the velocity of the larger fish immediately after lunch? (Ignore water resistance.)

$$v = \frac{(5 \times 1) + (1 \times 0)}{5 + 1} = \frac{5}{6} \approx 0.83\ \mathrm{m/s}$$

Q: A 1000 kg car moving at 10 m/s collides and couples with a 1500 kg truck at rest. What is their final velocity after the collision?

$$v = \frac{(1000 \times 10) + (1500 \times 0)}{1000 + 1500} = \frac{10{,}000}{2500} = 4\ \mathrm{m/s}$$

Question 1

A 1500 kg car is moving at 20 m/s. What is its momentum?

Accepted Answer

$$1500 	imes 20 = 30{,}000\ \mathrm{kg\cdot m/s}$$

Question 2

Which of the following best describes the law of conservation of momentum?

Accepted Answer

If no net external force acts on a system, the total momentum of the system remains unchanged.

Question 3

A 0.2 kg baseball is pitched at 40 m/s and is hit back at 60 m/s. What is the change in momentum of the ball?

Accepted Answer

$$\Delta p = m(v_f - v_i) = 0.2 	imes (60 - (-40)) = 0.2 	imes 100 = 20\ \mathrm{kg\cdot m/s}$$

Question 4

If the speed of an object is doubled, its momentum will:

Accepted Answer

Double

Question 5

A 5 kg fish swims at 1 m/s and swallows a 1 kg fish at rest. What is the velocity of the larger fish immediately after lunch? (Ignore water resistance.)

Accepted Answer

$$v = \frac{(5 	imes 1) + (1 	imes 0)}{5 + 1} = \frac{5}{6} \approx 0.83\ \mathrm{m/s}$$

Question 6

Which of the following is true about impulse?

Accepted Answer

Impulse is the product of force and the time interval over which it acts: $$J = F t$$

Question 7

A karate expert breaks a board with a quick strike. Why is the force on the board so large?

Accepted Answer

Because the time interval of contact is very short, resulting in a large force for a given impulse.

Question 8

In catching a fast-moving baseball, why is it better to extend your hand forward as you catch the ball?

Accepted Answer

It increases the time over which the ball's momentum is brought to zero, reducing the force felt.

Question 9

A 1000 kg car moving at 10 m/s collides and couples with a 1500 kg truck at rest. What is their final velocity after the collision?

Accepted Answer

$$v = \frac{(1000 	imes 10) + (1500 	imes 0)}{1000 + 1500} = \frac{10{,}000}{2500} = 4\ \mathrm{m/s}$$

Question 10

Which of the following is a characteristic of an elastic collision?

Accepted Answer

Colliding objects rebound without lasting deformation or generation of heat.

Question 11

A dish falls and lands on a carpet rather than a hard floor. Why is the change in momentum the same in both cases?

Accepted Answer

Because the initial and final velocities are the same, so $$\Delta p is $$unchanged.

Question 12

A bullet and a ship are both moving. Which could have more momentum?

Accepted Answer

The ship, because its mass is much greater, even if its speed is lower.

Question 13

If you push with the same force on something for twice the time, its change in momentum is:

Accepted Answer

Doubled

Question 14

A 2 kg cart moving at 3 m/s collides with a 1 kg cart moving at -2 m/s (opposite direction). What is the total momentum before the collision?

Accepted Answer

$$p_{	ext{total}} = (2 	imes 3) + (1 	imes -2) = 6 - 2 = 4\ \mathrm{kg\cdot m/s}$$

Question 15

Which of the following is NOT true about momentum?

Accepted Answer

Momentum is a scalar quantity.

Question 16

A freight car A (mass 2000 kg) moving at 5 m/s collides and couples with identical car B at rest. What is the speed of the coupled cars after collision?

Accepted Answer

$$v = \frac{(2000 	imes 5) + (2000 	imes 0)}{2000 + 2000} = \frac{10{,}000}{4000} = 2.5\ \mathrm{m/s}$$

Question 17

Which of the following scenarios best illustrates the concept of impulse reducing force in real life?

Accepted Answer

Bending your knees when landing from a jump.

Question 18

A 5 kg fish swims at 1 m/s and swallows a 1 kg fish swimming toward it at 4 m/s. What is the velocity of the larger fish immediately after lunch?

Accepted Answer

$$v = \frac{(5 	imes 1) + (1 	imes -4)}{5 + 1} = \frac{5 - 4}{6} = \frac{1}{6} \approx 0.17\ \mathrm{m/s}$$

Chapter 6: Momentum – Concepts, Impulse, and Conservation

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