FLOW resistance of simple fluids.Newton’s law of viscosity defines the shear stress between adjacent fluid layers as proportional to the velocity gradients between the two layers.The ratio of shear stress to shear RATE is a constant, for a given temperature and pressure, and is defined as the viscosity or COEFFICIENT of viscosity.Shear stress between two layers of fluid is directly proportional to the rate of CHANGE of velocity with respect to perpendicular distance from the fixed point (Velocity Gradient or strain rate or rate of deformation)\({\rm{\tau \;\alpha \;}}\frac{{{\rm{du}}}}{{{\rm{dy}}}}\)\({\rm{\tau }} = {\rm{\mu \;}}\frac{{{\rm{du}}}}{{{\rm{dy}}}}\)\({\rm{\mu }} = \frac{{\rm{\tau }}}{{\frac{{{\rm{du}}}}{{{\rm{dy}}}}}}{\rm{\;}}\)where, τ – Shear stress and \(\frac{{{\rm{du}}}}{{{\rm{dy}}}}\) – Velocity gradient or strain rate.