SEED Science

Floating and Stability: Center of Buoyancy

Find out more about
Floating and Stability

Center of Gravity 
General Stability
Buoyant Force
Floating and Sinking
Archimedes’ Principle
Center of Buoyancy
Stability and Center of Buoyancy
Stability in Sailboats

Center of Buoyancy

Now it is time to combine these ideas.

The center of buoyancy of a floating object is related to the center of gravity concept. When an object is partly or completely submerged in a liquid, some of that liquid is displaced. The volume of displaced liquid is exactly the same as the volume of whatever part of the object is below the surface. The shape of the volume of displaced liquid is also exactly the same as the shape of the part that is submerged, because the object itself is displacing the liquid.

Center of BuoyancyThe center of buoyancy of the object is located at the center of gravity of the volume of the displaced liquid. It is the point through which the upward buoyant force seems to act.

The diagram here shows a block of wood floating in water. The center of gravity (CG) of the block is at the geometric center of the block, as it was before the block was placed in the water. The center of buoyancy (CB) is at the center of gravity of the displaced water.

block
weight
Supporting force
buoyant
force
 

Virtual Lab: Center of Buoyancy game

Virtual Lab

Click here to
try for yourself

The challenge is for you to imagine the part of the block below the water level, shaded in gray, being replaced by water. Doing this, however, shows you exactly what the displaced water looks like. In this case, the shape of the displaced water is the shape of the part of the block that is beneath the water surface.

The block floats at a level that displaces exactly enough water to create an upward buoyant force equal to the downward weight of the block.

Note: The wood block floats because of Archimedes’ principle. It is able to displace enough water to balance its weight without having to submerge its entire volume to do so. Another way of saying this is that the density of the material in the block is less than that of water.