Definition
A physical law stating that any object immersed in a fluid is buoyed up by a force equal to the weight of the fluid the object displaces. In aviation, this principle explains why lighter-than-air craft such as balloons and airships rise: the surrounding air pushes up on the craft with a force equal to the weight of the air it pushes aside, and if that force exceeds the weight of the craft, the craft rises.
Plain English
When something sits in a fluid like air or water, the fluid pushes up on it with a force equal to the weight of whatever the object pushes out of the way. If that upward push is greater than the object's own weight, the object floats or rises.
Context Anchor
Seen when studying balloon and airship lift, and when discussing how seaplane floats or hulls support an aircraft on water.
Derivation
Named after Archimedes, a Greek mathematician from the 3rd century BC who, according to legend, discovered the principle while stepping into a bath and noticing the water rise. Knowing the origin reinforces that this is a general law of fluids, not just a fact about water -- it applies equally to air.
Why Pilots Care
It is the foundation of how balloons and airships fly. A hot-air balloon rises because heating the air inside it makes that air less dense, so the balloon and its contents weigh less than the surrounding air it displaces.
Analogy
A boat floats because it pushes water aside, and that displaced water pushes back upward. If the upward push is enough to support the boat's weight, the boat stays afloat.
Grounding Statement
If you push a beach ball underwater, you feel the water shoving it back up -- that shove equals the weight of the water the ball pushed aside. Air does the same thing to a balloon, just less obviously.
Intuition Check
Archimedes' Principle is not only about water. In aviation it also applies to air, because air is a fluid too.
Example Sentence 1
Archimedes' Principle explains why a hot-air balloon rises once the air inside it is heated and becomes lighter than the surrounding air.
Example Sentence 2
During preflight, the mechanic confirmed the buoyant force from the displaced water would keep the aircraft afloat at maximum gross weight.