Archimede's principle

Meaning: An object fully or partly submerged in a fluid feels an upward buoyant force equal in magnitude to the weight of the fluid it displaces. The force exists because pressure rises with depth, so the fluid pushes up on the bottom of the object harder than it pushes down on the top.

Equation: Fb = ρfluid × Vsub × g, where Fb is the buoyant force (N), ρfluid is the fluid density (kg/m³), Vsub is the volume of fluid displaced (m³), and g is the gravitational acceleration, 9.8 m/s² (m/s²).

On the MCAT: For a floating object, Fb equals its weight, so the fraction submerged is ρobject/ρfluid. A fully submerged object sinks if it is denser than the fluid and rises if it is less dense. A scale reading an object in fluid gives apparent weight = true weight − Fb. Expect passages on ships, hydrometers, floating ice, or a block hung from a spring scale in water, and questions asking how Fb or the submerged fraction changes in a denser or less dense fluid.

Common confusion: Students think a heavier object feels a larger buoyant force. Fb depends on the fluid's density and the displaced volume, not on the object's mass. Two objects of equal volume, both fully submerged, feel the same Fb. Fb equals the object's weight only when it floats or is neutrally buoyant.