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
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.