Why do rain drops not possess a velocity greater than a certain limit? Explain.

$A$ viscous fluid is flowing through a cylindrical tube. The velocity distribution of the fluid is best represented by the diagram

An object falling through a fluid is observed to have acceleration given by $a = g - bv$,where $g$ is the gravitational acceleration and $b$ is a constant. After a long time of release,it is observed to fall with a constant speed. What must be the value of this constant speed?

Two rain drops falling through air have radii in the ratio $1: 2$. They will have terminal velocity in the ratio

Why do dust particles settle down on the floor in a closed room? Explain.

$A$ gas bubble of $2 \ cm$ diameter rises through a liquid of density $1.75 \ g \ cm^{-3}$ with a fixed speed of $0.35 \ cm \ s^{-1}$. Neglect the density of the gas. The coefficient of viscosity of the liquid is (in $\text{poise}$)