The dynamics of a single soliton in a ring-shaped Josephson junction is studied experimentally. An externally applied magnetic field H forms a sinusoidal potential relief for the soliton in the ring. The effects of soliton pinning and trapping by the potential are observed and measured as a function of the potential height and losses in the junction. An analogy between the soliton dynamics and motion of a particle in a wash-board potential is clearly demonstrated. The experimentally found trapping threshold for small H fairly well agrees with the theoretical prediction using the dissipation-based power balance. With increasing H, the observed large discrepancy between theory and experiment is attributed to additional soliton energy losses due to radiation.