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Exploring Forces

The push a liquid gives

Push a plastic ball under water, then release it. It rises fast. What pushes it?

A ball is pushed under water.
Step 1 of 4
  1. A ball is pushed under water.
  2. The Earth pulls it down. That is its weight.
  3. The water pushes it up. That is upthrust. Here it is bigger than the weight.
  4. Weight bigger: it sinks. Both the same: it floats. Upthrust bigger: it is pushed up.

The water pushes it up. This push is called upthrust.

New word

  • Upthrust — the upward push a liquid gives an object in it. It is also called buoyant force.

The rule

A liquid pushes up on an object. This push is upthrust. Weight bigger than upthrust: the object sinks. Both the same: it floats.

Upthrust bigger than weight: the object is pushed up.

Compare the two numbers. Pick what happens.

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This track

  1. 0 Start here
  2. 1 A push or a pull
  3. 2 What a force does
  4. 3 Touching, or not
  5. 4 Two at once
  6. 5 When the forces balance
  7. 6 When they do not balance
  8. 7 The surface pushes back
  9. 8 Friction both ways
  10. 9 Weight and the spring balance
  11. 10 Mass stays, weight changes
  12. 11 The push a liquid gives
  13. 12 Checkpoint: weight, mass and upthrust
  14. 13 The big picture
  15. 14 Track quiz

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