What is Buoyancy?
Buoyancy is an upward force exerted by a fluid on an immersed object in a gravity field.
In fluids, pressure increases with depth; hence, when an object is immersed in a fluid, the pressure exerted on its bottom surface is higher than the pressure exerted on its top surface.
This difference in the pressure leads to a net upward force (buoyancy force) which opposes the gravity force and is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.
Thus, if the object is less dense than the fluid, the buoyancy force will be higher than its weight and the object will float; on the contrary, if the object is denser than the fluid, it will sink.
The static balance occurs when the weight of the immersed part of the object is the same as the weight of the displaced fluid, i.e. the densities coincide.
Remark that the density of the object is meant in an averaged way, as a simple ratio between mass and volume of the immersed part.
The most common case is the immersion of a solid into a liquid (e.g. ships in the sea), but that’s not all: a rising bubble (gas in liquid), a falling droplet (liquid in gas) and aerostats (warm air into cold air) are examples of phenomena ruled by buoyancy forces as well.
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