Pressure

Atmospheric Pressure: "Weighing" Air

If you have ever watched a weather forecast on the local news, you have probably heard of atmospheric pressure or barometric pressure. But, what are atmospheric pressure and barometric pressure?

Before we discuss atmospheric pressure, it is a good idea to first separate the concepts of pressure and weight. Weight is the downward force created by gravity on a mass; Pressure is the force created over a given area.

They may seem similar but here is the difference. When you stand on a bathroom scale, your weight will appear. Weight does not depend on how much area receives the force. The size of your feet will not affect weight.

Here is an example of two objects that have the same weight but create different pressure values. This should not be done as an experiment, just consider the concept.

Chris Ratley has two identical blocks of concrete, which are sitting side by side on the floor in front of him. Each block weighs 50 pounds. The bottom of each block is 1 foot across and 2 feet long. The area of the bottom of the block is 2 square feet.

We also have two identical wooden tables and one nail. The head of the nail is very small. The area of the nail's tip is very small, .01 square inches!

Chris places one block on a table and nothing happens. The table is sturdy and is able to hold the block of concrete.

Chris is now curious to see if he can balance the other block of concrete on top of the nail on the other table. The nail goes right through the table leaving a small hole.

Why did the nail go through the table? If you said, "because the nail is sharp" you are only partially correct. The answer lies in the difference in pressure created by the two examples.