Pressure is how much force there is on a given area. So, whilst an elephant weighs more than a horse, the horse pushes down with a bigger pressure. This is because the areas of their feet are different.

Pressure depends on two variables: force and area (see page 6).

They make an equation as follows:

An equation for pressure

Picture 3.2 An equation for pressure.

Picture 3.2 shows the equation for pressure. The force is on the top of the fraction. Therefore a bigger force will produce a bigger pressure - which is what we expected. Area is on the bottom of the fraction so a biggerarea will produce a smaller pressure.

What are the units?

We usually measure force in newtons (N) and area in square centimetres (cm^{2}). The unit for pressure will then be N/cm^{2}. Notice that we get the new unit by putting the old units into the equation (newtons ÷ centimetres squared = N/cm^{2}).

The pascal

The pascal (Pa) is a standard unit for pressure. One pascal is equivalent to 1 N/m^{2}. So, to get the pressure in pascals, you need to measure the force in newtons and the area in square metres (rather than square centimetres).