feedback on... perpetual motion

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MOTHER's readers shouldn't invest workshop time or material resources on the "gravity perpetual motion machine" suggested by Rainbow Atma in MOTHER NO. 33. This device, sketched on page 144 of that issue, is a sort of hollow wheel divided into pie-wedge compartments, each holding a ball weight which is free to roll toward the center or the rim. As Rainbow said, the idea is an old one . . . and it doesn't really work.

The trouble with this and all similar "unbalanced wheel" perpetual motion machines is that they eventually run down because of bearing friction at the hub, air friction on the wheel itself, and other losses. (To explain those last factors would require my writing a complete lecture on energy physics, which might get a bit tedious for all concerned.) The same thing happens in all machines, and there's no magical reason why this particular design should be exempt.

"Working" models of the unbalanced wheel have been built, and they appear to operate all right . . . provided that the observer doesn't hang around long enough to see them run down, or doesn't make a series of strobe-light tachometer readings that would detect the steady loss of rpm. The fact is that friction always catches up in the end.

The "perpetual motion" wheel spins because of the impetus given it by the person or device that set it in motion. Like any other well-balanced rotor on good bearings (wheel, motor armature, gyroscope rotor, whatever), it can go on spinning a heck of a long time on its inertia. Even an antique foot-powered grindstone, with its primitive bearings and its many friction-prone linkages, takes a good while to quit once you have it up to any speed at all.

"Aha!" say fans of this and similar designs. "You just called the perpetual motion wheel a 'well-balanced rotor' . . . but the whole point of the gadget is that it's an unbalanced rotor. There's more 'weight' on one side than the other, and therefore gravity will keep pulling harder on that side, and therefore the machine will continue to run. Right?"

Not so. Each ball in this machine exerts what engineers and physicists call a "moment of force". If you regard all moments of force on the "downward" side of the wheel as "positive", all the moments on the "upward" side are "negative". The negative moments of force, added up, equal the sum of the positive moments. The result is zero.

It follows that the situation is the same as with an ordinary roller-skate wheel you've set spinning with a twirl of your finger. All the moments of force in that rotor add up to zero, too . . . that is, if you have a nicely balanced wheel. (One in poor adjustment vibrates itself to a stop rather quickly.)

Now then. For fun, let's suppose that we somehow managed to maintain a slight dynamic imbalance in the perpetual motion wheel . . . so that gravity affected the machine's sides differently and supplied just enough energy to overcome all the frictional forces. Then the wheel should keep spinning "forever", no? Yes. Of course. In fact, we could expect it to start rotating by itself if we so much as breathed on it. But what good would it be?

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