Pressure depends on area — Physics, 11–13 years
The same push can have very different effects when it is spread over a large area or concentrated on a small one.
A push spread out
Pressure tells us how concentrated a force is on a surface. A force on a tiny area produces more pressure than the same force spread over a large area. That is why a sharp drawing pin enters wood more easily than its flat head.
Why spread the load?
People needed a way to predict whether a surface would be damaged or whether something would sink into it. Snowshoes spread a person’s weight over more snow, while a knife edge concentrates force for cutting. The useful rule is pressure = force ÷ area.
A box on the floor
A box pushes down with 60 N. First, measure the part touching the floor: 0.02 m². Next, divide: 60 N ÷ 0.02 m² = 3,000 Pa. If the box is turned so its base is 0.06 m², the pressure becomes 1,000 Pa, even though its weight has not changed.
More force is not always more pressure
It is reasonable to think that a heavier object must always create more pressure, because it pushes down harder. But pressure depends on two things: the force and the area receiving it. A heavy person in snow may sink less when wearing wide snowshoes than a lighter person in narrow shoes.
Designing with pressure
Pressure matters when people design shoes, needles, tyres, bridges and foundations. A stiletto heel can mark a floor because its small tip concentrates a person’s weight. A building foundation spreads the building’s load over a large area so the ground is less likely to sink.
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