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 Sci Advisor HW Helper Thanks P: 26,160 hi sacher! welcome to pf! let's start with the simplest case … a wheel with one cylindrical hole across a diameter, and a rod we can slide along it with the rod horizontal and with the brake on, we slide the bar to the left, then we release the brake … the wheel turns 180°, until the rod is at the right … now we apply the brake again, slide the rod through, release the brake, and so ad infinitem however i] this assumes no friction … in practice, because of friction, the rod won't quite return to horizontal ii] even if we add more rods (at different angles), pushing them out when they're exactly horizontal, the same thing will happen: the wheel will turn to the point where the centre of mass is at the original height (and if we push a rod out when it's above the horizontal, then we're supplying energy by doing so, aren't we? )