Pressure in fluids

Pressure equation

P = F / A

• P = pressure in pascals (Pa) — 1 Pa = 1 N/m².
• F = force in newtons (N).
• A = area in m².

A small force on a small area produces a large pressure (drawing-pin tip). A large force spread over a large area produces a low pressure (a snowshoe stops you sinking).

Pressure in a liquid

Liquids exert pressure in all directions because particles are free to move. Pressure increases with depth:

P = ρ × g × h

• ρ = density in kg/m³
• g = 10 N/kg (use the value given in the paper)
• h = depth in m.

A diver 10 m below sea level experiences ≈ 100 kPa from the water alone, plus atmospheric pressure on top.

Upthrust, floating and sinking

When an object is submerged, the bottom face is deeper, so pressure on it is greater than on the top → net upward force = upthrust. By Archimedes' principle, upthrust equals the weight of the fluid displaced.

• Object floats if upthrust ≥ weight (overall density less than the fluid).
• Object sinks if weight > upthrust (denser than the fluid).
• A steel ship floats because its hollow shape displaces enough water for upthrust to balance its huge weight.

Atmospheric pressure (Higher)

The atmosphere is gas held by Earth's gravity. Atmospheric pressure decreases with altitude because:

• There is less air above at higher altitude.
• The atmosphere is less dense higher up.

At sea level, atmospheric pressure ≈ 100 kPa. On Everest summit, ≈ 30 kPa.

Edexcel exam tip

Always quote the unit for pressure (Pa or N/m²). Calculations use SI units; if cm² is given, convert to m² (×10⁻⁴) before substituting.