The Buoyancy reference article from the English Wikipedia on 24-Jul-2004
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Buoyancy commonly describes the ability of an object to float or apparently become less heavy when immersed in fluid. More generally, it refers to a force which creates motion and displaces a fluid due to the differences in density and pressure it has with its environment until equilibrium is reached.

Table of contents
1 The upward force
2 Density
3 Acceleration and energy
4 See also

The upward force

In physics, buoyancy refers to the quantity of the upward force caused by its volume, regardless of whether the object floats or not. If the buoyancy exceeds the downward force due to gravity, the weight, then the object floats; if the weight exceeds the buoyancy, the object sinks. It was the ancient Greek Archimedes of Syracuse who first discovered the law of buoyancy, sometimes called Archimedes' principle:

The buoyancy is equal to the weight of the displaced fluid.


If the weight of an object is less than that of the fluid that the object would displace if it was fully submerged, then the object is less dense than the fluid and it floats at such a level that the weight of the object is equal to the weight of the displaced fluid. If the object weighs more than that of the fluid that the object would displace if it was fully submerged, then the object is more dense than the fluid and the object sinks.

An object of a material of higher density than the fluid, e.g. a metal object in water, can still float if it has a suitable shape that keeps a large enough volume of air below the surface level of the fluid. In that case, for the average density mentioned above, the air is included also, which may reduce this density to less than that of the fluid.

This is the principle of vessels such as boats, ships, balloons, and airships, discovered by Archimedes.

Acceleration and energy

Although Archimedes' principle gives the force on a buoyant object, it is generally not recognized that this does not determine the related acceleration of the object in the usual way over Newton's first law. This is because not only has the mass of the object to be accelerated but also the mass of the displaced fluid. One can compare the situation to a scale, where the weight on one side is given by the object, and the weight on the other side by the displaced fluid element. Depending on which of the two is heavier, one side of the scale will drop and the other rise, but since both sides are rigidly connected, both masses have to be accelerated together at the same rate (albeit in opposite directions).

It is obvious that without taking the displaced fluid element into account, energy would not be conserved during the buoyant motion of an object as it would gain both potential and kinetic energy when rising in the fluid.

See also