We present a study on teacher education and, in particular, on Physics. Our assumption is that General Didactics and Disciplinary Didactic constitute two fundamental components of teachers' professional development and therefore both must be used to interpret didactic phenomena adequately. This research involves a sample of 274 prospective teachers, enrolled in the Primary Education Sciences degrees in three Italian Universities (Milano Bicocca, Udine, Urbino). It concerns the study of the spontaneous representations produced by students to foster understanding of the force concept and on the reasons given to motivate their effectiveness on an educational level and disciplinary one. The research tools and methods were developed by crossing the literature of General Didactics and Physics Education, identifying analytical categories that emerge from two different and complementary perspectives. In the broad spectrum of students' conceptions, given by this double perspective, some aspects stand out: most of the representations do not include the representation of the forces at play, but rather imply a precise didactic approach inherent to the concept of force. This is confirmed by the analysis of the didactic and disciplinary motivations, in which a frequent identification of the two types of motivations emerges.