Q: A 0.15 kg baseball is pitched at 40 m/s. What is the magnitude of its momentum?

$$p = 0.15 \times 40 = 6\ \mathrm{kg\cdot m/s}$$

Q: A 60 kg astronaut floating in space pushes off a 90 kg astronaut. If the 60 kg astronaut moves away at 2 m/s, what is the velocity of the 90 kg astronaut after the push?

$$v = -\frac{60 \times 2}{90} = -1.33\ \mathrm{m/s}$$

Q: A 2 kg cart moving at 3 m/s collides elastically with a stationary 1 kg cart. What is the final velocity of the 2 kg cart after the collision?

$$v_{1f} = \frac{2 - 1}{2 + 1} \times 3 = 1\ \mathrm{m/s}$$

Q: A hailstone of mass 0.02 kg strikes a roof at 10 m/s and comes to rest in 0.005 s. What is the average force exerted by the roof on the hailstone?

$$F_{avg} = \frac{\Delta p}{\Delta t} = \frac{0.02 \times 10}{0.005} = 40\ \mathrm{N}$$

Q: A rocket ejects 5 kg of fuel per second at a speed of 2000 m/s. What is the thrust produced by the rocket?

$$F = \dot{m} v = 5 \times 2000 = 10000\ \mathrm{N}$$

Q: Two ice skaters, one of mass 50 kg and the other 70 kg, push off each other and move in opposite directions. If the 50 kg skater moves at 3 m/s, what is the speed of the 70 kg skater?

$$v = \frac{50 \times 3}{70} = 2.14\ \mathrm{m/s}$$

Question 1

A 0.15 kg baseball is pitched at 40 m/s. What is the magnitude of its momentum?

Accepted Answer

$$p = 0.15 	imes 40 = 6\ \mathrm{kg\cdot m/s}$$

Question 2

Which statement best describes the impulse-momentum theorem?

Accepted Answer

The impulse delivered to an object equals its change in momentum: $$\vec{J} = \Delta \vec{p}$$

Question 3

A 60 kg astronaut floating in space pushes off a 90 kg astronaut. If the 60 kg astronaut moves away at 2 m/s, what is the velocity of the 90 kg astronaut after the push?

Accepted Answer

$$v = -\frac{60 	imes 2}{90} = -1.33\ \mathrm{m/s}$$

Question 4

Which of the following is true for a perfectly elastic collision between two objects?

Accepted Answer

Both momentum and kinetic energy are conserved.

Question 5

A 2 kg cart moving at 3 m/s collides elastically with a stationary 1 kg cart. What is the final velocity of the 2 kg cart after the collision?

Accepted Answer

$$v_{1f} = \frac{2 - 1}{2 + 1} 	imes 3 = 1\ \mathrm{m/s}$$

Question 6

In a completely inelastic collision, which of the following statements is correct?

Accepted Answer

The colliding objects stick together after the collision.

Question 7

A hailstone of mass 0.02 kg strikes a roof at 10 m/s and comes to rest in 0.005 s. What is the average force exerted by the roof on the hailstone?

Accepted Answer

$$F_{avg} = \frac{\Delta p}{\Delta t} = \frac{0.02 	imes 10}{0.005} = 40\ \mathrm{N}$$

Question 8

Which of the following best describes the motion of the center of mass of a system of particles when acted on by external forces?

Accepted Answer

The center of mass moves as if all the mass were concentrated at that point and acted on by the net external force.

Question 9

A rocket ejects 5 kg of fuel per second at a speed of 2000 m/s. What is the thrust produced by the rocket?

Accepted Answer

$$F = \dot{m} v = 5 	imes 2000 = 10000\ \mathrm{N}$$

Question 10

Two ice skaters, one of mass 50 kg and the other 70 kg, push off each other and move in opposite directions. If the 50 kg skater moves at 3 m/s, what is the speed of the 70 kg skater?

Accepted Answer

$$v = \frac{50 	imes 3}{70} = 2.14\ \mathrm{m/s}$$

Question 11

Which of the following is NOT a condition for the conservation of momentum in a system?

Accepted Answer

The system must consist of only one particle.

Question 12

A 1 kg ball moving at 4 m/s collides elastically with a stationary 1 kg ball. What are their velocities after the collision?

Accepted Answer

First ball: $$0\ \mathrm{m/s}$$, second ball: $$4\ \mathrm{m/s}$$

Question 13

Which formula gives the position of the center of mass for a system of particles?

Accepted Answer

$$x_{cm} = \frac{\sum m_i x_i}{\sum m_i}$$

Question 14

A car crash is designed to be inelastic. Why is this beneficial for passenger safety?

Accepted Answer

Inelastic collisions absorb energy, reducing the force on passengers.

Question 15

A 3 kg object moving at 5 m/s collides with a 2 kg object at rest. If they stick together after the collision, what is their final velocity?

Accepted Answer

$$v_f = \frac{3 	imes 5}{3 + 2} = 3\ \mathrm{m/s}$$

Question 16

Which of the following best explains why a person should bend their knees when landing from a jump?

Accepted Answer

Bending the knees increases the time over which the momentum changes, reducing the force on the legs.

Question 17

A 1 kg ball moving at 10 m/s collides elastically with a 10 kg ball at rest. What is the velocity of the 1 kg ball after the collision?

Accepted Answer

$$v_{1f} = \frac{1 - 10}{1 + 10} 	imes 10 = -8.18\ \mathrm{m/s}$$

Question 18

Which of the following is true about the center of mass of a homogeneous sphere?

Accepted Answer

It is located at the geometric center of the sphere.

Chapter 08

Study Guide - Practice Questions

Study Guide - Flashcards