The differential models seen so far were obtained from wheels that roll along a planar surface. Many generalizations are possible by considering other ways in which bodies can contact each other. In robotics, many interesting differential models arise in the context of manipulation. This section briefly covers some other examples of velocity constraints on the C-space. Once again, dynamics is neglected for now. Such models are sometimes classified as quasi-static because even though motions occur, some aspects of the model treat the bodies as if they were static. Such models are often realistic when moving at slow enough speeds.