Cognitive Scientists have recently developed models of physicists' problem solving behavior. Their models propose a rich set of cognitive constructs including procedures (Heller and Reif, 1984), problem-solving schemata (Larkin 1983), categorization rules (Chi,Feltovich & Glaser, 1981), phenomenological primitives(diSessa 1983), forward and backward chaining (Larkin,McDermott, Simon, & Simon, 1980), and qualitative reasoning (deKleer, 1975, Forbus 1986, deKleer and Brown, 1986, and others in Bobrow, ed. 1986). These constructs have proved useful in understanding aspects of physics reasoning.This paper udll provide an analysis of physics problem solving skill that integrates cognitive constructs previously considered disparate. The main point is this:Commonsense reasoning about situations provides an indispensable resource for coping with physics problem solving complexity. More precisely, I will argue that the systematic integration of the deep structure of situational and theoretical knowledge can reproduce competent physics cognition. To support this claim I will discuss the capabilities of running computer programs, written in Prolog, that implement several representations and reasoning processes. In addition, I will show how the Prolog models capture the essence of a think-aloud protocol of a physicist recovering from an error while working a novel problem.