Pressure

Pressure can succinctly be defined as a measure of the amount of force distributed over an area. The pressure in a fluid acts equally in all directions and is perpendicular to all surfaces.

Units of Measurement
The SI unit used to measure it is N/m 2, or the pascal (Pa).

In imperial units, pressure can be measured in lb/in2. 1 Pa is equal to approximately 1.455E-4 lb/in2. Another common unit for measuring pressure is the bar, which is equal to 105 Pa. This unit is most often used in weather forecasting, as 1 bar is roughly equal to the value of atmospheric pressure.

Relations
Pressure is defined algebraically as: In this equation, P represents pressure, F represents amount of force, and A represents area.
 * P = F/A

In order to increase pressure, either the force applied to a given area must increase, or a given force must be applied to a smaller area. It logically follows that pressure will decrease if the force applied to a given area decreases or a given force is applied across a larger area.

Atmospheric Pressure and Gauge Pressure
Atmospheric pressure, represented as Pat, is defined as: Atmospheric pressure can also be expressed in imperial units as 14.7 lb/in2, and is equal to approximately 1 bar. It is important to examine pressure in the context of atmospheric pressure. After all, it wouldn't be accurate to say a deflated tire or basketball has a pressure of 0. Atmospheric pressure is still in play. Thus, to inflate a tire, basketball, or similar object to a certain pressure, it must actually be that amount of pressure greater than Pat. Because of situations like this, physicists rely on the concept of gauge pressure (Pg), which can be summarized as follows:
 * Pat = 1.01E5 Pa, or 101 kPa
 * Pg = P - Pat

Sources and Further Research
Walker, James S. Physics: Fourth Edition: AP Edition. Pearson Education, Inc., 2010.

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