Question

A rain drop of radius 0.4 mm falls through air with a terminal velocity of `50 cm s^(-1)` . The viscosity of air is 0.019 Pa-s`. Find the viscous force on the rain drop.

Answer 1

Here,
`r=0.4 mm = 0.4 xx 10^(-3)m=4 xx 10^(-4)m`,
`upsilon = 50 cm s^(-1) = 0.50 ms^(-1) , eta = 0.019 Pa-s`
`F= 6 pi eta r upsilon`
` = 6 xx 22/7 xx (0.019) xx (4 xx 10^(-4)) xx (0.50)`
`= 7.17 xx 10^(-5) N`.