Chapter 21, Problem 72P

An electron is projected with an initial speed v_l = 3.2 × 10⁵ m/s directly toward a very distant proton that is at rest. Because the proton mass is large relative to the electron mass, assume that the proton remains at rest. By calculating the work done on the electron by the electrostatic force, determine the distance between the two particles when the electron instantaneously has speed 2v_i.