Starting from established concepts that describe the dynamics of the cantilever in a scanning force microscope operated in intermittent contact mode as that of a driven harmonic oscillator. we introduce effective interaction parameters based on Fourier coefficients to describe the interaction forces. These interaction parameters are linear in the forces and separate conservative from dissipative contributions. They allow a qualitative description of the interaction process and enable a thorough discussion of the influence of experimental parameters on the measured data. Exemplary force spectroscopy data obtained from hard and soft polymeric model surfaces show that consistent data for the interaction parameters can be obtained which can be related to the material properties of the investigated surfaces. Images obtained in intermittent contact mode from a semicrystalline polymer as an exemplary nanostructured surface with hard-soft contrast illustrate that with the approach a clear identification of harder and softer domains is possible.