Density (ISO 31: volumic mass) is a measure of mass per unit of volume. The higher an object's density, the higher its mass per volume. The average density of an object equals its total mass divided by its total volume. A denser object (such as iron) will have less volume than an equal mass of some less dense substance (such as water).
- 1 kg/dm3 = 1000 g/1000cm3 = 1 kg/L.
Perhaps the highest density known is reached in neutron star matter. The singularity at the centre of a black hole, according to general relativity, does not have any volume, so its density would be be seen as either infinite or non-existent.
A table of densities of various substances:
Note the low density of aluminium compared to most other metals. For this reason, aircraft were made of aluminium in the past. Also note that air has a nonzero, albeit small, density.
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Relative density, formerly called specific gravity, is a dimensionless quantity defined as the density of a substance divided by the density of water at standard temperature and pressure. By definition, then, the relative density (or RD) of water is 1, and the RD of osmium is about 22.
Density may denote how much of a certain substance, object or occurrence is present per unit area or volume. Often used is population density, meaning how many people per square kilometre (or square mile) on average live in an area.
The density of discrete entities such as people is difficult to characterise as a continuous quantity.
Geographers and mathematicians have made a number of attempts to formalize the concept of population density.
Charge density is the electric charge per unit area or unit volume.
See also probability density function.