Newton's third law

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.

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.

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.

The Earth and a satellite point to each other in space with arrows of equal size.

Explaining Newton's third law

Science presenter Jon Chase explains Newton's third law