A freely falling spherical rain drop gathers moisture (maintaining its spherical shape all the way) from the atmosphere at a rate d m d t =k t^2 where t is the time and m is the instantaneous mass of the drop, the constant k=12 g m / s ^3 . If the drop, of initial mass m₀=2 g m , starts falling from rest, the instantaneous velocity of the drop exactly after 5 second shall be (ignore air friction and air buoyancy)