Spinning rubber ball with equatorial ridge I have this rubber ball with something like a very slight equatorial ridge (sort of like Saturn's moon Iapetus) which I often spin around on my desk. I keep noticing that no matter the inclination of this ridge, whenever I spin the ball around, the ridge (or rather the plane defined by this ridge) always ends up perpendicular to the ground. Is this a coincidence or is there actually a reason for this happening?
 A: I suspect this is an example of the spinning-egg problem, in which a prolate spheroid (such as an egg) spun on a table about one of its "short" axes will tend to "stand up" so that it's spinning about its long axis.  A few explanations have been proposed for this phenomenon, most notably:


*

*H. K. Moffatt & Y. Shimomura, "Spinning eggs — a paradox resolved".  Nature 416, pp. 385–6 (2002).

*K. Sasaki, "Spinning eggs—which end will rise?".  Am. J. Phys. 72, 775 (2004).


Roughly speaking, the frictional force between the table and the egg robs the egg of its energy;  the minimum-energy state turns out to the state in which the object spins about a its longest principal axis.  This has the effect of increasing the egg's potential energy is higher (since its center of mass is higher), but its kinetic energy is lower since it's now spinning about an axis with a lower moment of inertia.  The frictional force is essential for this to occur;  otherwise, the egg doesn't rise.
The only question to this interpretation is that it's not clear from the literature whether these results apply only to objects which are axisymmetric about their principal axis with the lowest moment of inertia.  Eggs do satisfy this property, but your ball does not.  Still, it seems like it might be a plausible explanation.
