**What is viscosity?**

Defined as the resistance of fluids to change of shape, being due to the internal molecular friction of molecule with molecule of the fluid producing the frictional drag effect.

**Dynamic Viscosity**

>Absolute (Dynamic) viscosity is numerically equal to the force to shear a plane of fluid surface of area of a unit square meter, over another plane surface at the rate of one meter per sec, when the distance between the surfaces is one meter.

>Or it is the tangential force per unit area required to move one horizontal plane with respect to the other at

unit velocity when maintained a unit distance apart by the fluid.

>The SI unit of Dynamic viscosity is N s/m2> Dynamic viscosity formula

*τ = μ dc/dy*

*where*

*τ = shearing stress*

*μ = dynamic viscosity*

**Kinematic viscosity**

>Kinematic viscosity is the ratio of the absolute viscosity to the density at the temperature of viscosity measurement.

>The SI unit of kinematic viscosity is m

^{2}/s.

>The cgs physical unit for kinematic viscosity is the

*stokes*

>Water at 20 °C has a kinematic viscosity of about 1 cSt

>Kinematic viscosity formula

*ν = μ / ρ*

*where*

*ν = kinematic viscosity*

*μ = absolute or dynamic viscosity*

*ρ = density*

**Viscosity index**

Viscosity index is a numerical value which measures the ability of the oil to resist the change in viscosity with the change in temperature. A high viscosity index shows a good resistance to the change in viscosity with change in temperature.