<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2014) (Learn how and when to remove this template message) </Td> </Tr> <P> In physics, buoyancy (/ ˈbɔɪənsi, - əntsi, ˈbuːjənsi, - jəntsi /) or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object . In a column of fluid, pressure 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 . This pressure difference results in a net upwards force on the object . The magnitude of that force exerted is proportional to that pressure difference, and (as explained by Archimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid . It is also known as buoyant force . </P> <P> For this reason, an object whose density is greater than that of the fluid in which it is submerged tends to sink . If the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat . This can occur only in a non-inertial reference frame, which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction . In a situation of fluid statics, the net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body . </P> <P> The center of buoyancy of an object is the centroid of the displaced volume of fluid . </P>

What is the name of the force that keeps things afloat