Archimedes' principle

Meaning: Pressure in a fluid increases with depth, so the upward push on the bottom of a submerged object is larger than the downward push on its top. The net upward force is the buoyant force, and it equals the weight of the fluid the object displaces. A floating object has a buoyant force equal to its weight, so the fraction submerged equals ρobject/ρfluid.

Equation: Fbuoy = ρfluid Vsub g, where Fbuoy is buoyant force (N), ρfluid is fluid density (kg/m³), Vsub is displaced fluid volume, equal to the submerged volume (m³), and g is 9.8 m/s².

On the MCAT: A block floats in water, then is moved to a denser liquid; it floats higher because less volume must be submerged. Another setup weighs an object in air and in fluid: apparent weight = true weight − Fbuoy, which yields the object's volume or density.

Common confusion: Buoyant force depends on the fluid's density and the displaced volume, not on the object's weight. A steel ship floats because its hull displaces a large volume of water. A floating object has Fbuoy equal to its weight; a fully submerged sinking object has Fbuoy less than its weight.