**Viscosity: **The property of a fluid by virtue of which an internal friction comes into play when the fluid is in motion and opposes the relative motion of its different layers is called viscosity.

**Newton’s Law of Viscosity, Coefficient of Viscosity**

P + 1/2 . ϱv^{2} = constant Consider a liquid flowing steadily over a fixed solid horizontal surface as shown in the figure. Every layer of liquid moves parallel to the fixed surface. The layer in contact with the fixed surface is at rest while the velocity of other layers increases uniformly upwards. Consider two layers AB and CD moving with velocity v and v + dv at a distance x and x + dx respectively from the fixed solid surface.

The change of velocity divided by the distance, dv/dx, in a direction perpendicular to the velocity is called the velocity gradient.

According to newton, the viscous force F depends upon the following factors:

- It is directly proportional to area A or the layers in contact. i.e; F α A
- It is directly proportional to the velocity gradient between the layers. i.e; F α dv/ dx

^{2}or poise in CGS system and N s m

^{-2}or decapoise for SI system. Dimensional formula of η is [M L

^{-1}T

^{-1}]

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