Newton's Third Law of Motion

Click for an introduction to forces


Newton's third law states

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

Simply stated
"To every action there is always an equal and opposite reaction"

One force we call the action force and the other we call the reaction force. It does not matter which force we call action and which we call reaction.

Sir Isaac Newton
It is important to note that all forces exist in pairs. We may not be aware of action and reaction forces around us but they happen even during the simplest act of walking. We push against the road surface with our shoes and the road pushes back with an equal force but in the opposite direction to the force that we exert on the road. It is the force the road exerts on us that causes us to move forwards. Similar to a bike tyre spinning in contact with the road, the tyre exerts a force on the road and the road exerts an equal force in the opposite direction pushing the bike forwards. A swimmer moves through the water only because he pushes water backward and the water pushes the swimmer forwards.
A rocket pushes gases out its exhaust nozzle. The force of the gases escaping the engine produces an equal and opposite force that accelerates the rocket skyward.


The recoil of a rifle or a cannon is due to Newton's Third Law of Motion. The force the cannon exerts on the cannon ball accelerating it out from the barrel is equal and opposite to the force the ball exerts on the cannon to recoil it backwards.


All missiles obey Newton's Third Law of Motion. Fuel is rapidly burnt to produce hot gases that expand quickly, escaping with a great force from the nozzle. The rocket pushes gases out the back and the gases push the rocket forward.

Click to see an exocet missile in flight.


Propellers are designed to push air backwards. The force with which the plane pushes air back is equal in magnitude and opposite in direction to the force the air pushes the plane forwards.


It is exactly the same with propellers that power ships and submarines. The difference is that water, rather than air, is pushed backwards.
Worksheet (junior science)