Question

why we say relative density also specific gravity“??

Answer 1

Density is itself the nature of a material . Like I would quote , we say something is sweeter than sugar . Why do we say so , is because we compare it with the sweetness of sugar . Actually it is sweet as much as it is but we compare it to sugar to give a sense of sweetness it carries . Similarly , pure water's density is taken as a parameter to compare densities , I.e. the density of water is taken as 1 gm /cm ^3
We actually don't know actual density . It's just taken as a reference . Physics itself is a reference . Nothing is certain . That's why we call it relative density . It is the density of a substance as compared to the water.