clipka <ano### [at] anonymousorg> wrote:
> Am 03.10.2018 um 02:34 schrieb Kenneth:
> > To take a break from my other POV-ray chores, I made a simple animated demo of
> > what an object looks like when it rotates (in POV-ray), as if free-falling under
> > gravity-- but discounting wind resistance or any other extra force. Its a
> > comparison between applying the rotations in two axes vs. three. I made the
> > animation my own purposes (to easily refer to later), but it might be of
> > interest to others as well.
>
> If free-fall without air resistance is what you want to model, you
> should use a /single/ rotation about an arbitrary axis.
>
> This is because without external forces, angular momentum is conserved,
> i.e. the axis of rotation doesn't change.
>
>
> When you combine it with a second rotation, two things can happen:
>
> (A) If both rotational axes remain stable in space and you apply the
> rotations simultaneously, the result is just a single constant rotation
> about a different axis.
>
> A real-life equivalent would be a ball set in motion by two rollers
> touching it on non-opposing points.
>
> (B) If you apply the second rotation after or before the first, the
> result is a precessing motion.
>
> A real-life equivalent would be a rotating object mounted in a gimbal,
> which in turn is also rotating.
>
>
> Thing you get if you specify a two-axis rotation in POV-Ray is (B): The
> rotation around the X axis is applied first, then the rotation around y
> is applied, effectively changing the axis around which the first
> rotation is applied.
>
> This violates conservation of angular momentum, and thus is only
> realistic when external forces are present, such as due to air
> resistance. In that context, a precessing motion would happen if the
> (asymmetric) wind resistance remained constant, which may be close to
> realistic for some comparatively symmetric objects.
>
> A rotation about a third axis adds yet another layer of complexity,
> turning the precessing motion into a tumbling motion. This should be the
> closest to realism at least for highly asymmetric objects.
>
> In either case - precessing or tumbling motion - the motion will only be
> a rough approximation of real physical behaviour.

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

you can search youtube with that term.

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