Principles of Hydrostatic Pressure

Principles of Hydrostatic pressure

UNIT PRESSURE, p. The unit pressure, meaning the intensity of pressure, at any point in a fluid is the amount of pressure per unit area.

If the unit pressure is the same at every point on any area, A, on which the total pressure is P,

p= P/ A
if, however, the unit pressure is different is different points, the unit pressure at any point is equal to the total pressure on a small differential area surrounding the point divided by the differential area, or

p= dP/dA

where there is no danger of ambiguity, the term pressure is often used as an abbreviated expression for unit pressure. The fundamental foot-pound-second unit for pressure is pounds per square inch is often used.

Direction of Resultant Pressure. The resultant pressure on any plane. In a fluid at rest is normal to that plane. Assume that the resultant pressure P, on any plane AB, makes an angle other than 90 degrees with the plane. Resolving P into rectangular components P1 and P2, respectively parallel with and perpendicular to AB, gives a component P1 which can resisted only by a shearing stress. By the definition, a fluid at rest cannot resist a shearing stress. By the definition, a fluid at rest cannot resist a shearing stress, and therefore the pressure must be normal to the plane. This means that there can be no static friction in hydraulics.


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