FHSST Physics/Pressure/Summary

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The Free High School Science Texts: A Textbook for High School Students Studying Physics
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Important Equations and Quantities

Important Equations and Quantities[edit]

Table 10.1: Units used in Pressure
Quantity Symbol Unit S.I. Units Direction
Pressure P pascal (Pa) No1

Non Si Units for pressure[edit]

PSI: PSI stands for pounds per square inch. One PSI is equal to 6,894.76 Pascals

Atm: Atm stands for Atmosphere. This is the atmospheric pressure at sea level. I atm is equal to 101,325 Pascals

Hydrostatic pressure (head pressure)[edit]

Hydrostatic pressure is the pressure due to the weight of a fluid.


  • ρ (rho) is the density of the fluid (i.e., the practical density of fresh water is 1000 kg/m3);
  • g is the acceleration due to gravity (approximately 9.81 m/s2 on earth's surface);
  • h is the height of the fluid column (in metres). Other units can be used if the rest of the units used in the equation are defined in a consistent way.

It is important to realize that air is a fluid. This equation can be used to find the pressure created by air. This also means that if we are trying to find the pressure of a liquid open to the atmosphere, we must add in the atmospheric pressure to the pressure of the liquid to find the total pressure.



Pressure is force divided by area, so in a sense it does have a direction as force has a direction. The term pseudovector is sometimes used for such quantities.