Question

A book is lying on a table.The weight of book & the normal force by table on the book are equal in magnitude and opposite in direction. Is this an example of Newton's third law?

• A. Definitely yes
• B. May be
• C. Never

Answer 1

The correct option is C Never

So the forces involved in the question are weight of book and normal on the book.

weight of book = mg

Force mg is on the book and is in downward direction. But which object is applying that mg on the book.

Oh yes the Earth is pulling the book. mg is the pull of earth's gravity on the book.

2nd force is Normal

Here the table is providing the normal in upward direction on the book

Force 1:- By earth on Book

Force 2:- By table on Book

Clearly these 2 forces are not action reaction pair as there is no body A, B

Plus both the forces are acting on the same body. Also action reaction pair should be of same fundamental nature.

What I mean is If I apply an electrostatic force on you then your reaction force on me can't be gravitation.

Here force 1 (mg) is gravitational

While force 2 (N) is electrostatic in nature.

Then what is the reaction force of mg & N

Think about it.

Force 1 action $\to$ Gravitation pull on book in downward direction (mg $\downarrow$)

Its by Earth and on Book

Reaction $\to$ Gravitation pull on Earth in upward direction (mg $\uparrow$)

Its by book on Earth

We know two objects with mass attract each other.

So even the book is attracting the Earth with mg

Force 2 action $\to$ Electrostatic force by table on book in upward direction (N $\uparrow$)

Reaction $\to$ Electrostatic force by book on table in downward direction (N $\downarrow$ )

This might clear out a lot of doubts you have.