Pressure and area — Science, 11–13
Pressure tells us how concentrated a force is over an area, explaining why the same push can have different effects.
What is pressure?
Pressure describes how much force acts on each unit of area. The same force creates greater pressure on a small area and less pressure on a large area. In a liquid or gas, pressure can spread in all directions, not just downwards, because its particles push on the container and on one another.
Why use pressure instead of force?
A force alone does not tell us how an object will affect a surface. A person’s weight may be spread by wide shoes or concentrated by a narrow heel, so the ground feels different effects. Pressure was developed to compare these situations fairly and to design objects that grip, cut, float or protect.
Calculating pressure
A box pushes down with a force of 600 newtons. On a base of 0.03 square metres, pressure = force ÷ area = 600 ÷ 0.03 = 20,000 pascals, or 20 kPa. Turn the box so its base is 0.30 square metres: 600 ÷ 0.30 = 2,000 Pa. The force stayed the same, but the larger area made one-tenth of the pressure.
Pressure is not force
A common mistake is to use pressure and force as if they were the same thing. That seems reasonable because both describe a push and are linked in the formula. But force is the total push, measured in newtons; pressure tells how concentrated that push is, measured in pascals. Area must be included before comparing pressures.
Pressure in everyday designs
Snowshoes spread a person’s weight over a large area, so they sink less into snow. A drawing pin has a wide head for your finger but a tiny point that creates high pressure in the wall. Hydraulic brakes and syringes use pressure in a liquid to transmit a push, showing that pressure is useful in moving and controlling things.
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