Density
Definition
How much mass is packed into a given volume. A bowling ball and a beach ball can be the same size, but the bowling ball is denser because it holds far more mass.
Symbol: · Unit: kilograms per cubic meter (kg/m³)
Technical definition
Mass per unit volume of a substance: ρ = m/V. An intensive property; the SI unit is kilograms per cubic meter (kg/m³). Density determines whether objects float or sink in a fluid.
Example
Water has a density of 1000 kg/m³. A 2 m³ block of wood with mass 1200 kg has density 600 kg/m³ — less than water, so it floats with 60% of its volume submerged.
Density is not the same as weight or heaviness. A massive ship is heavy, but its average density (steel hull plus air inside) is less than water, so it floats. Always compare densities, not weights, when predicting floating or sinking.