If you have ever seen a semi truck overturned, you have probably wondered how are they going to get it back to its upright position.  Well, one way that they can turn the semi truck upright again is by inflating several air bags underneath it.  If you think about this, it is very fascinating to know that air can lift a semi truck from the ground to its upright position.  Yet, this same idea should not be a surprise if you think about how a semi truck is supported.  A semi truck is usually supported by many tires that are inflated with air.  The air can support the truck  because the air in the tires is a gas, and gases exert pressure.

Although air pressure exists, many students, such as those the in the students' ideas page, do not believe that air exerts pressure.  Although it may seem as if the air we breathe does not exert pressure, the following activity, UP, UP, and AWAY! and the demonstrations, Egg in the Bottle and phenomena of a The Crushing Can can help you see with your OWN eyes that air does exert pressure!

So what does it mean to have gas pressure?

If you have ever played basketball or any other sport that involves using an inflated ball, you know that the air inside the ball will effect how the ball will bounce.  For example, if you squeeze the ball and it is soft, you know that it needs to be pumped with air so that it will bounce.  Before any extra air is added, the ball contains some air molecules that exert a pressure on the wall of the ball.

Pressure is a measurement of the force an object pushes against a given area.

Since we can squeeze the ball inwards, the pressure on the outside is much greater than the pressure that the gas exerts on the inside of the ball.

As a result, gas pressure is the force per unit area that the particles in the gas exert on the walls of their container.

If we pump more air into the basketball, you will increase the number of molecules inside.  As a result, more molecules strike the inner wall of the ball more often and the pressure increases.  As a result, the ball becomes firmer and bouncier!  The reason why we feel pressure is due to the fact that the molecules are exerting a pressure.  The faster they move, the more pressure they will exert.  The slower they move, the less pressure they will exert.

Since pressure is a variable of gases, it is one of the four variables that helps us to understand the properties of gases.