(The pressure at the bottom horizontal line is equal on both sides of the tube. The pressure of the gas is equal to a column of mercury of height 26.4 cm. The pressure of a sample of gas is measured with a closed-end manometer, as shown to the right. Let’s apply this concept to determine which exerts a greater pressure in Figure 9.3-the elephant or the figure skater? A large African elephant can weigh 7 tons, supported on four feet, each with a diameter of about 1.5 ft (footprint area of 250 in 2), so the pressure exerted by each foot is about 14 lb/in 2:Ĭalculation of Pressure Using a Closed-End Manometer Thus, pressure can be increased either by increasing the amount of force or by decreasing the area over which it is applied pressure can be decreased by decreasing the force or increasing the area. Note that pressure is directly proportional to force and inversely proportional to area. In general, pressure is defined as the force exerted on a given area: P = F A. If you actually perch a bowling ball on your thumbnail, the pressure experienced is twice the usual pressure, and the sensation is unpleasant. These may seem like huge amounts, and they are, but life on earth has evolved under such atmospheric pressure. At sea level, this pressure is roughly the same as that exerted by a full-grown African elephant standing on a doormat, or a typical bowling ball resting on your thumbnail. A dramatic illustration of atmospheric pressure is provided in this brief video, which shows a railway tanker car imploding when its internal pressure is decreased.Ī smaller scale demonstration of this phenomenon is briefly explained.Ītmospheric pressure is caused by the weight of the column of air molecules in the atmosphere above an object, such as the tanker car.
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