buoyancy

1. That property of an object that enables it to float on the surface of a liquid, or ascend through and remain freely suspended in a compressible fluid such as the atmosphere.

Quantitatively, it may be expressed as the ratio of the specific weight of the fluid to the specific weight of the object; or, in another manner, by the weight of the fluid displaced minus the weight of the object.

1. (Or buoyant force, buoyancy force;
   also called Archimedean buoyant force.) The upward force exerted upon a parcel of fluid (or an object within the fluid) in a gravitational field by virtue of the density difference between the parcel (or object) and that of the surrounding fluid.
   The magnitude of the buoyancy force F per unit mass may be determined by Archimedes's principle as

   

   where g is the acceleration of gravity, ρ the density of the buoyed fluid parcel or object, and ρ₀ the density of the surrounding fluid. In the atmosphere, a buoyant force on an air parcel may be attributed directly to a local increase of temperature and may be written

   

   where T and T₀ are the temperatures of the heated air and that of the environment, respectively. The coefficient (T/T₀ - 1) is sometimes called the buoyancy factor. The force F is sometimes called the reduced gravity.
   See free convection.