成人快手

Key points

  • When objects float on water the force keeping the object afloat is called upthrust and this balances the weight of the object.

  • When something is placed in the water, the water moves out of the way. We say the water is displaced.

  • For floating objects, the weight of displaced water is equal to the weight of the object.

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Watch to find out how to weigh an object without scales

Have a go

Image gallerySkip image gallerySlide 1 of 7, Image showing an orange, measuring jug, scales, calculator and sheets of paper, Click to see a step-by-step slideshow.
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What is upthrust?

When objects float, they push water out of the way. We say the water is .

The water pushes back with a force we call upthrust.

The water continues to be pushed out of the way until the upthrust equals the weight of the object and the object floats.

Using upthrust to find the weight of an object

If the water that is displaced is weighed, we find it has the same weight as the object that is floating.

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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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More on Forces and movement

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