Mental models help us understand the formation of objects, the development of the process and to state an opinion about it. Therefore, in this study, we understand a mental model as 'an internal representation, which acts out as a structural analogue of situations or processes. Although mental models are the representations of knowledge created during a cognitive activity, they are also reflective of personal beliefs. This study intends to exhibit prospective teachers' mental models for the arguments they use in describing the relations between the concepts of force and velocity. In line with this intention, the study employs phenomenographic study method. This study has found that prospective teachers have three different mental models for relating the concepts of force and velocity. One of them is a scientific model called "Newtonian Model". The other two models were determined to be unscientific models and called "Aristotelian Model" and "Impulsive Model" respectively. By bringing these models into light, we were informed of the scientific and unscientific arguments the prospective teachers used in interpreting the relationships between force and velocity. In other words, this study exhibited how prospective teachers organized and structured their knowledge about these concepts.