Buoyancy

Buoyancy , or upthrust, is the <a href="/facts/Force/LNAUWMwK">force</a> exerted by a <a href="/facts/Fluid/wLnMhBC0">fluid</a> opposing the <a href="/facts/Weight/9NMHCsQd">weight</a> of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, <a href="/facts/Pressure/eyc37GRG">pressure</a> increases with depth as a result of the weight of the overlying fluid. Thus, the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. The pressure difference results in a net upward force on the object. The magnitude of the force is proportional to the pressure difference, and (as explained by <a href="/facts/Archimedes%2527_principle/L4AsK5Jj">Archimedes' principle</a>) is equivalent to the weight of the fluid that would otherwise occupy the submerged volume of the object, i.e. the <a href="/facts/Displacement_(fluid)/tXFhoWUA">displaced</a> fluid.
For this reason, an object with average <a href="/facts/Density/92p6hh9I">density</a> greater than the surrounding fluid tends to sink because its weight is greater than the weight of the fluid it displaces. If the object is less dense, buoyancy can keep the object afloat. This can occur only in a <a href="/facts/Non-inertial_reference_frame/UGfKCASo">non-inertial reference frame</a>, which either has a <a href="/facts/Gravitational_field/K1uTmJFy">gravitational field</a> or is <a href="/facts/Acceleration/UVDmjYvR"> accelerating due to a force other than gravity</a> defining a "downward" direction.
Buoyancy also applies to fluid mixtures, and is the most common driving force of <a href="/facts/Convection/E8vsCPhu">convection</a> currents. In these cases, the mathematical modelling is altered to apply to <a href="/facts/Continuum_mechanics/vI7LpFDk"> continua</a>, but the principles remain the same. Examples of buoyancy driven flows include the spontaneous separation of air and water or oil and water.
Buoyancy is a function of the force of gravity or other source of acceleration on objects of different densities, and for that reason is considered an apparent force, in the same way that <a href="/facts/Centrifugal_force/CWgvMw6w">centrifugal force</a> is an apparent force as a function of inertia. Buoyancy can exist without gravity in the presence of an inertial reference frame, but without an apparent "downward" direction of gravity or other source of acceleration, buoyancy does not exist.
The center of buoyancy of an object is the <a href="/facts/Center_of_gravity/n4Lr9GAz">center of gravity</a> of the displaced volume of fluid.

Buoyancy open-in-new

Buoyancy