The relative density (or specific gravity) is the ratio of a fluid density to the density of a standard reference fluid maintained
at the same temperature and pressure:
kg / m 3
1000 kg / m 3
Viscosity is a measure of a fluid’s resistance to flow. The viscosity of a liquid is related to the ease with which the molecules
can move with respect to one another. Thus the viscosity of a liquid depends on the:
Strength of attractive forces between molecules, which depend on their composition, size, and shape.
The kinetic energy of the molecules, which depend on the temperature.
Viscosity is not a strong function of pressure; hence the effects of pressure on viscosity can be neglected. However, viscosity
depends greatly on temperature. For liquids, the viscosity decreases with temperature, whereas for gases, the viscosity
increases with temperature. For example, crude oil is often heated to a higher temperature to reduce the viscosity for
Consider the situation below, where the top plate is moved by a force F moving at a constant rate of V (m/s).
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The shear stress τ is given by:
τ = F/A
The rate of deformation dv (or the magnitude of the velocity component) will increase with distance above the fixed
τ = constant x (dv / dy)