Pressure

Pressure is a force spread over a certain area.

P = \cfrac{F}{A}

Variable Metric Unit Standard Unit
P Pressure Pa=\si{N/m^2}
(Pascals)		 psi=\si{lb/in^2}
(pounds per square inch)
F Force N (Newtons) lb (pounds)
A  Area m2 in2

An object can put pressure on another object because of its weight. Weight is the force due to gravity (not the same thing as mass).

Example: what is the pressure exerted on a table by a laptop that weighs 15.0 N if its base is 40 cm x 30 cm?

Solution:
Force = 15.0 N (the weight of the laptop).
Area = 0.40 m x 0.30 m = 0.12 m2.
(Note that we converted centimeters to meters, because pressure units are N/m2).

P = \cfrac{F}{A} = \cfrac{\SI{15.0}{N}}{\SI{0.12}{m^2}} = \SI{125}{ N/m^2} = \SI{125}{Pa}

Fluid Pressure is exerted by a gas on every side of its container. The pressure on every side of a container is the same.

Atmospheric Pressure is the pressure from the gas molecules in our atmosphere.

Earth’s atmospheric pressure at sea level equals 1 atm.
(“atm” is a unit that is short for “atmospheres.”)

1 atm = 14.7 psi = 760 mmHg = 101,325 Pa

Other Pressure Units

  • The height of a liquid represents a certain amount of pressure. “mmHg” is “millimeters of mercury.”
  • “mmHg” is the exact same thing as “torr.”
  • The atmosphere’s pressure is equal to 34 feet of water. Sometimes barometric pressure is expressed in inches of water.