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Overview
Buoyancy is the upward force exerted by a fluid on an object immersed in it. It explains why objects feel lighter in water, why ships float, why balloons rise, and why submarines can control depth. Buoyancy comes from pressure differences in a fluid.
Pressure Difference
Fluid pressure increases with depth. Because pressure is greater on the lower part of an immersed object than on the upper part, the fluid can exert a net upward force.
Archimedes' Principle
Archimedes' principle states that the buoyant force on an object equals the weight of the fluid displaced by the object.
Floating
An object floats when the buoyant force balances its weight before it is fully submerged, or when its average density is less than the fluid's density.
Sinking
An object sinks when its weight is greater than the maximum buoyant force the displaced fluid can provide.
Neutral Buoyancy
Neutral buoyancy occurs when an object's weight equals the buoyant force while fully submerged. Submarines and divers use buoyancy control to rise, sink, or hover.
Connection to Density
Density determines whether an object tends to float or sink. Average density compared with fluid density is the key condition.
Buoyancy in Air
Buoyancy also occurs in gases. Hot-air balloons rise because the heated air inside is less dense than the surrounding cooler air.
Common Mistakes
A common mistake is thinking buoyancy only exists in water. Any fluid, liquid or gas, can exert buoyant force.
Why This Matters in Physics
Buoyancy connects pressure, density, fluids, ships, balloons, submarines, weather, oceanography, and engineering design.