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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 where bodies are in equilibrium. A body is in equilibrium when the forces acting on it are all balanced, so there is no overall force acting on the body. This means that the body will either be stationary or moving at a constant speed.

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

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

A satellite in Earth's orbit

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

• the Earth pulls the satellite
• the satellite pulls Earth

Explaining Newton's third law