A rain drop of radius 0.3 mm falls through air with a terminal velocity of 1 ms^-1. The viscosity of air is $18\times {10}^{-5}$ poise. The viscous force on the rain drop is

Here, $\mathrm{r}=0.3\mathrm{mm}=0.03\mathrm{cm}\text{, }$
$v=1{\mathrm{ms}}^{-1}=100{\mathrm{cms}}^{-1},\eta =18\times {10}^{-5}\text{ poise }$
According to stokes law, force of viscosity on rain drop is $\mathrm{F}=6\pi \eta rv$
$\begin{array}{l}=6\times 3.142\times 18\times {10}^{-5}\times 0.03\times 100\text{ dyne }\\ =1.018\times {10}^{-2}\text{ dyne }\end{array}$