I think that what would be the best way to achieve a realistic rotating object
would be to do what you suggested, and write a particle system / physics engine
that would take things like mass, rotational inertia, and external forces into
account, so that the system could determine the rotations, not follow some
arbitrarily decided upon values.

That sort of inflexible set of arbitrary rules doesn't work in real life - be it
"Law" (rules) or market economics, so you're just seeing the same thing here.
You car runs itself - you just give it gas and steer it.   If you had to
manually monitor and control the fuel mixture, the sparking sequence, the valve
timing, etc, it would quickly become unmanageable.
If someone were trying to control your car remotely by a rigid set of inflexible
rules, then there wouldn't be the necessary leeway to deal with all of the
exigent circumstances that you experience and react to during even a short ride
- other drivers, squirrels, puddles, rain, fog, losing traction in snow, etc.

In the same way  - let the object "decide" what is best for it, and of course it
will look natural.


1 and 2 look natural (1 the most) because they are more in line with what you'd
expect from an object experiencing wind resistance parallel to it's direction of
motion, and so rotating around an axis perpendicular to the axis of translation.

I would do a few things if you wanted to explore this:

Define mass for the object(s) and then find the center of mass, or "reduced
mass" so that you can rotate around that point.

There should be some fairly simple equations that relate the deflection in
response to a force on a rotating object - the moment of inertia thing.

Rotation around any other axis should be a function of the rotational speed /
moment of inertia around the primary axis.
Then you can add an external force --- and this is important - in order for it
to look natural, you need to add a visual cue and time it correctly.

Look at some of the basic recommendations for making animations and adding
sound, and they will explain that even very slight changes in the timing of the
video and sound can make something look / feel very unnatural.   Your brain is
very sensitive to these cues.

So, your other examples _might_ just need some sort of visual indication of an
external force to make it look "right" - wind, a falling stream of particles...
something perpendicular to the extra axis of rotation.

Post a reply to this message