3. Pressure
Introduction and variables 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.

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.

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Question 6
Look at the pairs of tests below. In each case, we are trying to find something out about pressure. Some of them are fair tests and some are not. Decide whether each pair is a fair test or not and choose from the drop down menu.
A. A single acrobat on one foot. compared   with the same acrobat on both feet.
B. An elephant standing on all four feet. compared   with the same elephant standing on two feet.
C. An elephant standing on two feet. compared   with a horse standing on four feet.
D. A single acrobat on one foot. compared   with two piggy backing acrobats on two feet.
E. An acrobat on two feet compared   with a heavier acrobat on one foot.

Summary                                           Close
  • pressure is the force per unit area
  • pressure = force ÷ area