•    The tangential stress between two adjoining layers is directly proportional to velocity gradient in a direction perpendicular to the layers. This is known as law of viscosity. It is denoted by ζ.
 
ζ ∝ du / dy

ζ  = μ du / dy

Where μ  = Coefficient of dynamic viscosity

du / dy = Velocity gradient or rate of share strain

For example, a high viscosity liquid requires more power to pump than the fluid having a low viscosity. Therefore, it is use for designing pumping and piping systems.

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