Air molecules colliding with a surface create atmospheric pressure.

The idea

Air pressure is the result of countless molecules moving randomly at high speed and bouncing off any surface they meet. Each collision changes the molecule's momentum, so each one exerts a tiny force on the surface; averaged over the enormous number arriving every second, the result is a steady force spread over the area, and that is what pressure means. Because the motion is random the collisions arrive from every direction, so the push acts on a surface whichever way it faces and always at right angles to it. At sea level the pressure is about 100 kPa, so the force on an outstretched palm is about 1-2 kN, the weight of an adult or two, unnoticed only because air on the other side pushes back just as hard.

What earns the marks

• Air molecules colliding with the surface.
• Each collision exerting a force on the surface.
• Pressure as that total force spread over the area of the surface.
• The collisions arriving from all directions because the molecular motion is random.

Common mistakes and how to put them right

• Describing the air as pressing down only, so that a vertical surface would feel nothing.

• Saying air pressure is caused by the weight of molecules resting on the surface rather than by collisions.

• Confusing the number of collisions with the speed of the molecules when explaining a change.

• Leaving out the word area, which turns a pressure answer into a force answer.

Exam tips

• Three words earn the marks here: collisions, force, area.
• If a change in pressure is asked for, say whether the collisions become more frequent, harder, or both.