Question

A rain drop of radius $0.3\, mm$ has a terminal velocity in air is $1\, m/s$. The viscosity of air is $8 \times 10^{-5}$ poise. The viscous force on it is

• A. $45.2 \times 10^{-4}$ dyne
• B. $101.73 \times 10^{-5}$ dyne
• C. $16.95 \times 10^{-4}$ dyne
• D. $16.95 \times 10^{-5}$ dyne

Answer 1

Answer: (A)

Radius of drop, $r = 0.3\, mm = 0.03\, cm$
Terminal velocity, $v = 1 \,m/s = 100\, cm/sec$
Viscosity of air, $\eta=8\times10^{-5}$ poise
Viscous force, $f = 6\,\pi\eta rv$
$F=6\times3.14\times\left(8\times10^{-5}\right)\times0.03\times100=4.42\times10^{-3}$ dyne