Every pressure formula, depth calculations, atmospheric pressure and upthrust — all pressure question types for GCSE Physics explained.
Pressure is a topic that bridges the Forces and Motion unit with the Particle Model unit in GCSE Physics. It covers three distinct contexts — pressure in solids, pressure in liquids, and pressure in gases — each with its own formula and its own set of exam question types. This guide covers all three clearly with worked examples throughout.
Pressure is defined as force per unit area. When a force is applied to a surface, the pressure it creates depends on both the size of the force and the area over which it acts.
The unit of pressure is the pascal (Pa), where 1 Pa = 1 N/m². For the same force, a smaller contact area produces a greater pressure. This is why a drawing pin, a knife blade or a stiletto heel can create enormous pressure despite a relatively small force — the force is concentrated on a tiny area.
A box of mass 20 kg rests on the floor. Its base measures 0.5 m × 0.4 m. Calculate the pressure on the floor. (g = 10 N/kg)
Force = weight = mg = 20 × 10 = 200 N
Area = 0.5 × 0.4 = 0.2 m²
Pressure = 200 ÷ 0.2 = 1000 Pa
Always convert area to m² before substituting into P = F/A. If area is given in cm², divide by 10,000. If in mm², divide by 1,000,000. Unit errors here are one of the most common ways marks are lost in pressure calculations.
Liquids exert pressure in all directions — not just downward. The pressure at any point in a liquid depends on the depth below the surface and the density of the liquid.
This equation tells us several important things. Pressure increases with depth — the deeper you go, the greater the weight of liquid above and the higher the pressure. Pressure increases with density — a denser liquid (mercury vs water, for example) produces more pressure at the same depth. Pressure does not depend on the shape or volume of the container — only on depth and density.
This is a classic application exam question. Since pressure in a liquid increases with depth (P = ρgh), the water pressure is greatest at the bottom of the dam. The dam must be thicker at the base to withstand this greater force. At the top, pressure is much lower, so less structural strength is needed.
When an object is submerged in a fluid, the pressure on the bottom surface of the object is greater than on the top surface (because the bottom is deeper). This pressure difference creates a net upward force called upthrust.
Archimedes' Principle states that the upthrust on a submerged object equals the weight of fluid it displaces. An object floats if its weight equals the upthrust — i.e. if its average density is less than or equal to the density of the fluid. Ships float despite being made of steel because their overall density (including all the air inside) is less than the density of water.
If an object is fully submerged and stationary, the forces are balanced: upthrust = weight. If upthrust > weight, the object accelerates upward. If weight > upthrust, it sinks. For a floating object: upthrust = weight, and the object is only partially submerged — it displaces exactly its own weight of fluid. These three scenarios are the basis of most upthrust questions.
The atmosphere is a layer of gas surrounding the Earth. It has weight — and that weight exerts pressure on everything at the Earth's surface. Atmospheric pressure at sea level is approximately 101,000 Pa (101 kPa).
Atmospheric pressure decreases with altitude. Higher up, there is less atmosphere above — less weight of air pressing down. This is why it is harder to breathe at high altitude and why aircraft cabins are pressurised.
Atmospheric pressure also acts in all directions (just like liquid pressure). This is why a vacuum-sealed bag collapses when you open it — the atmospheric pressure outside crushes it when the air inside is removed and can no longer push back.
Gas pressure is caused by gas particles colliding with the walls of their container. This links directly to the Particle Model topic. The key relationships at GCSE:
For a fixed mass of gas at constant temperature, Boyle's Law applies: P₁V₁ = P₂V₂. Halving the volume doubles the pressure.
❌ A common error: saying temperature increases pressure because "particles have more energy". While technically true, the mark scheme wants: "particles move faster → collide more frequently and with greater force → pressure increases". Always describe the mechanism in terms of collisions.
The AQA pressure specification is at the AQA GCSE Physics specification page. Edexcel's is at the Edexcel GCSE Physics page.
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