Density

It is often useful to know not simply the mass of an object but rather the mass of a definite amount of the material of which it is made.

If you were to have two life-size statues made of you, one of wax and the other of bronze, then the one of bronze would be much heavier. A given volume of bronze is heavier than the same volume of wax.

The mass of a unit volume of a substance is called the DENSITY of the substance and is measured in kilogram per cubic metre (kg/m³) or gram per cubic centimetre (g/cm³).

To work out the density you have to divide the mass of an object by its volume.

Density = Mass/Volume

Example problems
1. What is the density of a metal if 4 m³ of it have a mass of 28 000 kg?
Density = 28000/4 = 7000kg/m³.

2. What is the mass of 0.5 m³ of copper? mass = density x volume 8930 x 0.5 = 4465 kg.

3. What is the volume of 9000 kg (about 9 tonnes) of concrete, density 3000 kg/m³
Volume = mass/density = 9000/3000 = 3m³.

Material	Density (kg/m³)	Material	Density (kg/m³)
Aluminium	2710	Water	1000
Lead	11340	Meths	791
Iron	7870	Mercury	13 600
Wood	600	Air	1.2
Ice	920

Relative density

The density of a substance is often compared with that of water and this is called the RELATIVE DENSITY of the substance.

Relative density = mass of substance/mass of an equal volume of water

On this scale, iron would have a relative density of 7.87, methylated spirit 0.79 etc.

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