Third law of motion

According to Newton's third law of motion, whenever two objects interact, they exert equal and opposite forces on each other.

This is often worded as 'every action has an equal and opposite reaction'. However, it is important to remember that the two forces:

• act on two different objects
• are of the same type (eg both contact forces)

Examples of force pairs

Newton's third law can be applied to examples of equilibrium situations.

When the mountain climber pulls down on the rope, the rope pulls up on the mountain climber.

A cat sits on the ground

There are contact gravitational forces between Earth and the cat:

• the cat pulls the Earth up
• the Earth pulls the cat down

These forces are equal in size and opposite in direction.

The runner experiences Newton’s third law. (a) A force is exerted by the runner on the ground. (b) The reaction force of the ground on the runner pushes him forward.

Pushing a pram

There are contact forces between the person and the pram:

• the person pushes the pram forwards
• the pram pushes the person backwards

These forces are equal in size and opposite in direction.

Car tyre on a road

There are contact forces between the tyre and the road:

• the tyre pushes the road backwards
• the road pushes the tyre forwards

These forces are equal in size and opposite in direction.

A satellite in Earth orbit

There are non-contact gravitational forces between Earth and the satellite:

• the Earth pulls the satellite
• the satellite pulls Earth

These forces are equal in size and opposite in direction.

When the swimmer exerts a force on the wall, she accelerates in the opposite direction; in other words, the net external force on her is in the direction opposite of F feet on wall.   This opposition occurs because, in accordance with Newton’s third law, the wall exerts a force F wall on feet on the swimmer that is equal in magnitude but in the direction opposite to the one she exerts on it. The free-body diagram shows F wall on feet w (the gravitational force), and BF, which is the buoyant force of the water supporting the swimmer’s weight. The vertical forces w and BF cancel because there is no vertical acceleration.