Buoyancy

The resultant force acting on a submerged body

Buoyancy is defined as the tendency of a body to float or rise when submerged in a fluid. The resultant force acting on a submerged body by the fluid is called the buoyant force and can be expressed as

F = V γ

= V ρ g                  (1)

where

F = buoyant force (N)

V = body volume (m3)

γ = specific weight of fluid (N/m3)

ρ = density of fluid (kg/m3)

g = acceleration of gravity (= 9.81 m/s2)

The buoyant force acts upwards.

Archimedes' principle indicates that

"the upward buoyant force that is exerted on a body fully or partially submerged in a fluid - equals to the weight of the fluid that the body displaces"

• if the body weighs more than the fluid - it sinks
• if the body weighs less than the fluid - it floats

Example - Buoyant Force Floating Box

A plastic box with length 0.3 m and width 0.4 m is submerged 0.1 m into water.  The water density is 1000 kg/m3.

The buoyant force acting on the box can be calculated with (1)

F = (0.3 m) (0.4 m) (0.1 m) (1000 kg/m3) (9.81 m/s2)

= 119 N

Related Topics

• Fluid Mechanics - The study of fluids - liquids and gases. Involves velocity, pressure, density and temperature as functions of space and time

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