This paper intends to go on with the study of problem solving in a compatible way with the theories of conceptual fields (TCC) of Vergnaud (1990,1994,1998) and mental models of Johnson-Laird (1983,1990). Together with findings of another study (Escudero & Jaime 2007), some achievements and difficulties of freshmore engineering students when solving problems of the motion of rigid body in terms of the knowledge-in-action are analysed. The research methodology under a qualitative paradigm grouped data into categories which are not provided a priori by the theoretical framework. It can be said that the quality of the conceptual representation has been explicit in the quality of the proposed solution. Some meanings introduced by students in their problem solving activities can be characterized as operational invariants
