Question

A raindrop of radius 0.3 mm has a terminal velocity in air $=1 \, m \, s^{- 1}$ . The viscous force on it is (Given the viscosity of the air is $18 \, \times 10^{- 5} \, g \, cm^{- 1} \, s^{- 1} \, $ )

• A. $101.73\times 10^{- 4} \, dyne$
• 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)

$F=6 \, \pi \, \eta \, r \, v$
$=6\times 3.14\times \left(\right.18\times \left(10\right)^{- 5}\left.\right)\times 0.03\times 100$
$=101.73\times 10^{- 4} \, dyne$