Floating Bodies: Buoyancy

The pressure in a fluid at rest increases with depth, any body immersed in the fluid will have a greater pressure exerted over its lower surfaces than over its upper surfaces. Consequently, there will be a net upwards force on the body and this is known as the buoyancy force (FB). If buoyancy force is equal to the weight of the body, then the body will float, and the body is said to be buoyant.

This buoyancy force is expressed by the two laws of buoyancy discovered by Archimedes:

  1. A body immersed in fluid experiences a vertical buoyant force equal to the weight of the fluid it displaces.
  2. A floating body displaces its weight in the fluid in which it floats.

The buoyancy force has no horizontal component, because the horizontal thrust in any direction is the same as on a vertical projection of the surface of the body perpendicular to that direction, and the thrusts on the two faces of such a vertical projection balance exactly.

Buoyancy is known from elementary physics that the buoyant force, ‘FB‘, is given by the equation:

FB = γ.V

In the formula, ‘γ’ is the fluid’s specific weight and ‘V’ is the volume of the fluid displaced by the body. The buoyant force is vertically upward and has a magnitude equal to the weight of the fluid displaced by the body. The buoyant force passes through the centroid of the fluid displaced volume by the body and the point through which the buoyant force acts is called the center of buoyancy.

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