A Nate on Dynamics and Kinematics*

At the Fourth International Conference on Event Perception and Action in Trieste this past summer. Karl Newell. Bruce Kay. and I had a brief discussion in which we agreed upon a need for clarification of the meaning and use of the term "dynamic." The concern was that there may be some confusion because "dynamic" is used in nontechnical parlance to denote activity or change. In this usage. "dynamic" connotes motion. However. the technical use of the term in mechanics contrasts "dynamic" with "kinematic." where the latter term connotes motion and the former is used to signify physical constraints responsible for particular forms of motion. If technical usage is mistaken for the informal expression. then the significance of the contrast between dynamic and kinematic is lost. Kinematics and dynamics have become a common and important part of theoretical and experimental explorations reported in the psychological literature. Related work in perception and cognition includes studies in naive physics as well as studies on the kinematic specification of dynamics and. more generally. on event perception. Included under this heading is a growing body of work in social psychology on person perception. expectation. gender and age perception. and the perception of emotion. Also included are studies in speech and sign language perception. Related work in action includes studies on control and coordination as well as on skill acquisition and motor memory. Despite its importance to problems in human action and perception. the relation between kinematics and dynamics as descriptive systems has never been very thoroughly explored or explicated in the collective literature. On occasion. brief reference has been made to standard mechanics textbook classifications. However. given the focal importance of this relation to our developing understanding of action and perception. a more careful examination is to be desired. Ideally. the classification should reflect the structure of dynamics. Kinematics and dynamics refer to descriptive systems that are used to describe alternative. but related aspects of events. Dynamical deSCription is not independent of kinematic description. A standard classification provided in the textbooks and in dictionaries is as folloWS. (For the follOWing discussion. see Becker. 1954; Halliday & Resnick. 1981; Mach. 1893/1960; Maxwell. 1877/1920; Merriam-Webster. 1949; Sommerfield. 1942; Stein. 1981; Tuma. 1974a. 1974b; and Whittaker. 1904/1959.) ClaSSical mechanics is divided into Statics and Dynamics. Statics treats the stability of force structures in which there are no changes in position over time. that is. no motion. Dynamics is used to describe the behavior of force structures in which there is change of position over time. Dynamics is subdivided into kinematics and kinetics where kinematics is