I can't figure this one out, but I bet someone here can really quickly. I hope I can explain it right. In 2D, if a shaft mounted wheel is connected to a weightless arm and at the end of the arm is a mass then the wheel will rotate because of gravity until the mass loses it's energy. Now lets say we put 8 of these arms around the same wheel and we are given full control over the length of each arm at any given time (dynamically). Then, decrease the length of those on the right side and increase the length of those on the left side so that we still get a torque on the system. If would seem to be possible to change the lengths so that the potential energy would remain constant, yet we would get a net torque. (By having symmetrical lengths across the x axis, but non-symmetrical lengths across the y-axis) Since we have full control over the lengths, couldn't the lengths change accordingly as the wheel is rotating as to keep the net torque on the left side of the wheel causing accelerated rotation?