Unit 7, lesson 34. About 12 minutes.

Floating and sinking

Why do you feel lighter when you get into a swimming pool?

Play

Getting the experiment ready…

Explain

Water pushes things up.

Wide hollow things float best.

Water pushes up on anything in it. This force is called upthrust.

The more water something pushes aside, the bigger the upthrust. So big, hollow shapes float.

A Greek thinker called Archimedes worked this out long ago, and it is said that he shouted "Eureka!" in his bath.

Archimedes' principle: upthrust equals the weight of the fluid pushed aside.

An object floats when it settles with upthrust equal to its weight. If it is fully under water and upthrust is still less than weight, it sinks.

Worked example: 10 litres of water has a mass of 10 kg, so the upthrust is 10 × 9.8 = 98 N.

Object pushes water aside
Water pushes up
Up equals down: it floats

Want the full story?

The Eureka story may not be true, though the principle certainly is. Upthrust comes from water pressure being larger on the bottom of an object than on the top. The sim compares weight with the biggest possible upthrust. A floating object only needs part of its volume under water.

The Formula

upthrust = water density × volume pushed aside × gravity

U = \rho \times V \times g

$U$: the upthrust, measured in newtons (N)
$\rho$: the density of the water, 1000 for fresh water, measured in kilograms per cubic metre (kg/m³)
$V$: the volume of water pushed aside, measured in cubic metres (m³)
$g$: the strength of gravity, 9.8 on Earth, measured in newtons per kilogram (N/kg)

Try the numbers

Volume under water: 10 litres

Upthrust if fully under water: 98.1 N

Try it at home

Ask a grown-up to help

You need a ball of modelling clay, a bowl of water.

You need: a ball of modelling clay, a bowl of water.

Make playdough into a boat. Does it float?

Drop a ball of clay in water. It sinks.
Shape the same clay into a boat.
Float it, and add small toys as cargo.

What's happening: the boat shape pushes much more water aside, so upthrust is greater.

Physics is life

Your body

You feel light in the pool.

Your body is nearly the density of water, so upthrust holds up most of your weight.

Daily life

Armbands help you float.

Armbands and life jackets add a large volume with little mass, so they give you more upthrust.

Nature

Fish use air bags inside.

Many fish have a swim bladder full of gas to float at the depth they want.

Space

Astronauts train under water.

Astronauts practise for spacewalks in big pools, where upthrust acts a bit like being weightless.

Quick check

Question 1 of 3

What is upthrust?

Words to know

Upthrust: The upward push of a liquid or gas on something in it.

Upthrust: The upward push of a liquid or gas on something in it.
Buoyancy: The ability to float, thanks to upthrust.
Density: How much mass is packed into a certain space.

Meet the scientist

Archimedes

Next up

Air pressure and flight