Very often, efficient problem solving depends on the ability to construct, to shift between, and to coordinate different mental problem representations. The cognitive simulation program Sepia has been developed to investigate the complementary roles qualitative and quantitative mental domain representations play in physics problem solving. It reconstructs characteristic differences in the problem solving behavior of those subjects who coordinate their qualitative and quantitative physics knowledge and those subjects who do not. Various model-based measures to supplement traditional instructional techniques are set forth. The results of an empirical study underline the importance a coordinated use of knowledge as modeled by Sepia plays in physics problem solving.