Pressure
Section: Motion, Forces & Energy | Syllabus: Cambridge IGCSE Physics (0625)
Pressure
- pressure is defined as the force acting per unit area
- Formula:
- p = (F/A)
- Where:
- p = pressure (Pa)
- F = force (N)
- A = area (m²)
- 1 pascal (Pa) = 1 newton per square metre, so 1 Pa = 1 N/m^2
Pressure, force and area formula triangle
Pressure, Force and Area
- for a given force, a smaller area produces a larger pressure, and a larger area produces a smaller pressure
- spreading a force over a large area lowers the pressure; concentrating a force onto a small area raises the pressure
- everyday examples:
- tractors have large tyres, which spread their weight over a large area to lower the pressure on the ground and stop them sinking into soft mud
- drawing pins have a sharp point with a very small area, which concentrates the force to create a large pressure that pushes the pin into a wall
- a stiletto heel has a small area, creating a large pressure that can sink into soft ground, while a flat shoe spreads the same weight over a larger area, creating a much lower pressure
The same force produces a much higher pressure when it acts over a smaller area
Worked Example
- Museli's box weighs 240 N and rests on a base of area 0.4 m². Calculate the pressure it exerts on the floor.
Museli's box: weight 240 N, base area 0.4 m²
- p = (F/A) = (240/0.4) = 600 Pa
Pressure in a Liquid
- the pressure beneath the surface of a liquid increases with depth, because at greater depths there is more liquid above pressing down
- the pressure beneath the surface of a liquid also increases with the density of the liquid - a denser liquid exerts a greater pressure at the same depth
- at a given depth, pressure acts equally in all directions
- dams are built thicker at the base, where the water pressure is greatest
Water jets from holes at different depths show that pressure increases with depth
Change in Pressure with Depth
Extended Only
- Formula:
- Δ p = ρ g Δ h
- Where:
- Δ p = change in pressure (Pa)
- ρ = density of the liquid (kg/m³)
- g = gravitational field strength (N/kg)
- Δ h = change in depth (m)
Worked Example: Change in Pressure with Depth
- Muchindu dives from a depth of 1 m to a depth of 6 m in a swimming pool of water, density 1000 kg/m³. (Take g = 10 N/kg.) Calculate the change in pressure.
Muchindu dives from a depth of 1 m to a depth of 6 m
- Δ p = ρ g Δ h = 1000 × 10 × 5 = 50\,000 Pa
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