> You could replace these weights with a single weight that slides along the radial bar toward and away from the center axis. I don't think this is the same as

> You could replace these weights with a single weight that slides along the radial bar toward and away from the center axis. I don't think this is the same as what's in the paper, as the axial position of the vertical motors appears to be symmetric relative to the angular position of the horizontal motors. EDIT: Scratch that, just watched the video -- I misunderstood how the vertical motors were moving. They are indeed changing the moment of inertia. You're right -- I don't understand why the authors need to resort to Coulomb forces to explain this behavior. (Aside -- you can use the above phenomenon to rotate in a swivel chair without relying on external forces. Start with legs and arms outstretched. Then: arms left, legs in, arms right, legs out, repeat.)