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

Here, `r=2 mm=2xx10^(-3) m`,
`v=6.5 cm s^(-1)= 6.5xx10^(-2) ms^(-1)`,
`eta=? , sigma=1.5xx10^(3) kg//m^(3)`
As `v=(2r^(2)(rho-sigma)g)/(9eta)` or `eta=(2r^(2)(rho-eta)g)/(9v)`
`=(2xx(2xx10^(-3))^(2)xx[8.9-1.5]xx10^(3)xx9.8)/(9xx6.5xx10^(-2))`
`0.992` deca poise