1. The problem statement, all variables and given/known data Pretend two links with mass are connected together by a massless motor that provides a torque between the links. The whole thing is just floating in deep space. The motion of each link depends on the other one being connected (if one link were removed, the motor rotor or housing would just rotate and the remaining link wouldn't move). So it'd seem that torque is a relative quantity with a frame assigned to it, and a base that it has to act against. But torque (unlike a moment) has no origin, so isn't the situation above (the motor connecting the two links) the same as two non-rotatable motors floating in the lab frame, and driving each free of the the two links that are now connected by a pin joint?