viscosity SI unit pascal second (Pa.s) or N.s.m^-2. In the CGS system its unit is poise. The frictional force within a fluid is known as viscosity. The frictional forces within the liquid act between the layers and resist the flow of liquid. The flow is said to be streamline or laminar if the successive particles while passing a given point have the same velocity. It is turbulent if the velocity at a given point varies rapidly.

The tangential force between two parallel layers of area A, dy distance apart, moving with relative velocity dv in a liquid in streamline flow is,

F= h A[dv/dy]

where h is the coefficient of viscosity (or just the viscosity) of the liquid. Liquid for which equation (v3) is applicable is called Newtonian fluid. See also Stokes law.