大象传媒

Key points

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

  • Newton鈥檚 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.

Video: How to explain forces

Have a go

Image gallerySkip image gallerySlide 1 of 5, , WHAT YOU NEED: Some sticky tape, a long piece of thread, a balloon and a straw.
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Forces

Newton鈥檚 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
Image caption,
The balloon travels forwards by pushing the air backwards.

How does Newton's Third Law apply to us?

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 from the ground that pushes us forwards.

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鈥檚 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.

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Quiz - Test your knowledge

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Play the Atomic Labs game! game

Try out practical experiments in this KS3 science game.

Play the Atomic Labs game!
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