(III) Two positive charges are affixed rigidly to the x axis, one at and the other at A third charge of mass m, which is constrained to move only along the x axis, is displaced from the origin by a small distance and then released from rest. (a) Show that (to a good approxima- tion) will execute simple harmonic motion and deter- mine an expression for its oscillation period T. (b) If these three charges are each singly ionized sodium atoms at the equilibrium spacing typical of the atomic spacing in a solid, find T in picoseconds.