IGCSE Physics / Density and pressure
Pressure revision notes
Edexcel International GCSE Physics (4PH1), chapter 18: Density and pressure, specification point 5.5. Free lesson.
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Pressure measures how concentrated a force is on a surface. Use pressure = force ÷ area to calculate pressure or force, and remember that a mass must be converted to weight before you use it.
What pressure measures
Pressure tells you how concentrated a force is on a surface. The equation is p = F/A. Pressure is measured in pascals, and 1 Pa = 1 N/m². The force is measured in newtons and the area in square metres. p = pressure, in pascals (Pa) F = force pressing on the surface, in newtons (N) A = area pressed on, in square metres (m²) If you are given a mass, find its weight first: W = m × g.
How to convert area and calculate pressure
The pressure equation needs area in m². Square centimetres must be converted before substitution. Because 1 m² = 100 cm × 100 cm = 10 000 cm², divide an area in cm² by 10 000, not by 100.
Why a larger area gives less pressure
For the same force, spreading it over a larger area lowers the pressure because p = F/A. A small area concentrates the force, so pressure is higher. Snowshoes and wide tractor tyres spread the force over a large area. This gives low pressure, so they do not sink into snow or mud. A knife edge and a drawing pin point have a tiny area. This gives high pressure, so they cut or push in easily.
How to calculate a force from pressure
When pressure and area are known, rearrange p = F/A to find the force. Multiply pressure by area, then give the answer in newtons.
Choosing the face that gives greatest pressure
If an object has the same weight on different faces, the smallest face gives the greatest pressure. The force stays the same, while dividing by a smaller area gives a larger result. Use the object's weight as the force in newtons. Choose the smallest area when asked for the greatest pressure.