Question

A rain drop falls near the surface of the Earth with almost uniform velocity because

• A. Its weight is negligible
• B. The force of surface tension balances its weight
• C. The force of viscosity of air balances its weight
• D. The drops are charged and atmospheric electric field balances its weight

Answer 1

The correct option is C

The force of viscosity of air balances its weight

Various forces acting on a drop during its descent are

(i)Its weight$\omega$, downwards

(ii)Buoyant force by air, upwards

(iii)Viscous force by air, upwards

Now when drop is falling with uniform velocity,

Forces are balanced [acceleration = 0]

$\Rightarrow \omega =F_B+F_v$
But due to very low density of air and very small size of the drop
$FB\simeq 0$
$\Rightarrow \omega =F_v$
Hence (c)