Q.A book is at rest on a table, which exerts a normal force on the book. If this normal force is considered to be the reaction force, what is the action force according to Newton's third law?

Newton's Third Law Pairs

The Intuition: Forces Never Act Alone

Imagine you are standing on a skateboard and you push against a wall. The wall does not move, but you roll backward. You pushed the wall, and the wall pushed back on you — at the very same instant, with the very same strength.

This is the core idea behind Newton's Third Law: whenever one object exerts a force on a second object, the second object exerts an equal and opposite force back on the first. These two forces are called an action-reaction pair (or "Third Law pair").

The Precise Statement

Newton's Third Law: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.

Both forces have the same magnitude and act along the same line, but point in opposite directions — and, crucially, they act on two different objects.

The Key Rule for Spotting a Third Law Pair

A genuine Third Law pair must satisfy two conditions:

1. The two forces act on different objects (never on the same object).
2. They arise from the same interaction (the same pair of bodies in contact, or the same gravitational/electric attraction).

If a pair of forces acts on the same object, they are not a Third Law pair — even if they happen to be equal in magnitude and opposite in direction.

Why Equal and Opposite Forces Don't Cancel

This confuses many students: if the two forces are always equal and opposite, why does anything ever move?

Because the two forces act on different objects — they never appear together in the same free-body diagram. When you push against a wall, the wall pushes back on you. That reaction force acts on you, not on the wall, so it can accelerate you backward even though the wall itself does not move (the wall is anchored to the Earth, which is far too massive to notice the force).

A Careful Worked Example: A Book on a Table

A book sits at rest on a table. Two different force pairs are involved here, and it is important not to mix them up:

• Pair 1 (gravitational interaction): Earth pulls the book downward with a force equal to the book's weight, $mg$. By the Third Law, the book pulls the Earth upward with the same magnitude, $mg$. This pair acts on two different bodies — the book and the Earth.
• Pair 2 (contact interaction): The table pushes up on the book with a normal force $N$. By the Third Law, the book pushes down on the table with a force of the same magnitude, $N$. This pair also acts on two different bodies — the book and the table.

More Everyday Examples

• Walking: Your foot pushes backward on the ground; the ground pushes forward on your foot. That forward push is what propels you ahead. …