Neat Piece on Perpetual Motion

Many readers that have seen bogus examples of perpetual motion machines may recall the device shown at left.

In the video linked above is something that looks similar; masses (of children) slide back and forth, closer to and farther away from the center point of rotation, and the device keeps spinning.

The difference between the two is that the kids shown in this remarkable piece are putting in work against the force of gravity so as to maintain a constant torque.