• Archimedes’ Principle states that the buoyant force has a magnitude equal to the weight of the fluid displaced by the body and is directed vertically upword.
  • Buoyant force is a force that results from a floating or submerged body in a fluid.
  • The force results from different pressures on the top and bottom of the object

Module 7 Lessson 11 Archimedes principal

W is the weight of the shaded area

F1 and F2 are the forces on the plane surfaces

FB is the buoyant force the body exerts on the fluid

The force of the fluid on the body is opposite, or vertically upward and is known as the Buoyant Force.

  • The force is equal to the weight of the fluid it displaces.

  • The buoyant forces acts through the centroid of the displaced volume

Module 7 Lessson 11 Center of buoyancy

Stable Equilibrium: if when displaced returns to equilibrium position.

Unstable Equilibrium: if when displaced it returns to a new equilibrium position.

Module 7 Lessson 11 Stable unstable Equilibrium


  • If the Centre of Gravity is below the centre of buoyancy this will be a righting moment and the body will tend to return to its equilibrium position(Stable).

  • If the Centre of Gravity is above the centre of buoyancy, an over turning moment is produced and the body is unstable.

  • Note that, As the body is totally submerged, the shape of displaced fluid is not altered when the body is tilted and so the centre of buoyancy unchanged relative to the body.

Last modified: Monday, 16 September 2013, 6:46 AM