# Properties of Fluids

The physical properties of fluids are as follows:

1.3.1 Mass Density or Density

Mass density or simply density of a fluid is defined as the mass per unit volume. It is denoted as p (rho). If 6.m and Si are mass and volume respectively of a fluid element enclosing a point P, then p is mathematically written as

Fig. 1.1 shows a fluid element enclosing point P.

S.I. (Standard International) unit of p is given by

P water = 1000 kg / m3

1.3.2 Specific Weight or Weight Density

The specific weight or weight density of a fluid is defined as the weight of the fluid per unit volume. It is denoted as w. It is mathematically expressed as

w  = gp or w = pg

where                          dW = weight of fluid element.

= gdm.

W water = 9.81 x 1000 N/m3

The volume of a fluid per unit mass is called specific volume. It is denoted as v. It is mathematically expressed as

u = 1 /p

In S.I unit it is m3 / kg. We encounter v frequently in case of gases only.

1.3.4 Specific Gravity

The specific gravity of a liquid is defined as the ratio of weight density or density of the liquid to the weight density or density of pure water at pressure 101 KN/m2 and temperature at 4°C. It is denoted as S. It is mathematically expressed as

S = w liquid / w std.water = w liduid / 9.81 x 1000 N/m3

or                                 w liquid = S x 9.81 x 1000 N/m2

S can also be expressed as

S = p liquid / p std.water = p liduid /  1000 kg/m3

or                                 p liquid = S x 1000 kg/m2