Pressure is the force over an area applied on an object in a direction perpendicular to the surface, denoted by symbol P.|
When a force is applied perpendicular to a surface area, it exerts pressure on that surface equal to the ratio of F to A,
where F is the force and A the surface area. Hence, the formula for pressure is:
P = F/A
One interesting consequence of this ratio is the fact that pressure can increase or decrease without any change
in force i.e in other words, if the surface becomes smaller, the pressure becomes larger, and vice versa.
The principle SI unit is called a pascal (Pa) or 1 N/m2.
A Newton (N), the SI unit of force, is equal to the force required to accelerate 1 kilogram of mass at a rate of 1 meter per second squared. Thus a Pascal is equal to the pressure of 1 newton over a surface area of 1 square meter.
In the English or British system, pressure is measured in terms of pounds per square inch, abbreviated as lbs/in2. This is equal to 6.89 x 103 Pa or 6890 Pa.
Another important measure of pressure is the atmosphere (atm), which the average pressure exerted by air at sea level. In English units, this is equal to 14.7 lbs/in2, and in SI units to 1.013 x 105 Pa i.e 101300 Pa.
There are also two other specialized units of pressure measurement in the SI system: the Bar, equal to 105 Pa and the Torr, equal to 133 Pa.
Meteorologists, scientists who study weather patterns, use the millibar (mb) which is equal to 0.001 bars. At sea level, atmospheric pressure is approximately 1013 mb.
The instrument for measuring atmospheric pressure, the barometer is calibrated to read zero when there is a complete vacuum. Hence the pressure indicated by the instrument is therefore called absolute pressure. The term pressure gauge is commonly applied to the other instruments used for measuring pressure.
Pressure plays a number of important roles in daily life, among them its function in the operation of pumps and hydraulic presses.
The maintenance of ordinary air pressure is essential to human health and well-being, the body is perfectly suited to the ordinary pressure of the atmosphere, and if that pressure is altered significantly, a person may experience harmful or even fatal side-effects.