Buoyancy

When a body is partially or fully immersed in a fluid, it pushes aside (displaces) a certain volume of that fluid, and the displaced fluid in turn exerts an UPWARD force on the body, opposing its weight; this upward force is called UPTHRUST or the BUOYANT FORCE, and the phenomenon itself is called BUOYANCY. ARCHIMEDES' PRINCIPLE states this precisely: when a body is partially or wholly immersed in a fluid, it experiences an upward thrust equal to the weight of the fluid it displaces, and this upthrust acts through the centre of gravity of the displaced fluid -- in symbols, upthrust = weight of liquid displaced. LAW OF FLOATATION. A direct consequence of Archimedes' principle is the law of floatation: a body will float in a liquid if the weight of the liquid displaced by the immersed part of the body equals the weight of the body itself (for instance, a wooden object weighing about 3000 N floats in water by displacing 3000 N worth of water). When an object floats, the volume of fluid it displaces equals the volume of the object that is submerged, and the PERCENTAGE of the object's volume that is submerged equals the object's relative density with respect to the fluid it floats in -- an ice cube of density 0.9 g cm$^{-3}$ floating in fresh water (density 1.0 g cm$^{-3}$) has 90% of its volume submerged, but the same ice cube floating in denser sea water (density 1.3 g cm$^{-3}$) has only about 69.2% of its volume submerged, since it needs to displace less volume of the denser sea water to support the same weight. Everyday examples of floating and buoyancy include: swimming is easier in denser sea water than in river water; ice floats on water because ice is less dense than liquid water; and a steel ship floats despite steel itself being denser than water, because its hull is hollowed into a concave shape that displaces a large volume of water relative to the ship's actual weight. Submarines control their depth by adjusting their own average density usin …