Consider a $\mathrm{U}$-shaped frame with a sliding wire of length $l$ and mass $m$ on its arm. It is dipped in a soap solution that taken out and placed in vertical position as shown in the figure. Choose a minimum value of $m$, so that the wire does not descend (surface tension of soap solution is $S$):

An oil drop falls through the air with a terminal velocity of $5 \times {10}^{–4} \mathrm{m} {\mathrm{s}}^{-1}$. The radius of the drop will be: $\eta =3.6\times {10}^{-5 }\mathrm{N} \mathrm{s} {\mathrm{m}}^{-2}, {\mathrm{\rho }}_{\mathrm{oil}}=900 \mathrm{kg} {\mathrm{m}}^{-3}$.,$g=10 \mathrm{m} {\mathrm{s}}^{-2}$

Neglect the density of air as compared to oil.

A spherical ball is dropped in a long column of viscous liquid. Which of the following graphs represent the variation of

(i) gravitational force with time.

(ii) viscous force with time.

(iii) net force acting on the ball with time.