"green" <rov### [at] gmailcom> wrote:

> interesting reading on this topic
> https://en.wikipedia.org/wiki/Poinsot's_ellipsoid
>

That's new to me-- and indeed it IS interesting. Especially the section titled
"Special Case":

"In the general case of rotation of an unsymmetric body, which has different
values of the moment of inertia about the three principal axes, the rotational
motion can be quite complex unless the body is rotating around a principal axis.
As described in the 'tennis racket' theorem, rotation of an object around its
first or third principal axis is stable, while rotation around its second
principal axis (or intermediate axis) is not."

And,
"One of the applications of Poinsot's construction is in visualizing the
rotation of a spacecraft in orbit." (!) Or more generally, a free-falling
object.

Clipka's detailed comments are beginning to make sense to me now.

I've also come across the concept of 'reduced mass'...
https://en.wikipedia.org/wiki/Reduced_mass
..... mainly the part about "Moment of inertia of two point masses in a line."
That may seem to be tangential to the discussion here, but it helped me.

I already see that my notions of free-fall rotation have been rather simplistic,
especially regarding my attempts to mimic the movements using ONLY the simple
notion of rotate <...>, without taking ANYTHING else into account. There are
many things to consider-- an important one being 'symmetric' objects vs.
non-symmetric ones (relating to the location of their centers of mass-- i.e.,
where they should rotate from.) I guess you could say that my own 'expectation'
of ALL free-fall movement has been that of a spinning tennis racket! (Or,
another example: A gymnist doing a whirling dismount from a cross beam or
'horse'.) But not every object is like that, and a 'generic' set of rotations
may not fit every situation.

Much food for thought. Thanks for the nudge! Also thanks to Clipka and Le
Forgeron for their input; the concepts are 'deep', but I'm slowly catching on
;-)

BTW, I made another animation example to post-- more experiments-- but it looks
rather 'quaint' now, in light of these newer concepts.

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