Since pressure is equal to the force per unit area,set A equal to 1. Then the formula pressurebecomesP = h DEquation 2-5.It is essential that h and D be expressed in similarunits. That is, if D is expressed in pounds percubic foot, the value of h must be expressed infeet. If the desired pressure is to be expressed inpounds per square inch, the pressure formula,equation 2-5, becomesEquation 2-6.Pascal was also the first to prove byexperiment that the shape and volume of acontainer in no way alters pressure. Thus in figure2-8, if the pressure due to the weight of the liquidat a point on horizontal line H is 8 psi, thepressure is 8 psi everywhere at level H in thesystem. Equation 2-5 also shows that the pressureis independent of the shape and volume of acontainer.Pressure and Force in Fluid Power SystemsFigure 2-9.—Force transmitted through fluid.of the shape of the container. Consider the effectof this in the system shown in figure 2-9. If thereis a resistance on the output piston and the inputpiston is pushed downward, a pressure is createdthrough the fluid, which acts equally at rightangles to surfaces in all parts of the container.If force 1 is 100 pounds and the area of theinput piston is 10 square inches, then the pressurein the fluid is 10 psiRecall that, according to Pascal’s law, anyforce applied to a confined fluid is transmittedin all directions throughout the fluid regardlessNOTE: Fluid pressure cannot be createdwithout resistance to flow. In this case, resistanceFigure 2-8.—Pressure relationship2-6with shape.
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