Buoyant force
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
Fb = rhofluid × V × g (Archimedes); object floats if ρobject < ρfluid.
Buoyant force in plain English
Buoyant force is the upward push a fluid gives anything sitting in it — it's why a beach ball pops up when you let go underwater. The rule (from Archimedes) is F_b = ρ_fluid × V × g, meaning the push equals the weight of the fluid your object shoves out of the way: ρ is the fluid's density (how tightly packed it is), V is how much of the object is submerged, and g is gravity. The key insight: compare densities. If the object is less dense than the fluid (ρ_object < ρ_fluid) it floats, and if it's denser it sinks — which is why a steel nail drops but a wooden log rides high.
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