Pressure is any force applied to an object. When you press on an object, you're applying pressure to it. Your car's tires are kept firm by the pressure of compressed air pushing out on the inside. Scales measure weight by reading the increase of pressure on top of it. As divers, we measure pressure to determine how full our cylinders are and how deep we are.

Pressure is also the cause of many diving injuries. Ear squeezes, lung overexpansion, and decompression sickness are just a few examples of injuries caused by pressure changes. For this reason, it's important to understand pressure and how it affects air spaces.

In this lesson you'll learn about atmospheric pressure, pressure underwater, and how pressure changes affect the volume and density of air.