Pressure

Pressure describes how concentrated a force is, measuring the force that pushes perpendicularly against a surface divided by the size of that surface. The same force feels very different depending on the area it acts on: spread over a large surface it is gentle, but squeezed onto a tiny point it becomes intense. That is why a sharp knife cuts easily while a blunt one does not, and why lying on a bed of many nails is survivable when standing on a single one is not. In the metric system pressure is measured in pascals, where one pascal equals one newton spread over one square metre. Because a pascal is very small, everyday values are often given in kilopascals, bars, or atmospheres.

Pressure is everywhere in daily life and engineering: it inflates our tyres, drives water through pipes, holds up hydraulic lifts, and keeps aircraft cabins comfortable at altitude. The atmosphere itself presses on us constantly, and changes in air pressure help forecasters predict the weather. Understanding pressure lets engineers design safe boilers, dams, and pressure vessels, and helps doctors interpret blood pressure. In short, knowing the force and the area over which it acts tells you how much strain a surface must withstand.