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Calculating Pressure

 
  
 
Figure 9-3.-Fluids exert pressure in all directions. 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  P = f  and  substitute  the  proper values for F and A. In this case, F= 15,000 pounds; the area of the bottom in square inches is 10 x 6 x 144, since 144 square inches = 1 square foot. P  = 15,000 1 0 x 6 x 1 4 4 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  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 chest is  2,160  pounds  per  square  foot-more  than  a  ton pushing against an area of 1 square foot. If no air were inside  your  chest  to  push  outward  with  the  same pressure,  you’d  be  flatter  than  a  bride’s  biscuit. MEASURING FLUID PRESSURE All  fluids-both  liquids  and  gases—exert  pressure. A fluid at rest exerts equal pressure in all directions. As shown in figure 9-3, water will push through a hole in a submarine, whether it is in the top, the bottom, or in one of  the  sides. Many jobs aboard ship will require you to know the pressure exerted by a gas or a liquid. For example, knowing  the  steam  pressure  inside  a  boiler  is  always important. You can use three different gauges to find the pressure  of  fluids:  Bourdon  gauge,  Schrader  gauge,  and diaphragm  gauge. 9-3


   


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