Castor oil at $20^{\circ}\;C$ has a coefficient of viscosity $2.42\;N\,s\, /m^2$ and density $940\; kg/m^3$. Calculate the terminal velocity of steel ball of radius $2\;mm$ falling under gravity in the oil, taking density of steel as $7800\;kg/m^3$.
Castor oil at $20^{\circ}\;C$ has a coefficient of viscosity $2.42\;N\,s\, /m^2$ and density $940\; kg/m^3$. Calculate the terminal velocity of steel ball of radius $2\;mm$ falling under gravity in the oil, taking density of steel as $7800\;kg/m^3$.