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.
Watch to find out how to weigh an object without scales
NARRATOR:
You're about to find out how to weigh a floating object without scales, which is also an excuse to splash around a bit and make some mess.
And here's everything you'll need; a measuring jug, an unpeeled orange, calculator, a piece of paper and a pen and some scales… just to check. And you're ready to begin.
Fill your measuring jug with enough water to float an orange and note down the volume in millilitres. Place your orange in the water. The orange should be floating. If not, your orange is probably broken and you should take it back to the shop for a refund.
Take the new volume measurement of the water and note it down. And calculate the difference. The difference is the second volume minus the first.
Now this is the clever part… Every 1 millilitre of water is 1 gram, so the difference in volume is the same as the weight of the water pushed up by the orange, which makes the maths easy.
Let's test it. Weigh the orange, best to weigh in grams. Grab that note you made, it's the same. The weight of the floating orange is the same as the weight of the water displaced. It's magic… No, wait, it's science!
Have a go
Image caption, 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 To move something out of the way by taking its place..
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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