The interaction between a magnet and a high T_c superconductor (HTSC) is investigated by a novel method. A permanent magnet mounted at one end of a stiff rod hanging by a knife's edge forms a mechanical pendulum. When a HTSC is placed below the magnet the frequency v and damping factor Gamma will be changed due to the interaction between the magnet and the HTSC. The experiments show that, as the separation between the magnet and the HTSC decreases, the flux lines start to penetrate the sample via intergranules and grain boundaries, which results in a horizontal restoring force even for the field range B<<B_c1. The present experimental results are consistent with the theoretical model of weak links for bulk HTSC. The power-law-like potential V_eff, and force, F_eff, are introduced to describe the interaction. The behaviour of V_eff and F_eff obtained from the present measurements is in disagreement with the dipole-image model even for the field range B<<B_c1.