Q: A 1 kg ball is dropped from rest. The earth exerts a gravitational force on the ball, and the ball exerts an equal force on the earth. If the ball’s acceleration is $$a_{ball} = 9.8\ \mathrm{m/s^2$}$$, what is the earth’s acceleration due to the ball’s force?

$$a_{earth} = 2 \times 10$$^{-24}$$\ \mathrm{m/s^2$}$$

Question 1

Which statement best describes Newton’s third law?

Accepted Answer

For every action, there is an equal but opposite reaction; these forces act on different objects.

Question 2

A person pushes a crate across a rough floor. Which of the following is NOT an action/reaction pair according to Newton’s third law?

Accepted Answer

The person pushes on the crate and the floor pushes up on the crate.

Question 3

A mosquito collides head-on with a moving truck. According to Newton’s third law, during the collision:

Accepted Answer

The mosquito and the truck exert equal and opposite forces on each other.

Question 4

A 1 kg ball is dropped from rest. The earth exerts a gravitational force on the ball, and the ball exerts an equal force on the earth. If the ball’s acceleration is $$a_{ball} = 9.8\ \mathrm{m/s^2$}$$, what is the earth’s acceleration due to the ball’s force?

Accepted Answer

$$a_{earth} = 2 	imes 10$$^{-24}$$\ \mathrm{m/s^2$}$$

Question 5

Two blocks, A and B, are connected by a massless string and pulled across a frictionless table. Block B has a larger mass than A. Which statement about the tensions in the strings is correct?

Accepted Answer

The tension in string 2 is less than the tension in string 1.

Question 6

A student pulls horizontally with a 100 N force on a rope attached to a wall. In a second scenario, two students pull on opposite ends of a rope with 100 N each. What is the tension in the rope in both cases?

Accepted Answer

The tension is 100 N in both cases.

Question 7

A car is parked at rest on a horizontal road. The upward force of the road on the car (normal force) is equal in magnitude to the downward force of gravity. Which principle explains this?

Accepted Answer

Newton’s first law, because the car is in equilibrium.

Question 8

In a tug-of-war, you pull on a rope and your opponent pulls back. If you exert a force of 300 N, what is the force exerted by your opponent on you?

Accepted Answer

300 N

Question 9

A block hangs from a massless rope. The tension in the rope is due to:

Accepted Answer

The stretching of spring-like molecular bonds within the rope.

Question 10

Two blocks, A and B, are connected by a massless string and move together. If block A moves to the right with acceleration $$a_{Ax}$$, block B moves down with acceleration $$a_{By}. $$What is the acceleration constraint?

Accepted Answer

$$a_{Ax} = -a_{By}$$

Question 11

A block of mass $$m is $$accelerated across a frictionless table by a falling mass $$m$$ connected by a massless string over a massless, frictionless pulley. What is the tension $$T in $$the string compared to $$mg$$?

Accepted Answer

$$T \u003c mg$$

Question 12

Which of the following best describes the role of a massless string in a system of interacting objects?

Accepted Answer

It transmits force between objects without changing the magnitude of the force.

Question 13

A block is pulled by a rope over a frictionless pulley. The rope and pulley are massless. If the block is at rest, what can be said about the tension in the rope?

Accepted Answer

The tension is equal throughout the rope.

Question 14

A person tries to walk across a frictionless floor. Why does their foot slip?

Accepted Answer

Because there is no static friction to propel them forward.

Question 15

In a system where two blocks are connected by a massless string and pulled by a hand, which force is responsible for accelerating both blocks?

Accepted Answer

The force exerted by the hand on the first block.

Question 16

Which of the following is NOT a requirement for the tension in a string to be constant throughout its length?

Accepted Answer

The string must be frictionless.

Question 17

A block is accelerated across a frictionless table by a falling mass $$m$$ over a massless, frictionless pulley. If the mass of the block is increased, what happens to the acceleration of the system?

Accepted Answer

The acceleration decreases.

Question 18

In an interaction diagram, what do the lines connecting objects represent?

Accepted Answer

The forces of interaction between the objects.

Question 19

A scale is attached to a massless, frictionless pulley system holding two masses at rest. If each mass is 5 kg, what does the scale read?

Accepted Answer

5 kg

Question 20

Which of the following is true about action/reaction pairs?

Accepted Answer

They are equal in magnitude and opposite in direction.

Newton’s Third Law and Interacting Objects: Study Guide

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