A giant log floats but a tiny pebble sinks — so it isn't just about being heavy. Today you'll learn the real rule, then you will predict whether each object floats or sinks.
Before we start — a huge steel ship floats, yet a small coin sinks — so how can it just be about being heavy?
A huge log floats, but a small steel nail sinks. So "heavy sinks, light floats" can't be right. The real question is how tightly packed an object is — that's its density.
Density is how much stuff (mass) is packed into a space. Compare an object to water: if it's less dense than water, it floats; if it's more dense, it sinks.
Water pushes up on anything in it — that push is called upthrust (buoyancy). If the upthrust can balance the object's weight, it floats. Clever shapes (like a boat hull) trap air and spread the weight out, lowering the overall density.