Physics > Fluid Mechanics > 1.0 Introduction
Fluid Mechanics
1.0 Introduction
1.1 Ideal liquid
1.2 Density of a liquid $\left( \rho \right)$
1.3 Relative density of a liquid $(RD)$
1.4 Density of a mixture of two or more liquid
1.5 Density variation with temperature
1.6 Density variation with pressure
2.0 Fluid pressure
2.1 Atmospheric pressure
2.2 Pressure variation with depth
2.3 Measurement of pressure
2.4 Pressure difference in accelerating fluids
3.0 Pascal's law
4.0 Buoyant force
5.0 Flow of fluids
6.0 Viscosity
7.0 Stoke's law
8.0 Intermolecular forces
9.0 Angle of contact
1.3 Relative density of a liquid $(RD)$
1.2 Density of a liquid $\left( \rho \right)$
1.3 Relative density of a liquid $(RD)$
1.4 Density of a mixture of two or more liquid
1.5 Density variation with temperature
1.6 Density variation with pressure
2.2 Pressure variation with depth
2.3 Measurement of pressure
2.4 Pressure difference in accelerating fluids
Relative density of a substance is defined as the ratio of its density to the density of water at $4^\circ C$.
It is denoted by $RD$ $${\text{Relative density = }}\frac{{{\text{Density of a liquid}}}}{{{\text{Density of water at 4^\circ C}}}}$$ or $$RD = \frac{{{\rho _S}}}{{{\rho _W}}}$$
For example:
Density of water at $4^\circ C$, ${\rho _W} = 1\;gm/cc$
Density of copper, ${\rho _S} = 8.92\;gm/cc$
So, relative density of copper is given by, $$RD = \frac{{8.92\;gm/cc}}{{1\;gm/cc}} = 8.92$$
Note:
- Relative density is a scalar quantity and it is always positive
- It is a dimensionless quantity
- Relative density is also known as specific gravity
- Relative density is also known as ratio of the weight of body in air and decrease in weight of the body in water $$\begin{equation} \begin{aligned} RD = \frac{{{\text{Weight in air}}}}{{{\text{Weight in water}}}} \\ RD = \frac{{{W_{actual}}}}{{{W_{actual}} - {W_{apparent}}}} \\ RD = \frac{{{\rho _b}{V_g}}}{{{\rho _b}{V_g} - \left( {{\rho _b} - {\rho _W}} \right){V_g}}} \\ RD = \frac{{{\rho _b}}}{{{\rho _W}}} \\\end{aligned} \end{equation} $$
