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In physics, buoyancy is an upward force on an object immersed in a fluid, enabling it to float or at least to appear to become lighter. If the buoyancy exceeds 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 buoyant force is equal to the weight of the displaced fluid.

1 Density

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, ballooncards or flowers. A balloon is a flexible bag normally filled with air or gas. Some balloons are purely decorative, while others are used for specific purposes. Early balloons were made of dried animal bladders. Balloons as flying machines Large balloonss, and airshipUSS Akron in flight, November 2nd 1931 An airship is a buoyant (" lighter-than-air") aircraft that can be steered and propelled through the air. Unlike aerodynamic ("heavier-than-air") aircraft which stay aloft by moving an airfoil through the air in ordes, discovered by Archimedes.

2 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 accelerationIn physics, acceleration (symbol: a is defined as the rate of change (or time derivative) of velocity. It is thus a vector quantity with dimension length/ time². In SI units, this is metre/second². To accelerate an object is to change its velocity over a 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 scaleA scale is either a device used for measurement of weights, or a series of ratios against which different measurements can be compared. The latter need not always be a linear ratio, and is often logarithmic. A draughtsman's scale is a ruler-like device, o, 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.

Buoyancy compensatorA buoyancy compensator (or buoyancy control device BC or BCD is a piece of diving equipment worn by divers to provide: life saving emergency buoyancy both underwater and on the surface. the ability to adjust and control the overall buoyancy of the diver a
Diving weighting systemDivers wear weighting systems weight belts or weights generally made of lead, to counteract the buoyancy of other diving equipment, such as diving suits and aluminium diving cylinders. The weights provide a useful rescue mechanism they can be dropped in a
FlotationFlotation is a method for the separation of mixtures. Flotation is a separation technique used widely in the minerals industry, for paper deinking and water treatment amongst others. The technique relies apon differences in the surface properties of diffe
Hull (ship)
Hydrometer
Lighter than air
Naval architecture
Pontoon
Quicksand

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Topics: Buoyancy, Density, Acceleration And Energy, See Also...

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