Buoyancy

Buoyancy is the upward force exerted on an object that is partially or completely immersed in a fluid. In order for a buoyant force to exist, the object must be of lower density than the surrounding fluid and both fluid and object must be subject to gravity.

Buoyancy is the method by which ships are made to float. Likewise, buoyancy is used to keep aircraft such as balloons and airships aloft.

Archimedes was the first person to describe buoyancy. As a result, the fundamental description of buoyancy is called Archimedes' principle.