A water drop of radius $1\mathrm{\mu m}$ falls in a situation where the effect of buoyant force is negligible. Coefficient of viscosity of air is $1.8\times {10}^{-5}{ \mathrm{Nsm}}^{-2}$ and its density is negligible as compared to that of water $\left(i.e, {10}^6 {\mathrm{gm}}^{-3}\right)$. Terminal velocity of the water drop is:  (take acceleration due to gravity =10ms^-2 )