40. The potential energy function for a system of particles is given by U(x) = -x3 + 2x2 + 3x, where x is the position of one particle in the system. (a) Determine the force F on the particle as a function of x. (b) For what values of x is the force equal to zero? (c) Plot U(x) versus x and F versus x and indicate points of stable and unstable equilibrium