The attraction of a spherical shell (idealized as a spherical surface of radius R) of mass M per unit area on a unit particle inside is 0. The attraction of the shell on a unit particle outside is the same as if the mass were concentrated at the center. If the unit particle outside is r units from the center, the force of attraction is 4πR²M / r². If the unit particle is just on the shell, the attractive force of the shell on the particle can be shown to be 2πM. Graph the force as a function of the distance r of the unit particle from the center of the shell for r ≥ 0.