$A$ rain drop of diameter $1 \ mm$ falls with a terminal velocity of $0.7 \ ms^{-1}$ in air. If the coefficient of viscosity of air is $2 \times 10^{-5} \ Pa \cdot s$,the viscous force on the rain drop is

$A$ lead sphere of mass $m$ falls with a terminal velocity $V$ in a viscous liquid. With what terminal velocity will another lead sphere of mass $8m$ fall in the same liquid?

What is the terminal velocity of a rain drop of radius $0.02 \ mm$ (in $cm \ s^{-1}$)? [Note that the coefficient of viscosity of air is $1.8 \times 10^{-5} \ N \ s \ m^{-2}$,density of water is $1000 \ kg \ m^{-3}$. Use $g = 10 \ m \ s^{-2}$ and density of air can be neglected in comparison with the density of water.]

The terminal velocity of a metallic ball of radius $6 \ mm$ in a viscous fluid is $20 \ cm/s$. The terminal velocity of another ball of same material and having radius $3 \ mm$ in the same fluid will be . . . . . . $cm/s$.

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.

Consider two solid spheres $P$ and $Q$ each of density $8 \ g \ cm^{-3}$ and diameters $1 \ cm$ and $0.5 \ cm$,respectively. Sphere $P$ is dropped into a liquid of density $0.8 \ g \ cm^{-3}$ and viscosity $\eta = 3 \ \text{poiseuille}$. Sphere $Q$ is dropped into a liquid of density $1.6 \ g \ cm^{-3}$ and viscosity $\eta = 2 \ \text{poiseuille}$. The ratio of the terminal velocities of $P$ and $Q$ is: