Pressure: why sharp things cut and wide things float on mud

Press your palm gently against a balloon and nothing happens. Press a pin against it with the same gentle push and it pops. The force was the same. What changed was the area the force was spread over. That idea is called pressure.

Force and area

Pressure is how much force pushes on each piece of a surface. You work it out by dividing the force by the area it acts on.

A person standing in bare feet on soft sand spreads their weight over their whole sole. Stand on the thin heel of a shoe and the same weight is squeezed onto a tiny area, so it sinks deep.

Tools shaped by pressure

Look at almost any tool and you will find two ends: a wide end where your hand pushes, so it does not hurt, and a narrow end that meets the material, so the pressure there is huge. A hammer has a wide handle and a small striking face. A spade has a flat top for your foot and a thin blade for the soil.

Māori tool makers understood this long before the word pressure was used. A toki (adze) was ground from stone such as pounamu or basalt to a fine edge, so each swing concentrated its force onto a thin line and could shape a tōtara log into a waka. A ngira (needle), often made of bone, had a fine point so a small push could pierce and thread material. Today these taonga are treasured, and museums and iwi help us learn how they were made and used.

Try it yourself

Next time you cut an apple, notice that the sharp edge slides in while the flat side just squashes. Same you, same apple, same force. Only the area changed.