The dipole sum rule is used to investigate the effect of nuclear forces on the integrated photonuclear cross section σint of a large nucleus, for a Serber potential containing a hard core. A Jastrow wave function is assumed to describe the nuclear ground state, and the first term in the cluster expansion of the resultant expression for σint evaluated under two different sets of assumptions for the potential and correlation function, corresponding to two different studies of the nuclear matter problem. The presence of a repulsive core in the potential is found to enhance considerably the calculated σint, compared with the result for a potential without a core which fits the same (low-energy) two-nucleon data.