Q: A dog drags two sleds, A (100 kg) and B (80 kg), across snow with a coefficient of friction $$\mu = 0.1. If $$the tension in rope 1 (attached to sled A) is 150 N, what is the tension in rope 2 (attached to sled B)?

$$T_2 = 150$$\ \mathrm{N} - (100\ \mathrm{kg} \times 9.8\ \mathrm{$$m/s^2$$} \times 0.1) = 52\ \mathrm{N}$$

Q: A 4 kg box is on a frictionless 35° slope and is connected via a massless string over a massless, frictionless pulley to a hanging 2 kg weight. What is the acceleration of the system after release?

$$a = \frac{2 \times 9.8 - 4 \times 9.8 \times \sin 35^\circ}{2 + 4} = 0.36\ \mathrm{m/s^2$}$$

Question 1

Which of the following statements best describes Newton's third law of motion?

Accepted Answer

For every action, there is always an equal and opposite reaction; the two forces act on different objects.

Question 2

A 1 kg block, a 2 kg block, and a 3 kg block are lined up in a row on a frictionless table. A 12 N force is applied to the 1 kg block. What is the acceleration of the system?

Accepted Answer

$$a = \frac{12}{1 + 2 + 3} = 2\ \mathrm{m/s^2$}$$

Question 3

If block A exerts a force $$\vec{F}_{A\ on\ B} on $$block B, what is the force that block B exerts on block A according to Newton's third law?

Accepted Answer

$$\vec{F}_{B\ on\ A} = -\vec{F}_{A\ on\ B}$$

Question 4

Which of the following is NOT a correct example of an action/reaction force pair?

Accepted Answer

The normal force a table exerts on a book and the gravitational force Earth exerts on the book.

Question 5

A dog drags two sleds, A (100 kg) and B (80 kg), across snow with a coefficient of friction $$\mu = 0.1. If $$the tension in rope 1 (attached to sled A) is 150 N, what is the tension in rope 2 (attached to sled B)?

Accepted Answer

$$T_2 = 150$$\ \mathrm{N} - (100\ \mathrm{kg} 	imes 9.8\ \mathrm{$$m/s^2$$} 	imes 0.1) = 52\ \mathrm{N}$$

Question 6

A painter (70 kg) sits on a 10 kg chair and pulls himself upward using a massless rope over a frictionless pulley. What force must he exert on the rope to accelerate upward at $$0.5\ \mathrm{m/s^2$}$$?

Accepted Answer

$$F = \frac{(70 + 10)(9.8 + 0.5)}{2} = 410\ \mathrm{N}$$

Question 7

A 4 kg box is on a frictionless 35° slope and is connected via a massless string over a massless, frictionless pulley to a hanging 2 kg weight. What is the acceleration of the system after release?

Accepted Answer

$$a = \frac{2 	imes 9.8 - 4 	imes 9.8 	imes \sin 35^\circ}{2 + 4} = 0.36\ \mathrm{m/s^2$}$$

Question 8

In a system of two blocks connected by a massless rope over a frictionless pulley, which statement about the tension in the rope is correct?

Accepted Answer

The tension is the same at both ends of the rope.

Question 9

Which of the following is a correct procedure for solving interacting-objects problems using Newton's laws?

Accepted Answer

Draw a free-body diagram for each object, showing only forces acting on that object.

Question 10

A horizontal force $$F$$ acts on a frictionless wedge of mass $$M$$ with a block of mass $$m$$ resting on it. What is the condition for $$m to $$not slip up or down the wedge?

Accepted Answer

$$a_y = a_{wedge}$$

Question 11

Which of the following best explains why action/reaction force pairs do not cancel each other out?

Accepted Answer

They act on different objects.

Question 12

A person pushes a wall with a force of 100 N. According to Newton's third law, what force does the wall exert on the person?

Accepted Answer

100 N in the opposite direction.

Question 13

Which of the following is a correct statement about the forces in an action/reaction pair?

Accepted Answer

They are equal in magnitude and opposite in direction.

Question 14

A block of mass $$m is $$pulled by a rope with tension $$T on a $$frictionless surface. What is the acceleration of the block?

Accepted Answer

$$a = \frac{T}{m}$$

Question 15

If two ice skaters push off against each other, what can be said about the forces they exert?

Accepted Answer

Each exerts a force on the other that is equal in magnitude and opposite in direction.

Question 16

Which of the following is a real-world example of Newton's third law?

Accepted Answer

A swimmer pushes water backward and moves forward.

Question 17

In the context of Newton's third law, what is the relationship between the force Earth exerts on the Sun and the force the Sun exerts on Earth?

Accepted Answer

They are equal in magnitude and opposite in direction.

Question 18

When analyzing a system of interacting objects, what is the first step you should take?

Accepted Answer

Identify which objects are part of the system and which are the environment.

Newton's Third Law: Action-Reaction and Interacting Objects

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