Viscosity and Newton’s law of Viscous Force
In case of steady flow of a fluid slips or tends to slip on adjacent layers in contact, the two layer exert tangential force on each other which tries to destroy the relative motion between them. The property of a fluid due to which it opposes the relative motion between its different layers is called viscosity
(or fluid friction or internal friction) and the force between the layers opposing the relative motion motion is called visous force.
or F = -nA dv/dx