IGCSE Physics revision · Mechanics
Pressure
Two equations and a lot of everyday explanations: snowshoes, knife edges, dams thicker at the bottom. The explain questions all reduce to the same sentence — same force, different area, different pressure.
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What the syllabus demands
- —Define pressure and calculate it with p = F ÷ A
- —Explain everyday high- and low-pressure applications
- —Calculate the pressure due to a liquid column with Δp = ρgΔh
- —Describe how pressure in a liquid varies with depth and density
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Pressure
- The force acting per unit area, at right angles to the surface: p = F ÷ A. Measured in pascals (1 Pa = 1 N/m²).
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Confusing force and pressure in explanations. A camel's wide feet do not reduce the force on the sand — the weight is unchanged. They increase the area, which reduces the pressure.
- 02
Using the wrong area units: cm² must become m² (divide by 10,000, not 100) before p = F/A gives pascals.
- 03
Forgetting that liquid pressure depends only on depth and density — not on the width or shape of the container. Examiners draw odd-shaped vessels to test exactly this.
- 04
Missing that liquid pressure acts in all directions at a given depth, not just downwards.
One worked example, done properly
Question
Calculate the pressure due to sea water (density 1030 kg/m³) at a depth of 25 m. (g = 9.8 m/s²)
Method
- 1.Δp = ρgΔh = 1030 × 9.8 × 25.
Δp ≈ 252,000 Pa ≈ 250 kPa