Buoyancy

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 = ρ g = 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 acting on a 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/m ³.

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