Question

Why is density a derived unit?

Answer 1

Density is a derived unit because it is defined in terms of other units.

Explanation:

Density is measured in `kg*m^-2`. This means that it is derived from fundamental units which are the kilogram and metre.

The kilogram is defined as the mass of a platinum-iridium cylinder which is kept in France. This is the only fundamental unit which is still defined by an artefact.

The second is defined at the time take for `9,192,631,770` transitions of a caesium 133 atom. The speed of light is defined to be exactly `299,792,458 ms^-1`. Now the metre is defined to be the distance light travels in `1/(299,792,458)` seconds.

hence the kilogram and the metre are SI units and density is a derived unit from these units.