Pressure
Definition & Formula
Pressure is the force acting per unit area.
P = F / A (Units: N/m² or Pa)
Pressure by Solids
- For the same force, a smaller area gives a larger pressure (e.g., sharp nails, knives).
- For the same area, a larger force gives a larger pressure.
Heavy equipment (e.g., cement mixers) are put on wide boards to spread the weight and reduce pressure on soft ground.
Quick Example (solid)
A 4400 N load sits on an 0.50 m² plate. P = 4400 / 0.50 = 8800 Pa (8.8 kPa).
Fluid Pressure
In liquids, pressure increases with depth and depends on density.
P = ρ g h where ρ is density (kg/m³), g is 9.8–10 N/kg, h is depth (m)
- At the same depth in a liquid, pressure is the same in all directions.
- Higher density ⇒ higher pressure at the same depth.
Quick Example (liquid column)
Water of depth 20 m: P = 1000 × 10 × 20 = 2.0 × 10⁵ Pa (200 kPa). On a 0.50 m² gate, F = P × A = 1.0 × 10⁵ N.
Applications
- Hydraulic brakes: small force at the master cylinder creates a pressure transmitted to larger-area pistons at the wheels, giving a larger force.
- Sharp tools (nails, needles, knives): small contact area gives high pressure to pierce or cut easily.
- Reducing pressure: wide skis/tractor tyres/spreading boards increase area to avoid sinking.
Disadvantages / Safety
People confined to bed can develop bed sores where the body’s weight presses on small skin areas; relieve by turning regularly and using soft, wide supports.
Units & Conversions
- 1 Pa = 1 N/m²
- 1 kPa = 1000 Pa, 800 kPa = 800 000 Pa