Three important findings of research on expertise in formal sciences such as physics are that novices and experts differ with respect to (1) how they structure their domain knowledge, (2) how they mentally represent problems and (3) how they approach problems. Though first attempts have been made to account for the acquisition of knowledge structures as possessed by experts, the reconstruction of the involved learning mechanisms in a psychologically plausible and instructionally fruitful way still remains a challenge. In an experimental study, we investigated how tenth graders acquire and successively relate qualitative and quantitative problem representations in classical mechanics. Initially, subjects were taught either qualitative or quantitative aspects of classical mechanics. Afterwards, two subjects, who were taught differently, collaborated on problems which were beyond the competence of each of them separately. Before and after the collaboration subjects had to work on multi-component tests. In addition, protocols were taken of the subjects' verbal exchange of information during collaborative problem solving. An analysis of variance of the multi-component tests revealed that the subjects successfully learned to interrelate qualitative and quantitative problem representations. A protocol analysis further indicated that subjects gradually shifted their focus from quantitative problem representations to qualitative problem representations during collaborative problem solving.