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.

A good way to state it is 'If body A exerts a force on body B then body B will exert the same force on body A but the force will be in the opposite direction'.

Examples of force pairs

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

Throwing a heavy ball whilst wearing roller skates

There are contact forces between the person and the heavy ball:

  • the person pushes the ball forward the ball pushes the person backwards

A car tyre on a road

There are contact forces between a car tyre and the road:

  • the tyre pushes the road backwards
  • the road pushes the tyre and hence the car forwards

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

When two forces aren't a Newton's third law pair

It is possible to confuse a pair of forces. For example, with a book on a table there is the weight of the book and the contact reaction force of the table. These forces are not a Newton's third law pair - they are not the same type of force, and they act on the same object. When you take away the table the weight of the book remains. The third law pair could be, for example, the gravitational pull of the Earth on the book and the gravitational pull of the book on the Earth.

Explaining Newton's third law

Science presenter Jon Chase explains Newton's third law with the help of some skateboarders