Newton's Third Law

Key Stage 4

Meaning

Newton's Third Law states that "whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction".

About Newton's Third Law

The two forces referred to in Newton's Third Law are on the separate objects which are interacting.

When an object "A" exerts a force on object "B" then object "B" will exert also be exerting a force on object "A". These two forces will be equal in magnitude and opposite in direction.

The terms "action force" and "reaction force" are sometimes misleadingly used in this context as they simply refer to which object the observer was interested in first, eg Did the ball hit the head or did the head hit the ball? However, the forces in such a force pair do not cause each other, they both exist simultaneously.

Newton's Third Law is related to the law of Conservation of Momentum.

Examples

The gravitational pull of the Earth on The Moon is equal in magnitude to the gravitational pull of The Moon on the Earth.

The gravitational pull of the Earth on you is equal in magnitude to the gravitational pull of you on the Earth. That is to say that the Earth is equally pulled towards you as you are to the Earth.

The normal contact force of a book on a table is equal to the normal contact force of the table on the book. Both objects exert the same force on one another.

When a rocket expells gas from its engine the force of the gas on the rocket is equal to the force of the rocket on the gas.