Hydrostatic Pressure

  • A fluid’s own weight causes the fluid to be pressurized.
  • Within the fluid, each fluid molecule exerts repulsive forces against all its neighboring molecules.


  • Against a foreign object, the fluid exerts repulsive forces in the direction perpendicular to the surfaces of the foreign object.


  • Each point in a fluid can be ascribed a quantity called the static fluid pressure p, which is the repulsive force per unit area exerted by the fluid at that point.

  • By considering the pressure force Fp required to support a vertical column of fluid’s weight W, we can deduce that

\begin{aligned}{{F}_{p}}&=W\\pA&=mg\\pA&=\rho Vg\\pA&=\rho hAg\\p&=h\rho g\end{aligned}


  • Amazingly, hydrostatic pressure at a point is not affected by the shape or volume of the fluid above it. It is only dependent on the vertical height h of the fluid above it.


  • And yes the atmosphere is also a fluid with weight. So the atmosphere is also pressurized. In fact, the atmospheric pressure at sea level is about 100 000 N per m2.

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