Q: A soccer player kicks a stationary ball, and the force-time graph for the kick is shown as a triangle with a maximum force of 300 N and a contact time of 8.0 ms. What is the impulse delivered to the ball?

$$J = \frac{1}{2} \times 300\ \mathrm{N} \times 0.008\ \mathrm{s} = 1.2\ \mathrm{N \cdot s}$$

Q: A 0.25 kg ball moving at $$1.3\ \mathrm{m/s} to $$the right hits a wall and rebounds to the left at $$1.1\ \mathrm{m/s}. $$What is the change in the ball’s momentum?

$$\Delta p = m(v_f - v_i) = 0.25\ \mathrm{kg} \times (-1.1\ \mathrm{m/s} - 1.3\ \mathrm{m/s}) = -0.60\ \mathrm{kg \cdot m/s}$$

Q: Two ice skaters, one with mass $$45\ \mathrm{kg}$$ and the other with mass $$80\ \mathrm{kg}$$, push off from each other on frictionless ice. If the $$45\ \mathrm{kg}$$ skater moves at $$2.2\ \mathrm{m/s}$$, what is the speed of the $$80\ \mathrm{kg}$$ skater?

$$v = \frac{45\ \mathrm{kg} \times 2.2\ \mathrm{m/s}}{80\ \mathrm{kg}} = 1.2\ \mathrm{m/s}$$

Q: A 1 kg box moving at $$3\ \mathrm{m/s}$$ collides with a stationary 2 kg box on a frictionless surface. If they stick together after the collision, what is their final velocity?

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

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

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

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

$$\Delta p = 0.15\ \mathrm{kg} \times (40\ \mathrm{m/s} - (-20\ \mathrm{m/s})) = 9.0\ \mathrm{kg \cdot m/s}$$

Q: A force acts on a hockey puck for $$0.2\ \mathrm{s}$$, changing its velocity from $$2\ \mathrm{m/s} to$$ $$6\ \mathrm{m/s}. If $$the puck has a mass of $$0.5\ \mathrm{kg}$$, what is the average force applied?

$$F_{\text{avg}} = \frac{0.5\ \mathrm{kg} \times (6\ \mathrm{m/s} - 2\ \mathrm{m/s})}{0.2\ \mathrm{s}} = 10\ \mathrm{N}$$

Question 1

A soccer player kicks a stationary ball, and the force-time graph for the kick is shown as a triangle with a maximum force of 300 N and a contact time of 8.0 ms. What is the impulse delivered to the ball?

Accepted Answer

$$J = \frac{1}{2} 	imes 300\ \mathrm{N} 	imes 0.008\ \mathrm{s} = 1.2\ \mathrm{N \cdot s}$$

Question 2

Which of the following best describes the law of conservation of momentum for an isolated system?

Accepted Answer

The total momentum of the system remains constant before and after an interaction.

Question 3

A 0.25 kg ball moving at $$1.3\ \mathrm{m/s} to $$the right hits a wall and rebounds to the left at $$1.1\ \mathrm{m/s}. $$What is the change in the ball’s momentum?

Accepted Answer

$$\Delta p = m(v_f - v_i) = 0.25\ \mathrm{kg} 	imes (-1.1\ \mathrm{m/s} - 1.3\ \mathrm{m/s}) = -0.60\ \mathrm{kg \cdot m/s}$$

Question 4

Two ice skaters, one with mass $$45\ \mathrm{kg}$$ and the other with mass $$80\ \mathrm{kg}$$, push off from each other on frictionless ice. If the $$45\ \mathrm{kg}$$ skater moves at $$2.2\ \mathrm{m/s}$$, what is the speed of the $$80\ \mathrm{kg}$$ skater?

Accepted Answer

$$v = \frac{45\ \mathrm{kg} 	imes 2.2\ \mathrm{m/s}}{80\ \mathrm{kg}} = 1.2\ \mathrm{m/s}$$

Question 5

A 1 kg box moving at $$3\ \mathrm{m/s}$$ collides with a stationary 2 kg box on a frictionless surface. If they stick together after the collision, what is their final velocity?

Accepted Answer

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

Question 6

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

Accepted Answer

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

Question 7

Which of the following statements about impulse is correct?

Accepted Answer

Impulse is equal to the area under the force versus time curve.

Question 8

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

Accepted Answer

$$\Delta p = 0.15\ \mathrm{kg} 	imes (40\ \mathrm{m/s} - (-20\ \mathrm{m/s})) = 9.0\ \mathrm{kg \cdot m/s}$$

Question 9

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

Accepted Answer

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

Question 10

A force acts on a hockey puck for $$0.2\ \mathrm{s}$$, changing its velocity from $$2\ \mathrm{m/s} to$$ $$6\ \mathrm{m/s}. If $$the puck has a mass of $$0.5\ \mathrm{kg}$$, what is the average force applied?

Accepted Answer

$$F_{	ext{avg}} = \frac{0.5\ \mathrm{kg} 	imes (6\ \mathrm{m/s} - 2\ \mathrm{m/s})}{0.2\ \mathrm{s}} = 10\ \mathrm{N}$$

Question 11

A figure skater spins with an initial angular velocity of $$0.8\ \mathrm{rev/s}$$ and a moment of inertia of $$2.34\ \mathrm{kg \cdot m^2$}. $$She pulls her arms in, reducing her moment of inertia to $$0.363\ \mathrm{kg \cdot m^2$}. $$What is her new angular velocity?

Accepted Answer

$$\omega_f = \frac{2.34\ \mathrm{kg \cdot m^2$} 	imes 0.8\ \mathrm{rev/s}}{0.363\ \mathrm{kg \cdot m^2$}} = 5.2\ \mathrm{rev/s}$$

Question 12

Which of the following is the correct expression for the angular momentum $$L of a $$rotating object?

Accepted Answer

$$L = I \omega$$

Question 13

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

Accepted Answer

Both have the same magnitude of change in momentum.

Question 14

A system of three particles has momenta $$\vec{p}_1$$, $$\vec{p}_2$$, and $$\vec{p}_3. $$What is the total momentum of the system?

Accepted Answer

$$\vec{P} = \vec{p}_1 + \vec{p}_2 + \vec{p}_3$$

Question 15

A 2.0 kg cart moving at $$-2\ \mathrm{m/s}$$ collides with a wall and rebounds at $$1\ \mathrm{m/s}. $$What is the change in momentum?

Accepted Answer

$$\Delta p = 2.0\ \mathrm{kg} 	imes (1\ \mathrm{m/s} - (-2\ \mathrm{m/s})) = 6\ \mathrm{kg \cdot m/s}$$

Question 16

Which of the following is true about external forces and momentum conservation?

Accepted Answer

If the net external force on a system is zero, the total momentum is conserved.

Chapter 9: Momentum, Impulse, and Conservation Laws – Study Guide

Study Guide - Practice Questions

Study Guide - Flashcards