Density, Specific Weight, Specific Gravity & Specific Volume

Density or Mass Density: It is the ratio of the mass to volume. Mass per unit volume. It is denoted by Greek letter ρ.

Concept: Every matter is made up of molecules; and molecules are combination atoms. When the molecules are together they become mass of that substance. Some of the molecules come together leaving voids between them and some are very compact. For example, take pebbles and sand. For the same volume, pebbles will have more voids and sand will have less voids.  Like this the molecule arrangement in some substances will be different from other substance.  Hence we have to find the mass available in particular volume. To make it common we take unit volume.  It may be 1 cm³ or 1m³.

Water is taken as Standard liquid and Air is taken as standard gas.

Specific Weight: Weight per unit volume   w= mg/V (N/m³) =W/V

Specific Gravity: 

Specific weight of a fluid/specific weight of standard fluid (water)

Specific weight of a gas /specific weight of standard gas (air)

This can also be ratios of densities.

Specific Volume: This is inverse of density v =1/ρ (m³/V)

Matter appears in the form of solids, semi-solids, and fluids.  In this we are dealing with only fluids. As already defined, a fluid is substance which deforms continuously under shear or tangential forces. It is also defined as the substance which flows.

Again the fluids are sub-divided into liquids and gases.  Some liquids flow fast and some liquids flow slowly.  Petrol flows faster than water and water flows faster than lubricant oil. This property which gives resistance to flow is called the viscosity, the internal resistance for flow between the layers.

Liquids are treated as incompressible fluid. It means, when compressive forces applied on liquids, the change or reduction in volume will be negligible and for all practical purposes they can be treated as incompressible.

Gases are compressible. When compressive forces applied, the change in volume is appreciable. Since mass did not change, the density changes when compressed. But when the compressive forces are small and flow velocities are small, gases are also considered as incompressible for easy computation.

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