A steel ship floats but a steel needle sinks — both are steel, so how?
Floating depends on average density, not the material alone.
Floating depends on average density, not the material alone. A ship is mostly hollow (steel + air), so its average density is less than water and it floats, displacing enough water to balance its weight (Archimedes). A solid needle has the density of steel throughout — far more than water — so it can't displace its own weight and it sinks.
Key point
Floats when average density < water's, i.e. it can displace its own weight.
Common mistake
judging floating by the material instead of the object's overall density.
Memory tip
shape the steel to trap air → lower average density → it floats.
Tap to watch or see it right here — the app stays open (it only opens a quick search if nothing embeddable is found).