The terminal velocity of a copper ball of radius `2mm` falling through a tank of oil at `20^(@)C` is `6.5cm//s`. Find the viscosity of the oil at `20^(@)C`. Density of oil is `1.5xx10^(3)Kg//m^(3)`, density of copper is `8.9xx10^(3)Kg//m^(3)`.

Explanation

We have `v_(t)=6.5xx10^(-2)ms^(-1),a=2xx10^(-3)m`.
`g=9.8ms^(-2),rho=8.9xx10^(3)kgm^(-3)`
`sigma=1.5xx10^(3)kgm^(-3)`. From Eq.
`eta=(2)/(9)xx((2xx10^(-3))^(2)m^(2)xx9.8ms^(-2))/(6.5xx10^(-2)ms^(-1))xx7.4xx10^(3)kgm^(-3)`
`=9.9xx10^(-1)kgm^(-1)s^(-1)`