Density or mass density, Weight density or specific weight, specific volume and specific gravity are important terms used in Hydraulics and Fluid Mechanics :

** I. Density or mass density.** It is defined as the mass per unit volume of a liquid at a standard temperature and pressure. It is usually denoted by p (rho). It is expressed in kg/m3. Mathematically, density or mass density,

where

m = Mass of the liquid, and

V = Volume of the liquid.

** 2. Weight density or specific weight.** It is defined as the weight per unit volume of a liquid at a standard temperature and pressure. It is usually denoted by w. It is expressed in kN/m3 or N/m3 or N/mm3. Mathematically, weight density or specific weight,

**Note :**** **For water, w = 9.81 kN / m3 = 9.81 x 103 N / m3 = 9.81 x 10-6 N / mm3.

** 3. Specific volume.** It is defined as the volume per unit mass of the liquid. It is denoted by v. Mathematically, specific volume,

** 4. Specific gravity.** It is defined as the ratio of specific weight of a liquid to the specific weight of pure water at a standard temperature (4°C). It has no units.

__Note:__** **The specific gravity of pure water is taken as unity.

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