Unit 7, lesson 32. About 12 minutes.

Density

Why does a huge ship float while a tiny coin sinks?

Play

Getting the experiment ready…

Explain

Some things float, and some sink.

Things that are light for their size float.

Density tells you how tightly mass is packed into a space.

A block of wood and a block of iron the same size have different masses. Iron is denser.

Things less dense than water float. Things denser than water sink.

ρ = m / V. Density is measured in kg/m³. Water has a density of about 1000 kg/m³.

The sim uses litres, where 1 litre = 0.001 m³.

Worked example: 6 kg in 10 litres means ρ = 6 / 0.01 = 600 kg/m³, so it floats.

Find the mass
Find the volume
Divide to get density

Want the full story?

A ship floats because its average density, including all of the air inside, is less than that of water. Density changes with temperature, which is why warm air rises. Salt water is denser, so things float more easily in the sea.

The Formula

density = mass ÷ volume

\rho = \frac{m}{V}

$\rho$: the density, written with the Greek letter rho, measured in kilograms per cubic metre (kg/m³)
$m$: the mass, measured in kilograms (kg)
$V$: the volume, where 1 litre is 0.001 m³, measured in cubic metres (m³)

Try the numbers

Mass: 6 kgVolume: 10 litres

Density: 600 kg/m³

Try it at home

Ask a grown-up to help

You need a bowl of water, small objects like a cork, a spoon and a grape.

You need: a bowl of water, small objects like a cork, a spoon and a grape.

Drop toys in water. Which ones float?

Fill a bowl with water.
Guess whether each object will float or sink.
Test them all.

What's happening: objects less dense than water float, and those more dense sink.

Physics is life

Your body

You float in the sea.

Your lungs full of air help you float, and you float more easily in salty water.

Daily life

Ships float even though they are metal.

Metal ships are full of air, so their average density is less than water.

Nature

Icebergs float, mostly under water.

About 90% of an iceberg is under the water, because ice is only a little less dense.

Space

Saturn could float on water.

Saturn is less dense than water, even though it is huge.

Quick check

Question 1 of 3

What does density tell you?

Words to know

Density: How much mass is packed into a certain space.

Density: How much mass is packed into a certain space.
Mass: How much stuff is in an object. It is measured in kilograms.
Upthrust: The upward push of a liquid or gas on something in it.

Next up

Pressure: snowshoes and drawing pins