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 CALCULATING PRESSURE To calculate pressure, divide the force by the area on which you apply force. Use the following formula:
or To understand this idea, follow this problem. A fresh
water holding tank aboard a ship is 10 feet long, 6 feet
wide, and 4 feet deep. Therefore, it holds 10 x 6 x 4, or
240, cubic feet of water. Each cubic foot of water weighs
about 62.5 pounds. The total force outside the tank’s
bottom is equal to the weight of the water: 240 x 62.5,
or 15,000 pounds. What is the pressure on the bottom of
the tank? Since the weight is even on the bottom, you
apply the formula
Now work out the idea in reverse. You live at the bottom of the great sea of air that surrounds the earth. Because the air has weight—gravity pulls on the air too—the air exerts a force on every object that it surrounds. Near sea level that force on an area of 1 square inch is roughly 15 pounds. Thus, the air-pressure at sea level is about 15 psi. The pressure gets less and less as you go up to higher altitudes. With your finger, mark out an area of 1 square foot on your chest. What is the total force pushing on
your chest? Again use the formula
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