Definition
A physical law stating that any object wholly or partially immersed in a fluid is buoyed up by a force equal to the weight of the fluid the object displaces. The principle applies to both liquids and gases, and it is the basis for understanding lift in lighter-than-air craft and the operation of certain aircraft instruments and components that rely on fluid displacement.
Plain English
When something sits in a fluid, the fluid pushes up on it with a force equal to the weight of the fluid the object pushes out of the way. If that upward push is greater than the object's weight, it floats or rises.
Context Anchor
Seen in aviation maintenance discussions of buoyancy, liquid density, floats, and tools that measure how heavy a liquid is for its volume.
Derivation
Named after Archimedes, a Greek mathematician of the 3rd century BC, who is said to have discovered the principle while noticing that water rose in his bath as he stepped in. Knowing the origin helps because the principle is genuinely about displacement — how much fluid is pushed aside — not about the object itself.
Why Pilots Care
It explains why hot-air balloons rise (heated air is less dense, so the balloon displaces a weight of cooler outside air greater than the balloon's own weight), and it underlies how a hydrometer reads battery charge by floating higher or lower in the electrolyte.
Analogy
If you push a sealed empty bottle into a bucket of water, the water pushes back upward. The more water the bottle pushes aside, the stronger that upward push becomes.
Grounding Statement
Picture pushing a beach ball underwater: the water pushes back up hard, and that upward push equals the weight of the water you shoved out of the way.
Intuition Check
Archimedes’ principle is not based only on how heavy the object is. It is based on how much fluid the object displaces and how heavy that displaced fluid is.
Example Sentence 1
By Archimedes' principle, the balloon lifts off once the heated air inside is light enough that the surrounding cooler air it displaces weighs more than the entire balloon and basket.
Example Sentence 2
The same principle explains why certain fuel or hydraulic components must be designed around the weight of fluid they displace during normal operation.