Pressure (A Level)


  • Force acting per unit area
  • P = Force/Area
  • Pressure due to a column of liquid of depth h(without atm pressure)  $p = h \rho g$
  • Actual press at base of column of fluid  $p = h \rho g$ + atmospheric pressure at liquid surface

Upthrust or buoyancy force

  • An upward force on a body produced by the surrounding fluid (i.e., a liquid or a gas) in which it is fully or partially immersed, due to the pressure difference of the fluid between the top and bottom of the object
  • Net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body
  •  Upthrust = Vρg Or Upthrust = mass of liquid displaced x g = mg

Archimedes’ Principle

  • States that the upthrust experienced by an object partially or entirely immersed in a fluid is equal to the weight of the fluid displaced by the object
  • Can be applied to objects of all shapes and sizes
  • An object floats in equilibrium if the upthrust = weight of object
  • An object sinks if upthrust < weight of object

Upthrust (in equilibrium)

= weight of object supported by water/fluid
= weight of fluid displaced
= volume of object submerged in water X density of fluid X g

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