Question

A raindrop of radius $0.3mm$ has a terminal velocity of air $1m/s$. The viscosity of air is $18\times {10}^{-3}$ poise. The viscous force on it is

• A. $101.78\times {10}^{-2}$ dyne
• B. $101.37\times {10}^{-5}$ dyne
• C. $16.95\times {10}^{-5}$ dyne
• D. $16.95\times {10}^{-4}$ dyne

Answer 1

The correct option is A $101.78\times {10}^{-2}$ dyne

Radius of drop $r=0.3$ $mm=0.03$ $cm$

Terminal velocity $v_c=1$ $m/s=100$ $cm/s$

Viscosity of air $\eta =18\times {10}^{-3}$ poise

Viscous force $F=6\pi \eta r{v}_c$

$\therefore$ $F=6\pi \times 18\times {10}^{-3}\times 0.03\times 100=1.0178$ $dyne$