mass per unit volume, measured in kg/m3
determines buoyancy: objects float when less dense than the surrounding fluid
water density at 4 C: 1000 kg/m3, the standard reference
varies with temperature and pressure; gases are highly compressible
air at sea level: ~1.225 kg/m3
Earth average: ~5515 kg/m3, indicating a dense metallic core
relative density (specific gravity) compares a substance to water
used in material selection, fluid mechanics, geophysics, and astrophysics
population density and information density extend the concept beyond physics