Equations of motion when masses move according to Newton's law of gravity. In general relativity, rotational motion is described by the relativistic angular momentum tensor, including the spin tensor, which enter the equations of motion under covariant derivatives with respect to proper time. The Mathisson–Papapetrou–Dixon equations describe the motion of spinning objects moving in a gravitational field. Unlike the equations of motion for describing particle mechanics, which are systems of coupled ordinary differential equations, the analogous equations governing the dynamics of waves and fields are always partial differential equations, since the waves or fields are functions of space and time. For a particular
Three Laws of Robotics inevitable result of the "Three Laws of Humanics". This story does not fit within the overall sweep of the "Robot" and "Foundation" series; if the George robots "did" take over Earth some time after the story closes, the later stories would be either redundant or impossible. Contradictions of this sort among Asimov's fiction works have led scholars to regard the "Robot" stories as more like "the Scandinavian sagas or the Greek legends" than a unified whole. Indeed, Asimov describes "—That Thou Art Mindful of Him" and "Bicentennial Man" as two opposite, parallel futures for robots that obviate the Three Laws