Two atoms will interact via electronical forces between their protons and electrons. One fairly good approximation to the electrical energy is the Lennard-Jones formula U(r)=k[(a/r)^12 - 2(a/r)^6] where r is the center-to-center distance between the atom and k is a positive constant. a) plot U vs r. b) determine the force that goes with this potential. c) show that the equilibrium separation is a. d) what is the effective spring constant about the point r=a?