1Force spread over an area
Pressure tells you how concentrated a force is. The same weight on a small area sinks into snow; spread over a large area, it doesn't.
pressure
Lesson 21.1 notes
Pressure on surfaces
Pressure = force ÷ area, the pascal, and why sharp things cut while snowshoes stop you sinking.
2The pressure equation
pressure = force ÷ area. Pressure is measured in pascals (Pa): 1 Pa is a force of 1 N on every square metre. A 600 N crate on 2 m² gives 300 Pa.
pascal
3Rearranging the equation
Cover the one you want in the triangle: force = pressure × area and area = force ÷ pressure.
4Small area, big pressure
A drawing pin’s point has a tiny area, so your thumb’s push makes a huge pressure on the wood and the point goes in. Its wide head keeps the pressure on your thumb low.
5Large area, small pressure
Spreading a force over a large area lowers the pressure. Wide tyres stop tractors sinking into mud, and wide foundations stop buildings sinking into the ground.
6At right angles to the surface
Pressure always pushes at right angles to a surface: the normal direction. The air in a balloon pushes outwards at right angles to the rubber everywhere.
normal
7Same brick, different pressures
A brick weighs the same whichever way up it stands. On its smallest face, its weight is on the smallest area, so the pressure is greatest.
Explore: Snow walker
Choose who is walking and what is on their feet. Watch how far they sink into the snow, and see the pressure worked out.
Common mistake: Pressure isn't the same as force
Your weight is the same in trainers or in snowshoes, so the force doesn't change. The pressure changes, because the area changes: a small area gives a big pressure.
Remember
- pressure = force ÷ area. Pressure is measured in pascals (Pa): 1 Pa = 1 N/m².
- A small area gives a large pressure (a knife edge); a large area gives a small pressure (snowshoes).
- Pressure always pushes at right angles to a surface.
Going further at GCSE