Pressure Definition and Examples

Pressure is defined as a measure of the force applied over a unit area. Pressure is often expressed in units of Pascals (Pa), newtons per square meter (N/m² or kg/m·s²), or pounds per square inch. Other units include the atmosphere (atm), torr, bar, and meters sea water (msw).

In equations, pressure is denoted by the capital letter P or the lowercase letter p.

Pressure is a derived unit, generally expressed according to the units of the equation:

P = F / A

where P is pressure, F is force, and A is area

Pressure is a scalar quantity. meaning it has a magnitude, but not a direction. This may seem confusing since it's usually obvious the force has direction. It may help to consider pressure of a gas in a balloon. There is no obvious direction of the movement of particles in a gas. In fact, they move in all directions such that that the net effect appears random. If a gas is enclosed in a balloon, pressure is detected as some of the molecules collide with the surface of the balloon. No matter where on the surface you measure the pressure, it will be the same.

Usually, pressure is a positive value. However, negative pressure is possible.

A simple example of pressure may be seen by holding a knife to a piece of fruit. If you hold the flat part of the knife against the fruit, it won't cut the surface. The force is spread out of a large area (low pressure). If you turn the blade so the cutting edge is pressed into the fruit, the same force is applied over a much smaller surface area (vastly increased pressure), so the surface cuts easily.