How to explain forces

  • A force is a push or pull that causes a change in speed, direction or shape.

  • Newton’s third law says that for every action there is an equal and opposite reaction.

  • This means that when one object pushes on another with a force, the other object pushes back on the first object with the same force.

Have a go

WHAT YOU NEED: Some sticky tape, a long piece of thread, a balloon and a straw.

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Forces

Newton’s third law says that for every action there is an equal and opposite reaction.

In the case of the balloon in the video, the action is the balloon pushing the air backwards; the reaction is the air pushing the balloon forwards. It is this force that makes the balloon move.

Balloon traveling forwards showing forces action and reaction
The balloon travels forwards by pushing the air backwards.

How does Newton's third law apply to us?

Did you know that we cannot move independently? We require a force acting upon us from elsewhere.

When we want to move, our feet try to push the ground backwards. But it is the friction from the ground that pushes us forwards.

If we tried the same on ice we wouldn't move because there is very little friction. We just slip because the ice is not going to push us anywhere!

Who needs to know about forces?

Engineers designing transport need to consider forces. Something always needs to be pushed backwards if the vehicle is to move forwards.

An aeroplane’s thrust depends on how much and how quickly its engines can push back the air. Boats need to consider how much and how quickly water can be forced backwards.

Where next?

Revise - Forces
revision-guide
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