The ISAAC program, written in 1975, solves rigid body statics problems stated in English. It can read, understand, solve, and draw a diagram for a textbook physics problem in a fraction of a second. Our more recent research in physics problem solving involves formalizing a problem that is stated informally and then deriving a solution from the formal model. Programs have been written to understand problems stated by a combination of English text and a diagram. Our current research involves representation of the formal models chosen for a problem and selection of appropriate models for an informally stated problem.