Rotational stability of an m&m

[ailijic]

I was bored at work and started spinning m&m's, specifically peanut butter m&m's.

If I spin the candy about its minor axis it will rotate about said axis until it reaches a certain velocity and then will rotate and start spinning about its major axis and then as it slows it will rotate back down to the minor axis and come to rest to the logical and most stable orientation.

Why does this happen?
How is it different than a rotating sphere or say a quarter?

 

[Archosaur]

When you spin the m&m, it's going to wobble a bit. In your imagination, keep track of the point of contact between the m&m and the table. When you spin the m&m on its minor axis, a little wobble makes the point of contact trace out a circle. At every point on that circle, there is a force on that part of the m&m that is directed toward the center, which acts as a torque on the m&m. Granted, gravity acting on the center of mass of the m&m is another torque in the opposite direction, but, as you have experienced, that can be overcome if you spin the m&m fast enough. Essentially, the center of contact is pulled to the center, and the m&m is righted. Once it looses enough energy, it will fall back down.

How is this different from a sphere or quarter?
Well, a sphere has no major/minor axis, so this won't happen with a sphere, but it can happen with a quarter! You'd have to spin it really fast, though.