# Floating and sinking

Let's look at why some things float and others sink.

It is all to do with density – the mass of a cubic metre of the material.

If the density of the object you are trying to float is greater than the density of the liquid that you are trying to float it in then it will sink.
If the density of the object you are trying to float is less than the density of the liquid that you are trying to float it in then it will float.

For example a lump of wood (density 800 kg/cubic metre) will float in water (density 1000 kg/cubic metre) but a lump of iron (density 7300 kg/cubic metre) will sink. Ice floats in water because its density is only 900 kg/cubic metre – less than water.

A ship floats in water because its average density (steel plus the air in the ship) is less than that of water.

There is another way of looking at the problem and this involves Archimedes principle. This is to do with the upthrust on an object in a liquid. If the upthrust is equal to the objects weight it will float and this can only happen if the object can displace its own weight if liquid without being completely submerged.

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