GCSE Physics Pressure in fluids revision

Pressure is the force acting per unit area: P = F/A, force in newtons, area in square metres, pressure in pascals (Pa) — one pascal is one newton spread over one square metre. The same force produces very different pressures depending on the area: a 300 N force over 2.0 m² gives 150 Pa, but the same 300 N balanced on a stiletto heel of a few square millimetres would dent a wooden floor. Engineers choose deliberately: knives, nails and drawing pins concentrate force into tiny areas to cut or grip, while skis, snowshoes and wide tractor tyres spread the same weight over large areas so the person or machine does not sink.

A liquid exerts pressure at right angles to every surface it touches — walls, floor, or your eardrums — and the pressure grows with depth because there is more fluid above pressing down. At any one depth the pressure is equal in all directions, which is why a dam wall is built thickest at the bottom. The Higher-tier equation P = h ρ g quantifies it: at depth h, in a liquid of density ρ, with g = 9.8 N/kg. Water (ρ = 1 000 kg/m³) adds about 9 800 Pa — nearly 10 kPa — for every metre you descend.

Immerse anything in a fluid and it feels an upward force: upthrust, equal to the weight of fluid displaced (pushed aside). An object floats when the upthrust balances its weight — which is exactly what happens when the object is less dense than the fluid: it sinks until it has displaced its own weight of fluid, then sits there. A steel ship floats because its hull encloses mostly air, making the *average* density less than water. A submarine dives by flooding ballast tanks (raising its average density above sea water) and surfaces by blowing them out with compressed air.

The atmosphere is a fluid, and its weight presses on everything: about 100 kPa at sea level, acting in all directions (otherwise we would be crushed sideways). Drinking through a straw works because you lower the pressure inside the straw and the atmosphere pushes the liquid up; suction cups stick for the same reason. As you climb, there is less air above you, so both the density of air and the atmospheric pressure fall — which is why aircraft cabins are pressurised, mountaineers carry oxygen on the highest peaks, and a sealed crisp packet puffs up on a mountain drive. Barometers measure this pressure by the height of a mercury column it can support (mercury’s large density, 13 600 kg/m³, keeps the column a handy 760 mm rather than ten metres of water).