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Floating and sinking
Beginner
An object floats if its average density is below the fluid's.
An object floats if its average density is less than that of the fluid; a floating body displaces its own weight of fluid. Steel ships float as the hollow hull lowers the average density.
Ice floats with 92% submerged.
Weight vs buoyancy, not mass vs volume alone.
In more detail
An object floats if its average density is less than the fluid's, and sinks if it is more. A floating object displaces its own weight in fluid.
Real-world uses
- Boats are designed to float by enclosing air to lower their average density.
- Icebergs float with about nine-tenths submerged, because ice is slightly less dense than seawater.
- A ship made of steel floats because its hollow shape encloses a large volume of air, lowering its average density.
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Frequently asked questions
Why does a steel boat float?
Because its hull encloses air, so its average density is less than water.
What fraction of an iceberg is submerged?
About 90%, since ice is roughly 90% as dense as seawater.
Why does a steel ship float but a steel nail sinks?
The ship's average density, including the air inside, is less than water, whereas a solid nail is much denser.
Related topics in Fluids & Elasticity
Gauge and absolute pressure · Buoyancy · Archimedes' principle · Apparent weight in fluid · Stability of floating bodies · Hydrometer