A charged oil (density 880 ~kg ~m^ -3 ) drop is held stationary...

A charged oil (density $880 \mathrm{~kg} \mathrm{~m}^{-3}$ ) drop is held stationary between two parallel horizontal metal plates 6.0 mm apart when a potential difference of $\mathrm{V}=103$ volt is applied between the two plates. When the electric field is switched off, the drop falls. At a certain time the drop is seen to fall a distance of 2.0 mm in 35.7 s and next 1.2 mm in 21.4 s . (The upper plate in the experiment is at higher potential). Given that the viscosity of air $=1.80 \times 10^{-5} \mathrm{Nsm}^{-2}$ and density of air $=1.29 \mathrm{~kg} \mathrm{~m}^{-3}$