Pressure, including pressure in liquids
Understand why spreading a load helps, why deeper water presses harder, and how to choose the right force, area and pressure difference.
Core: p = F/A, force/area comparisons and qualitative liquid depth/density. Supplement / Extended: Δp = ρgΔh and calculations using it. This is Cambridge IGCSE, not Egyptian Baccalaureate / Thanaweya Amma or Year 3.
Pressure means force concentrated over an area
Pressure is the force acting at right angles to a surface per unit area: p = F/A. Use force in newtons (N) and area in square metres (m²) to get pascals (Pa). One pascal is one newton per square metre. If contact pressure varies across a surface, F/A gives its average.
Worked example
A 600 N crate rests on 0.30 m²: p = 600/0.30 = 2000 Pa = 2.0 kPa. The floor supports the stationary crate, and the crate presses down on the floor. Use the force pressing on the chosen surface, not an unrelated force.
Think first: Is a force of 600 N enough information to calculate pressure?
No. The same force could act over a large or a tiny area. You need the contact area as well.
A reliable pressure method
- Identify the surface, perpendicular force and total contact area. Is the question about pressure, a pressure change, or net force?
- Convert area to m². Convert depth to metres and density to kg/m³ if needed.
- Core contact pressure: p = F/A. Rearrange before substituting for force or area.
- Supplement liquid change: Δp = ρgΔh. Use vertical depth difference, not container width or sloping distance.
- Keep unrounded values until the final step, give the requested unit and check the direction of the change.
Core worked example: both feet versus one foot
A stationary student weighs 650 N. Both feet together contact 0.032 m²: p = 650/0.032 = 20312.5 Pa, or 20000 Pa to 2 significant figures. One foot contacts 0.016 m²: p = 40625 Pa, or 41000 Pa to 2 significant figures. Before rounding, the pressure exactly doubles because the area halves. The rounded answers do not look exactly double; avoid rounding early.
Supplement worked example: change between two depths
In freshwater, moving a sensor from 0.20 m to 0.50 m changes depth by 0.30 m. With density 1000 kg/m³ and g = 9.8 N/kg, Δp = 1000 × 9.8 × 0.30 = 2940 Pa = 2.94 kPa. Equal surface pressure cancels when comparing the two readings. Using 0.50 m would give the increase from the surface, not from the starting point.
Contact pressure laboratory
Spread the same load
Core activity. The square shows a model contact footprint viewed from above. Its drawn area scales with the chosen contact area. The force presses perpendicular to this footprint. Assume the load is stationary and the pressure is uniform.
Keep force fixed and double area: predict the change in pressure. Next keep area fixed and double force. Explain the different results. Each control change clears your previous prediction.
Liquid pressure laboratory
Go deeper, then change the container
Core: compare depth, density and width qualitatively. Supplement / Extended: predict the pressure increase in kPa using g = 9.8 N/kg. The tank is open, liquid is still, and density is uniform. Both marked points are at the same vertical depth.
Investigate: keep depth and density fixed, then widen the tank. Pressure at the marked points is unchanged. Change only depth, then only density. Can you explain the change before revealing the number?
Practise, then explain
Four multiple-choice questions, four numerical responses and four written tasks. Each question is labelled by tier. Liquid-pressure equation calculations are Supplement / Extended. Contact pressure calculations and qualitative depth/density comparisons are Core. Numerical answers are checked automatically; written answers use a self-assessment guide. The suggested marks are for this original practice, not an official Cambridge mark scheme.
Practice stays only while this page is open. Refreshing or leaving clears these responses. Copy useful written work into your notes before leaving.
Which statement defines pressure correctly?
Optional hint
Think about how concentrated a force is on a surface.
Source clarifications and safe examples
The original PDF is unchanged. Its common-mistake box reverses the cm² error: entering the numerical area in cm² and labelling F/A as Pa gives a number 10000 times too small, not too big. The correct conversion is 1 cm² = 0.0001 m².
The source’s diving-window answer uses the pressure increase times area. That is the net pressure force only when internal pressure equals the external surface pressure and pressure is effectively uniform over the window. Our question makes those assumptions explicit. Double depth means double liquid pressure increase, not necessarily double total pressure.
Figures19.1–19.2 illustrate contact area and depth. Liquid pressure is a scalar; force on a wall acts normal to it. Sharp objects can injure even with broad handles, so do not try the source’s pin-on-finger suggestion. Use safe diagrams or teacher-supervised apparatus instead.
Source alignment: supplied Physics-0625 Volume A, PDF pp.132–137 / printed pp.124–129, Figures19.1–19.2; official Cambridge 2026–2028 syllabus, section1.8. Original learning text, activities and questions. Original source-download access is unchanged.