Pressure

A book lying on a table exerts a force F on the table. The same book also exerts a pressure p on the table. What is the difference?

Pressure refers to the force acting on a unit area of the surface. They are related by P = F/A, where A is the area of the book that is in contact with the table.

Exercise. The weight of a book is 2 N. The area of one side is 20 cm2. If this side lies on the table, what pressure does the book exert on the table? Answer. F = 2 N, A = 20 cm2 = 0.002 m2.. Pressure P = F/A = 2 / 0.002 = 1000 Pa.

What about pressure from water or gas? Think of the air in a balloon. The inner wall of the balloon must feel pressure from the air inside, or else the balloon would not stay inflated. We know of course that the balloon is inflated because someone blows air inside. Why does having air in the balloon give pressure to everywhere on the inner surface? It is not because of the weight of the air, which acts downwards, and which is very small.

We can find the reason if we recall that air is made up of molecules moving around randomly, in all directions. Many molecules would hit the inner wall of the balloon every second - up, down, left or right. This is the reason for the air pressure.

The same idea is true for any gas, and for liquids as well. In fluids (gas or liquid), pressure acts in all directions - not just downwards like the book.




Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 5 October 2010.