A computational model of musical dynamics is proposed that complements an earlier model of expressive timing. The model, implemented in the artificial intelligence language LISP, is based on the observation that a musical phrase is often indicated by a crescendo/decrescendo shape. The functional form of this shape is derived by making two main assumptions. First, that musical dynamics and tempo are coupled, that is, ‘‘the faster the louder, the slower the softer.’’ This tempo/dynamics coupling, it is suggested, may be a characteristic of some classical and romantic styles perhaps exemplified by performances of Chopin. Second, that the tempo change is governed by analogy to physical movement. The allusion of musical expression to physical motion is further extended by the introduction of the concepts of energy and mass. The utility of the model, in addition to giving an insight into the nature of musical expression, is that it provides a basis for a method of performance style analysis.