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Because we cannot see air we may think of it as nothing, but you need only to stand in front of a fan or pedal your bicycle into the wind to know that air has substance. Air has weight because it is matter and is attracted by the Earth's gravity. Weight is explained in Figure 2-1a.
Notice that the direction of the atmosphere's weight is toward Earth's center, which is the center of gravity. Refer to the 1 inch x 1 inch x 60 miles column of air shown in Figure 2-1a. This column of air has a weight of 14.7 pounds. This is caused by gravity pulling all air molecules within the column toward Earth's center with that many pounds of force. It is the weight of the air which produces atmospheric pressure.
Always remember that WEIGHT is in one direction, while PRESSURE is in all directions.
Mass is the amount of material as shown in Figure 2-1b.
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Updated: March 12, 2004