J. Gao and J. H. Weiner in their study of the origin of stress on the atomic level in dense polymer systems (Science 266,748 (1994)), observe that the tensile force required to maintain the length, I, of a long linear chain of N freely jointed links each of length a, can be interpreted as arising from an entropic spring. For such a chain, S(l) = -3kI2/2Na2 + C, where k is the Boltzmann constant and C is a constant. Using thermodynamic relations of this and previous chapters, show that the tensile force obeys Hooke's law, f = -krl, if we assume that the energy U is independent of l.