Pressure
Pressure as force per unit area, pressure in liquids and gases, and the effect of depth.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 1.8.1Define pressure and recall and use p = F / A.
- 1.8.2Describe how pressure in a liquid varies with depth and density, and recall and use Δp = ρgΔh.Supplement
6 minute read
Pressure
Pressure is force per unit area: p = F / A, measured in pascals (Pa), where 1 Pa = 1 N/m².
For the same force, a smaller area gives a larger pressure. This is why a drawing pin has a sharp point (small area, large pressure driving it into a surface) and why skis or snowshoes have a large area (spreading your weight, reducing pressure so you do not sink into snow).
Pressure in a liquid
Pressure in a liquid increases with depth, because there is a greater weight of liquid pressing down from above. It also increases with the density of the liquid.
The relationship is Δp = ρgΔh: the pressure difference between two depths equals density × gravitational field strength × the change in depth.
Two useful facts about liquid pressure:
- At a given depth, pressure acts equally in all directions — up, down and sideways.
- Pressure depends only on depth and density, not on the shape or width of the container.
Think of it like this
Pressure from a liquid is like the weight of a stack of books pressing down on a table. The deeper you go, the taller the "stack" of liquid above you, and the harder it presses — regardless of how wide the stack is.
Worked examples
Method, step by step
A box of weight 60 N rests on a surface through a base of area 0.30 m². Calculate the pressure it exerts.
- 1p = F / A
- 2p = 60 / 0.30
p = 200 Pa
Calculate the pressure due to water at a depth of 5.0 m. (density of water = 1000 kg/m³, g = 10 N/kg)
- 1Δp = ρgΔh
- 2Δp = 1000 × 10 × 5.0
Δp = 50 000 Pa
Common misconceptions
- Believing a wider container has higher pressure at the bottom. Pressure at a given depth depends only on the depth and the density of the liquid, not the width or shape of the container.
- Forgetting that pressure acts in all directions, not just downwards — a submerged object feels pressure pushing in from every side.
- Mixing up force and pressure. The same force spread over a larger area gives a smaller pressure.
In the exam
- When asked to increase pressure with the same force, the answer is always "decrease the area" — state it explicitly rather than just describing a shape.
- For liquid pressure questions, always check whether the question wants the pressure due to the liquid alone or the total pressure including atmospheric pressure above it.