A water drop of radius \( 1 \, \mu \text{m} \) falls in a situation where the effect of buoyant force is negligible. Co-efficient of viscosity of air is \( 1.8 \times 10^{-5} \, \text{N s m}^{-2} \) and its density is negligible as compared to that of water \( 10^{6} \, \text{g m}^{-3} \). Terminal velocity of the water drop is: (Take acceleration due to gravity \( = 10 \, \text{m s}^{-2} \))