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I have a Mendocino motor, a toy with magnets, solar panels, and coils. Such a thing requires careful balancing (eg. with tiny bits of tape) to achieve sufficient balance that it will start rotating just from normal light (as opposed to the direct beam of a flashlight).

I cannot balance the rotor so that it will rest in any position (I don't think that's reasonably possible outside of a lab), but it is fairly close -- in fact, it will come to rest in two opposing positions.

My request is this: Please explain to me the physics and/or geometry of why it's possible to have two balance positions? Assume that, outside of sufficient light, there should be a negligible magnetic effect between the coils and the center magnet.

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  • $\begingroup$ I don't think the 'negligible magnetic effect' assumption is safe: It might be correct, but it might well not be. Is it possible to remove the magnet and see what happens? $\endgroup$ – tfb Dec 18 '18 at 10:47
  • $\begingroup$ I'm not a physicist, so I can make bad assumptions. ;-) But no, the magnet is firmly glued into an acrylic plate (see pics at the wikipedia link, those are mine). $\endgroup$ – KlaymenDK Dec 18 '18 at 10:49
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    $\begingroup$ OK. I am not sure if something like this without being mucked around with by a magnet can have two stable positions: I am sure someone who is better at classical mechanics than me can answer that though. One possibility is that the bearings (magnetic bearings) are 'notchy' meaning they have positions they like to sit in. This is often the case for ball races & other contact bearings, but I don't have a good intuition about magnetic bearings. Sorry not to have any reasonable answer. $\endgroup$ – tfb Dec 18 '18 at 10:53

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