# Kinematic And Dynamic Viscosity

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 m2/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.