Question

A book weighing 5 N rests on top of a table

Part A

A downward force of magnitude 5 N is exerted on the book by the force of

Part B

An upward force of magnitude _____ is exerted on the _____ by the table.

Part C

Do the downward force in Part A and the upward force in Part B constitute a 3rd law pair?

Part D

The reaction to the force in Part A is a force of magnitude _____, exerted on the _____ by the _____. Its direction is _____ .

Part E

The reaction to the force in Part B is a force of magnitude _____, exerted on the _____ by the _____. Its direction is _____.

Part F

Which of Newton's laws could we have used to predict that the forces in Parts A and B are equal and opposite?

Part G

Which of Newton's laws could we have used to predict that the forces in Parts B and E are equal and opposite?

Answer 1

Concepts and reason

The required concept to solve the problem is Newton’s laws of motion.

First, find which quantity exerts the force on the book using the concept of action and reaction.

Using the same concept, find the magnitude of the force exerted by the table and on which body the force is acting.

Check if the downward force in Part A and upward force in Part B constitute a 3^rd law pair using the Newton’s third law.

Newton’s third law can be used to find the reaction force, the body acting and the body receiving and the direction of the force in Part A and Part B.

Find which of the laws can be used to predict that the forces in Part A and Part B are equal and opposite.

Find which of the laws can be used to predict that the forces in Part A and Part E are equal and opposite.

Fundamentals

Forces always act in pairs. Whenever a force is applied on a body, the body exerts an equal magnitude force on the other body. The directions on the forces will be opposite.

Newton’s third law states that for every action, there is an equal and opposite reaction. The applied force is the action and the force exerted in the opposite direction by the other object is the reaction.

(A)

A downward force of magnitude $5\,{\rm{N}}$ is exerted on the book by the force of gravity. The book, when placed on the table, exerts a force on the table due to its weight.

(B)

As the weight of the body acts on the table, there is an equal force acting on the book by the table. The $5\,{\rm{N}}$ force acting on the table is the action force and the reaction force is $5\,{\rm{N}}$ acting on the book by the table.

The upward force exerted on the book by the table.

(C)

No, they do not constitute a third law pair. The above forces are not an action reaction pair.

(D)

The reaction to the force in Part A is a force of magnitude $5\,{\rm{N}}$ exerted on the Earth by the book in the upward direction.

The reaction force in part A is exerted on the Earth.

The reaction force acts on the book by the book.

The direction of reaction force in part A is upward.

[Part D.4]

Part D.4

(E)

The reaction to the force in Part B is a force of magnitude $5\,{\rm{N}}$ exerted on the table by the book in the downward direction.

The reaction force in part B is acts on the earth.

The reaction force in part B is exerted on earth by the book.

[Part E.3]

Part E.3

The direction of reaction force in part B is downwards.

[Part E.4]

Part E.4

(F)

To predict that the forces in Parts A and B are equal and opposite, Newton’s first or second law can be used. As the book is at rest, we can use these laws to tell that the net force acting on the book is zero.

(G)

To predict that the forces in Parts B and E are equal and opposite, Newton’s third law can be used. The force in Part E represents the reaction force to the force in Part B.

Ans: Part A

The downward force on the book is force of gravity.