Pressure on a surface
Pressure is the force acting per unit area, at right angles to a surface. The same force spread over a smaller area gives a larger pressure — this is why a sharp knife or a drawing pin cuts easily. The SI unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m².
Key idea
Pressure on a surface: p = F / A (Pa). Pressure at depth h in a liquid: p = ρ g h, where ρ is density (kg/m³), g is gravitational field strength (N/kg) and h is depth (m).
Pressure in liquids
In a liquid, pressure increases with depth because more liquid weighs down from above. At a given depth the pressure acts equally in all directions and does not depend on the shape of the container. This extra pressure is p = ρgh.
Worked example
A box exerts a force of 200 N on the floor through an area of 0.50 m². Find the pressure. Then find the water pressure at a depth of 3.0 m (ρ = 1000 kg/m³, g = 10 N/kg).
p = F/A = 200 / 0.50 = 400 Pa.
p = ρgh = 1000 × 10 × 3.0 = 30000 Pa.
Remember
- Smaller area → larger pressure for the same force.
- Liquid pressure depends on depth and density, not container shape.
- 1 Pa = 1 N/m².