#"Density, "rho# #=# #"Mass"/"Volume"#, and usually is quoted with units #g*mL^-1# or #g*cm^-3#. Relative density or specific gravity ( https://en.wikipedia.org/wiki/Relative_density) is the density compared to a reference substance, that is water, which has a density of #1*g*cm^-3#; the relative density is thus a dimensionless quantity. If relative density is #># #1#, the object should sink; if relative density is #<# #1#, the object should float. Why?