3. Pressure
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Introduction and units P.6 of 13 

What is pressure?
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.

Let's see what effect they have on pressure.

Acrobat graphics for examples
Picture 3.1 How do the pressures compare?

Looking at force
To make it a fair test, we will keep the area constant (the area of the acrobats' feet).

Look at the acrobats in picture 3.1.

The brother at the front is on his own.

The twin brothers behind are doing a piggy back. So the force on the ground is bigger than for their brother - it has been doubled.

What effect will this have on the pressure?

Show answer

The pressure will be doubled.
If we had three brothers, the pressure would be tripled. We can say:

pressure is proportional to force.

Looking at area
To make it a fair test, we will keep the force the same by using just one acrobat each time.

One of the twins stands on one foot whilst another one stays on two feet.

Which one produces a bigger pressure?

Show answer

The one on two feet produces a smaller pressure.

A bigger area reduces the pressure. In fact, doubling the area halves the pressure. We say that:

pressure is inversely proportional to area.

An equation for pressure
Picture 3.2 An equation for pressure. Click here for help remembering formulae.
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 bigger area will produce a smaller pressure.
What are the units?
We usually measure force in newtons (N) and area in square centimetres (cm2). The unit for pressure will then be N/cm2. Notice that we get the new unit by putting the old units into the equation (newtons ÷ centimetres squared = N/cm2).
The pascal
The pascal (Pa) is a standard unit for pressure. One pascal is equivalent to 1 N/m2. 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).
Click here for help converting between cm2 and m2

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Question 6
Look at the pairs of fractions below. You need to decide which one (if either) is bigger? You should be able to do this without using a calculator. For example, in i, the numerator (top) is doubled, but the denominator stays the same. So what happens to the fraction?
Click here for help with fractions.
A.
B.
C.
D.
E.
F. In which case has the pressure:

i. doubled
ii. halved

Summary                                           Close
  • pressure is the force per unit area
  • we measure pressure in pascals (Pa)
  • pressure = force ÷ area
  • 1 Pa = 1 N/m2