Pressure

This guide covers subtopic 1.8 Pressure, from Topic 1, Motion, forces and energy, for Cambridge O Level Physics 5054, 2026–2028 series — the final subtopic of Topic 1.

Where this fits in 5054

Pressure closes out Topic 1 by applying the force ideas from Forces and Motion to surfaces and fluids — what happens when a force acts over an area rather than at a single point, and how a column of liquid or gas transmits force through its own weight.

Syllabus coverage

CAMBRIDGE O LEVEL PHYSICS 5054

• Define pressure as force per unit area; recall and use pressure = force ÷ area (1.8)
• Describe how pressure varies with force and area, in the context of everyday examples (1.8)
• State that the pressure at a surface produces a force in a direction at right angles to the surface, and describe an experiment to show this (1.8)
• Describe how the height of a liquid column in a liquid barometer may be used to determine atmospheric pressure (1.8)
• Describe, quantitatively, how the pressure beneath the surface of a liquid changes with depth and density of the liquid (1.8)
• Recall and use the equation for the change in pressure beneath the surface of a liquid, change in pressure = density × gravitational field strength × change in height (1.8)

5054 is not tiered — every candidate covers all of the above.

Pressure as force per unit area

Pressure is defined as force per unit area:

pressure = force / area        p = F / A

Worked example. A force of 600 N acts on an area of 0.5 m². Find the pressure.

p = F / A = 600 / 0.5 = 1200 Pa

For a given force, pressure increases as the area it acts over decreases, and decreases as the area increases — this is why a sharp knife (small contact area) cuts far more easily than a blunt one under the same applied force, and why snowshoes (large contact area) let a person walk on snow without sinking, spreading the same weight over a much larger area to produce a much lower pressure.

Direction of the force from pressure

The pressure at a surface produces a force acting at right angles (perpendicular) to that surface. This can be shown experimentally with a thistle funnel connected to a manometer: as the funnel’s open end is rotated to face different directions while held at the same depth in a liquid, the manometer reading (proportional to the force the liquid exerts on the funnel’s membrane) stays the same — showing the liquid pushes perpendicular to the membrane regardless of the membrane’s orientation.

Atmospheric pressure and the liquid barometer

A liquid barometer measures atmospheric pressure using a column of liquid (traditionally mercury) in a tube, closed at the top (with a vacuum above the liquid) and open at the bottom to a reservoir exposed to the atmosphere. Atmospheric pressure pushes down on the reservoir’s surface, supporting the column of liquid in the tube — the height of that column is a direct measure of atmospheric pressure: a taller column means a higher atmospheric pressure was needed to support it.

Pressure beneath a liquid surface

Pressure beneath a liquid’s surface increases with depth and with the liquid’s density — a direct, quantitative relationship:

change in pressure = density × gravitational field strength × change in height
Δp = ρgΔh

Worked example. Find the increase in pressure at a depth of 3 m in water (density 1000 kg/m³, g = 9.8 N/kg).

Δp = ρgΔh = 1000 × 9.8 × 3 = 29 400 Pa

A denser liquid produces a greater pressure increase for the same depth — this is why the same depth of mercury (density about 13.6 times that of water) produces a far greater pressure than the equivalent depth of water, which is exactly why mercury (rather than water) is the practical choice for a barometer: it gives a manageably short column for typical atmospheric pressures.

Common mistakes

• Using total length or diameter instead of area when calculating pressure from p = F/A — always confirm the value being used is genuinely an area (in m² or cm²), converting if the question gives a diameter or radius.
• Assuming a larger force always means a larger pressure. Pressure depends on both force and area — a huge force spread over a huge area can produce a small pressure, and vice versa.
• Forgetting pressure acts perpendicular to a surface, and trying to resolve it as if it were a force acting in a fixed, single direction regardless of the surface’s orientation.
• Mixing up which quantity increases pressure beneath a liquid. Both greater depth and greater density increase pressure — a shallow depth of a very dense liquid can produce more pressure than a deep column of a less-dense one.
• Forgetting units. Pressure calculated from p = F/A is in pascals (Pa) when force is in newtons and area is in m² — check units are consistent before calculating.

Quick revision checklist

• p = F/A, and how pressure changes with force and area independently
• Pressure acts at right angles to the surface it’s exerted on, and the experiment that shows this
• How a liquid barometer’s column height indicates atmospheric pressure
• Δp = ρgΔh, and that pressure beneath a liquid depends on both depth and density

Related resources

• Forces and Motion — the force ideas pressure builds on
• Measurement, Mass, Weight and Density — density, used directly in the liquid-pressure equation
• Cambridge O Level Physics subject hub

Written against Cambridge O Level Physics 5054, 2026–2028 series. Always check the current syllabus for your examination year.