Mass : The quantity of matter in a body is called its mass.The SI unit of mass is kilogram.It is written as kg.
Weight : The weight of a body on the earth is equal to the force with which the body is attracted towards the earth.Its unit is newton (N).
Density : The density of a substance is defined as the mass per unit volume of that substance.
density = mass ∕ volume
Units of density:
In the CGS system, the unit of density is g/cm³ and in the SI system, the unit of density is kg/m³.
1 g ∕ cm³ = 1000 kg ∕ m³
Relative Density : The relative density (R.D.) of a substance is defined as the ratio of the density of the substance to the density of water.
Relative Density = density of the substance ∕ density of water
Thus, relative density is also defined as the ratio of the mass of any volume of the substance to the mass of the same volume of water.
Since relative density is the ratio of the density of a substance and the density of water, it has no units.