## Buoyancy, principle of

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# Buoyancy, principle of

The principle of buoyancy is also called Archimedes principle, as it was discovered by this Greek mathematician in the third century BC. The principle states that the buoyant force acting on an object immersed in or floating on a fluid is equal to the weight of the fluid displaced by the object. An object completely immersed in a fluid (liquid or gas) displaces a volume of fluid exactly equal to its own volume; a floating object displaces only part of its volume. The weight of that volume of displaced fluid is the buoyant force acting on the object. In the case of a floating object, it is exactly equal to the objects weight.

Fluids such as water or air exert pressure in all directions. The amount of pressure depends on the depth of the fluid. The pressure on the bottom of an object immersed in a fluid will be greater than the pressure on the top of the object. The imbalance of pressure acting on the object creates an upward force called the buoyant force. If the buoyant force is greater than the weight of the object, the object will float. If the buoyant force is less than the weight of the object, the object will sink in the fluid.

The density of a fluid is its mass per unit of volume. (If Earth-normal gravity is assumed, units of weight, such as pounds [lb], are often substituted for units of mass, such as kilograms; this is done in the following text, where density is treated as pounds per unit volume.) Liquids and gases exhibit widely different densities. The buoyant force, or the weight of the volume of displaced fluid, will depend on the density of the fluid as well as the displaced volume. Freshwater has a

density of 62.4 lb per cubic foot (pcf), saltwater density is, on average, 64 pcf. Air at sea level has a density of 0.08 pcf and at 10,000 ft (3,050 m), 0.06 pcf. Saltwater is denser than freshwater because of its salt content, and, as a result, a swimmer is more buoyant in the ocean than in a freshwater lake. The density of saltwater depends on its salinity and varies around the world. The molecular structure of water expands when it freezes, therefore, ice is less dense than liquid water. As a result, ice cubes float and lakes freeze from the top down rather than the bottom up.