Question

Why is relative density called as specific gravity

Answer 1

Specific Gravity is also called Relative Gravity and is expressed by the formula: 

                Specific Gravity substance = ρ substance /ρ reference  

The reference material could be anything, but the most common reference is nearly always water at its densest, (4°C) for liquidsand for gases, air at room temperature, (21°C). That being stated temperature and pressure must be specified for both the sample and the reference.
                             Pressure is nearly always 1 atmequal to 101.325 kPa. Temperatures for both sample and reference vary from industry to industry.

Answer 2

Answer:

In Physics, the term 'specific' is used for a ratio of a physical quantity with another physical quantity of the same type with the latter taken as the standard.

The word 'gravity' here is used in reference to the weight of an object.

Explanation:

In the case of relative density, we compare the density of a particular object with a standard density (which is the density of water for solids and liquids). Since we compare density with density (i.e. divide the two or take their ratio) we use the word 'relative' (in relation) or 'specific' (in reference to) along with the word 'density' (for we are comparing densities) or 'gravity' (density has to do with the mass of an object hence it also has to do with it's weight which is a consequence of gravity).

Specific gravity with water as the standard for reference can be quite useful in determining whether an object will float, hover or sink when placed on it's surface. For example, the specific gravity of oil is lesser than 1 and it floats on water as it less denser than water, the specific gravity of gold is more than 1 and therefore it sinks in water. If the specific gravity of an object were equal to one the object would hover over water's surface (i.e. it would neither float nor sink).