Fluid pressure:

Hydrostatic pressure is the pressure exerted by the fluid at equilibrium because of the force of gravity. The hydrostatic pressure can be determined from a control volume analysis of an small cube of fluid.

The fluid pressure is pressure at some point or instant within the fluid, like water or air.

Fluid pressure arises in one of two situations:

1.  An open condition, like an ocean, a swimming pool, or the atmosphere; or

2.  A closed condition, like a water line or a gas line.

Pressure in open conditions can be approximated as the pressure in "static" or non-moving conditions, because the motions create only negligible changes in the pressure. Such conditions conform to principles of fluid statics. The pressure at any given point of a non-moving fluid is called as hydrostatic pressure.

The closed bodies of fluid are either "static," when the fluid is not moving, or "dynamic," when the fluid can move as in either a pipe or by compressing the air gap in the closed container. The pressure in closed conditions conforms to the principles of fluid dynamics. Because of the fundamental nature of fluids, a fluid cannot remain at rest under the presence of a shear stress. But, fluids can exert pressure normal to any contacting surface. If a point in the fluid is thought of as an small cube, then it follows from the principles of equilibrium that the pressure on every side of this unit of fluid must be equal. If this were not the case, fluid would move in the direction of resulting force. Thus, the pressure on a fluid at rest is isotropic; that is, it acts with equal magnitude in all the directions. This characteristic allows fluids to transmit force through the length of pipes or tubes; that is, a force applied to a fluid in a pipe is transmitted, by means the fluid, to the other end of the pipe.