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 forces act on two different objects at the same time.

Examples of force pairs

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

When drawing force diagrams, the size of the force is represented by the length of an arrow. A longer arrow indicates a larger force. Forces are vectors. This means they have both size and direction. The direction of the arrow shows the direction in which the force is acting.

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

Box on a table

There are contact forces between the box and the table:

  • the box pushes the table downwards
  • the table pushes the box upwards
A box rests on a table. Two arrows pointing in opposite directions act upwards and downwards from the point at which they meet on the table.

A satellite in Earth orbit

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

  • the Earth pulls the satellite and
  • the satellite pulls Earth
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