Pressure = force normal to a surface / area of that surface

The idea

A fluid, which can be either a liquid or a gas, exerts pressure on any surface it is in contact with. The particles in the fluid collide with the surface, creating a force that acts at right angles (normal) to it. The concentration of this force over a given area determines the pressure. For example, if the water pressure on a submarine window of area 0.20 m² is 500,000 Pa, the normal force exerted by the water is 100,000 N. Understanding this allows engineers to design containers, pipes, and submersible vehicles that can withstand the forces exerted by the fluids they hold or move through. Pressure is measured in pascals (Pa), where 1 Pa = 1 N/m².

What earns the marks

• Identify that a fluid (liquid or gas) exerts a force at right angles (normal) to any surface it contacts.
• Use the equation p = F / A to calculate pressure, force, or area in fluid contexts.
• Convert units correctly, ensuring force is in newtons (N), area in square metres (m²), and pressure in pascals (Pa).
• Rearrange the pressure equation to find the normal force exerted by a fluid on a given area.

Common mistakes and how to put them right

• Forgetting to convert area from cm² to m².

  Put it right: Divide the area in cm² by 10,000 (or multiply by 10⁻⁴) before calculating pressure.

• Assuming the force from fluid pressure acts downwards only.

  Put it right: Recognise that fluid pressure acts normally (at right angles) to all surfaces of an object or container, regardless of orientation.

• Confusing the units of pressure.

  Put it right: Remember that 1 Pa is exactly equivalent to 1 N/m², not N/m.

Exam tips

• When calculating the force on a container wall, ensure you use the area of that specific wall and the pressure acting on it.
• Always check if the area needs calculating first from given dimensions (e.g., a rectangular window) before substituting into the pressure equation.