What are density and buoyancy?

Density and buoyancy: F = ρ × V × g. Density is how much mass is packed into each bit of space. A fluid pushes up on anything in it with the weight of the fluid it pushes aside, so things less dense than the fluid float and denser things sink.

Read how it works: What are density and buoyancy? · The history of density and buoyancy · More boxes on Glassbox

Chapters in this interactive model

1. Density decides what floats: Same size, very different mass. The water pushes up with the weight of what you push aside.
2. Why a steel ship floats: Shape it right and even steel or clay floats. Load it wrong and it sinks.
3. Floating on air: balloons, geysers and AC: Warm air and warm water are a little lighter, so they rise. Cold sinks.
4. Swimmers, fish, submarines and eggs: A breath of air, a bag of gas, a tank of sea water or a spoon of salt tips the balance.
5. Ice on top, salty seas, space pools and myths: Water is weird at 4 °C, salt lets you lie on the sea, and heavy things can float.