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Science

What are forces?

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Forces occur when there is an interaction between two objects. These forces always happen in pairs – when one object exerts a force on another, it always experiences a force in return.

What are forces?

Stick figure holding up weight

The green arrow shows the force on the weights as the weightlifter pushes upwards.

The red arrow shows the downwards force on the weightlifter’s arm muscles.

These two forces are an interaction pair. They are equal in size, and opposite in direction.

You only have an interaction pair if the forces are caused by the interaction. In this case, the compression in the weightlifter’s muscles is caused by the weight pushing down, and the upwards force on the weight is caused by the weightlifter’s muscles.

Example: rocket engines

One common interaction pair of forces is found in a rocket or a jet engine:

  • As the fuel burns, exhaust gases are produced.
  • The rocket engine pushes these gases out backwards.
  • The gases push the space shuttle forwards, with the same size force in the opposite direction.

That’s how rockets work.

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