The Fourier transform procedure is used to determine the interaction potential experienced by a molecule adsorbed on a surface with a monoatomic step. The step is modeled as a perfect surface with a reduced set of lines or antilines describing the upper or lower terraces. The model is nearly analytic and, when additional terms issued from asymptotic approximations are considered, it appears to be fairly accurate. The behavior of the potential valleys close to the step is analyzed for ideal situations (Lennard-Jones potential, electric dipole, or quadrupole contributions), and it compares satisfactorily to exact calculations. This analysis allows us to understand the role of each interaction contribution in general situations of physisorption at a surface step.