Simulating Both Dynamic and Kinematic Behaviors o f Mechanisms

Humans, including children [7], can describe mechanisms using whatever knowledge and information they have . That is they often switch between dynamic and kinematic descriptions depending on th e information available to them . Furthermore, they seem to perform such reasonings subconsciously . For instance, they can describe how meshed gear trains work by just addressing how rotationa l velocities are transferred into velocities of opposite directions . Sometimes, however, they analyz e the motion propagation starting with the torque source and how the torque brings motion to th e rest of the system . One of the goals in qualitative physics is to achieve a cognitively more plausible account of ho w various physical systems behave so that both commonsense and more complex physical principle s can be captured . Theories in this field, however, tend to draw a clear line between the dynamic and kinematic behaviors of physical devices, thus deviating away from how humans normally describ e mechanisms . In particular, dynamic theories cannot be applied to produce kinematic behaviors . Neither can kinematic theories be applied to deal with dynamic devices . Our key observation is tha t humans use motion propagation as an unified way of accommodating both dynamic and kinematic descriptions .