Question

Choose the correct statement from the following.
A. Terminal velocities of rain drops are proportional to square of their radii
B. Water proof agents decrease the angle of contact between water and fibres
C. Detergents increase the surface tension of water.
D. Hydraulic machines work on the principle of Torricelli law.

Answer 1

Terminal velocities of rain drops are proportional to square of their radii;

- CORRECT STATEMENT

Reason:

The general expression for terminal velocity of raindrops is as follows :

$\boxed{ \sf{v_{t} = \dfrac{2 {r}^{2}( \sigma - \rho)g }{9 \eta} }}$

So, for constant density of medium , density of object and coefficient of viscosity , we can say that :

$\boxed{ \sf{v_{t} \propto \: {r}^{2}} }$

Other statements are incorrect as:

• Water - proof agents increases the angle of contact from acute to obtuse.

• Detergents decrease the surface tension between water and fibres.

• Hydraulic machines work on Pascal's Law.