Walker, Halliday & Resnick — Electric Potential: Problem 10

10 A thin, spherical, conducting shell of radius \(R\) is mounted on an isolating support and charged to a potential of \(-170 \text{ V}\). An electron is then fired directly toward the center of the shell, from point \(P\) at distance \(r\) from the center of the shell (\(r \gg R\)). What initial speed \(v_0\) is needed for the electron to just reach the shell before reversing direction?