Hydrostatics is referring to the study of fluids at rest, as opposed to hydrodynamics, which is the study of fluids in motion. A fluid can either be a liquid or gas, and no definitive shape. In order to discuss hydrostatics, we will first define several terms relating to the characteristics of a fluid, including the density, weight, specific gravity, and pressure.
Hydrostatic pressure, also called gauge pressure, is the pressure that is a result of being at a certain depth within a fluid. Consider a container filled to the brim with fluid. Within the container of fluid, imagine a flat sheet of metal that has a length (l) and width (w). This is important because the sheet of metal has a particular area that is going to be acted on by the fluid, creating pressure. Pressure is equal to force/area, and when the sheet of metal is submerged in water, the force acting on the metal is the weight of the fluid above it. Recall that we defined the weight of the fluid as its density multiplied by its volume. The farther down in the container the sheet is, the greater the weight of water acting on the sheet. The pressure acting on the sheet of metal is the force of gravity over the sheet across the area of the sheet. However, we can derive an even more specific equation. The volume of water above the sheet is equal to the length of the sheet, multiplied by the width of the sheet, multiplied by the depth (h) of the sheet below the water (essentially the height of the column of water above the sheet). The length by width of the sheet is equal to area, which cancels out with area in the denominator. The hydrostatic pressure of a fluid at a particular depth is therefore equal to the product of its density, the gravitational constant, and the depth at which it acts.
Hydrostatic pressure does not take into account the total pressure at a particular point in a fluid. The total pressure, or absolute pressure, at a particular point, takes into account two things: the hydrostatic pressure at that point, and the atmospheric pressure. The hydrostatic pressure is the pressure due to the weight of water acting across an area, and the atmospheric pressure is the pressure due to the atmosphere pushing down on the surface of the fluid. At the very surface of a fluid, there is no hydrostatic pressure, but there is atmospheric pressure due to the air above the fluid pressing downward. If an MCAT question specifically asks you to calculate the gauge or hydrostatic pressure at a point in a fluid, you would disregard atmospheric pressure. However, it is important to remember that it exists and contributes to absolute pressure.
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