$A$ small ball of mass $m$ and density $\rho$ is dropped in a viscous liquid of density $\rho_0$. After some time,the ball falls with constant velocity. The viscous force on the ball is:

Two spheres of equal masses but radii $r_1$ and $r_2$ are allowed to fall in a liquid of infinite column. The ratio of their terminal velocities is

In Millikan's oil drop experiment,what is the terminal speed of an uncharged drop of radius $2.0 \times 10^{-5} \; m$ and density $1.2 \times 10^{3} \; kg \; m^{-3}$? Take the viscosity of air at the temperature of the experiment to be $1.8 \times 10^{-5} \; Pa \; s$. How much is the viscous force on the drop at that speed? Neglect buoyancy of the drop due to air.

$A$ metal ball of radius $9 \times 10^{-4} \ m$ and density $10^4 \ kg/m^3$ falls freely under gravity through a distance $h$ and enters a tank of water. Considering that the metal ball has constant velocity in water,the value of $h$ is [coefficient of viscosity of water $= 8.1 \times 10^{-4} \ Pa \cdot s, g = 10 \ m/s^2$,density of water $= 10^3 \ kg/m^3$]. (in $m$)

In Millikan's oil drop experiment, a charged drop falls with terminal velocity $V$. If an electric field $E$ is applied in a vertically upward direction, then it starts moving in an upward direction with terminal velocity $2V$. If the magnitude of the electric field is decreased to $E/2$, then the terminal velocity will become

Two rain drops reach the earth with different terminal velocities having ratio $9:4$. Then the ratio of their volumes is