Figure 9-3.-Fluids exert pressure in all directions.CALCULATING PRESSURETo calculate pressure, divide the force by the areaon which you apply force. Use the following formula:orTo understand this idea, follow this problem. A freshwater holding tank aboard a ship is 10 feet long, 6 feetwide, and 4 feet deep. Therefore, it holds 10 x 6 x 4, or240, cubic feet of water. Each cubic foot of water weighsabout 62.5 pounds. The total force outside the tank’sbottom is equal to the weight of the water: 240 x 62.5,or 15,000 pounds. What is the pressure on the bottom ofthe tank? Since the weight is even on the bottom, youapply the formula P = f and substitute the propervalues for F and A. In this case, F= 15,000 pounds; thearea of the bottom in square inches is 10 x 6 x 144, since144 square inches = 1 square foot.P =15,0001 0 x 6 x 1 4 4Now work out the idea in reverse. You live at thebottom of the great sea of air that surrounds the earth.Because the air has weight—gravity pulls on the airtoo—the air exerts a force on every object that itsurrounds. Near sea level that force on an area of 1square inch is roughly 15 pounds. Thus, the air-pressureat sea level is about 15 psi. The pressure gets less andless as you go up to higher altitudes.With your finger, mark out an area of 1 squarefoot on your chest. What is the total force pushing onyour chest? Again use the formula P = f. Now sub-stitute 15 psi for P and 144 square inches for A. Then,F = 144 x 15, or 2,160 pounds. The force on your chestis 2,160 pounds per square foot-more than a tonpushing against an area of 1 square foot. If no air wereinside your chest to push outward with the samepressure, you’d be flatter than a bride’s biscuit.MEASURING FLUIDPRESSUREAll fluids-both liquids and gases—exert pressure.A fluid at rest exerts equal pressure in all directions. Asshown in figure 9-3, water will push through a hole in asubmarine, whether it is in the top, the bottom, or in oneof the sides.Many jobs aboard ship will require you to know thepressure exerted by a gas or a liquid. For example,knowing the steam pressure inside a boiler is alwaysimportant. You can use three different gauges to find thepressure of fluids: Bourdon gauge, Schrader gauge, anddiaphragm gauge.9-3

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