What is Pressure?

To minimize your confusion, it would probably be best to avoid a discussion of pressure altogether - but it's probably too late - you are probably already confused on the issue of force and pressure! Perhaps you think that force and pressure are really the same thing, like mass and inertia. Sorry!

Pressure depends on how much force is exerted - more force means more pressure. In fact, pressure is directly proportional to force. However, pressure is NOT the same thing as force. Pressure also depends on the area over which the force is applied. More area means less pressure - in fact, pressure is inversely proportional to area:

P = F/a

Pressure Units:

The SI unit of pressure is the Pascal - a force of 1 Newton applied over an area of 1 square meter produces a pressure of 1 Pascal, so a Pascal (abbreviated Pa) is a very small pressure. Another common unit of pressure is pounds per square inch (psi). Normal atmospheric pressure (1 atmosphere, abbreviated atm) = 14.7 psi = 1.0 x 105 Pa. Meteorologists use a unit of pressure based on the height of a column of mercury that the pressure will support, so on weather reports, you often hear atmospheric pressure reported in "inches of mercury", where 1 atm = 29.9 inches of Hg.

The Difference Between Force and Pressure:

When a force is exerted on an object, pressure is also exerted on the object, but force and pressure are not the same thing. Here's a "thought experiment" to illustrate the difference. Suppose that you are going to exert a force of 5 Newtons on your lab partner, and suppose your partner has the following choice:

Assuming your lab partner is not VERY strange, which would she pick? Of course, the book! But why? Both the pin and the book will exert the same force, and she will get the same acceleration from either force. Well, because the force exerted with the pin will hurt - a lot - and the force exerted with the book won't.

Although both the pin and the book exert the same force, they exert very different pressures. Pressure depends on the amount of force and the area over which the force is applied. More force - more pressure. More area - less pressure. In fact, pressure is directly proportional to force, and inversely proportional to area.

pressure = force/area

Force tells you how an object will accelerate. Pressure tells you how it will feel.


Force accelerates. Pressure cuts.


Pressure Examples

Example 1: Why does a sharp knife cut better than a dull knife?

sharp vs. dull knifeThe diagram at right shows a sharp knife and a dull knife in contact with a surface (maybe a nice, juicy steak!). You know that the sharp knife cuts more efficiently - Why?

Notice that the sharp knife has a very small area of contact with the surface, while the dull knife has a much larger area of contact. If both knives are pushed down with the same force, the sharp knife will exert a much greater pressure on the surface than the dull knife - and pressure cuts.

So, if you are "stuck" with a dull knife, you have to exert much more force in order to generate enough pressure to cut your steak, right?


Example 2: Ice skates vs. shoes

shoe vs. skateWhy do people have to use ice skates? Why don't shoes work just as well?

Look at the diagram on the right, which shows a shoe on the left and a skate on the right. The downward force that you exert on the ice in either case (assuming you are standing on two feet) would be half of your weight.

Notice, however, that the shoe distributes the force over a much larger area than the skate does. This means that the skate exerts a much higher pressure on the ice than the shoe does - it is this high pressure that makes ice skating possible!

Example 3: Why is a soft bed comfortable?

Why is it more comfortable (for most people!) to lie down on a soft mattress than to lie down on the floor?

First of all, no matter what you lie on, it must exert an upward (normal) force equal to your weight in order to support you. Therefore, the soft mattress and the hard floor exert the same upward force on you. However, the mattress conforms to your shape, so that it exerts the support force over a larger area than the floor does. Therefore, the mattress exerts less pressure on you than the floor. Pressure hurts.


last update December 27, 2005 by JL Stanbrough