Jet Ball Taiwan has interesting dynamics (try again)
Mike Crawford
shop at LA International Airport.
This is a clear plastic sphere about 4 cm in diameter. Inside the
sphere is a clear liquid, and inside of that is a solid sphere
printed to look like a bloodshot eyeball. The eyeball is waited
on the bottom, so you can hold it in your hand and wiggle it, and
the eyeball wiggles at you in a strange way, but remains generally
looking straight up (I'd rather it looked sideways...)
I have two of these, of course, for true stereoscopic vision.
I noticed these look particularly odd when I roll them on a tabletop.
They roll quite well as they weigh about 20 grams - so they have a good
deal of inertia. They look quite strange as the inner sphere continues
to look straight up, with just a little rocking around as I bounce
the balls against each other to demonstrate lab frame vs. center of
momentum experiments to the amazement and horror of my
non-physicist friends. Little bubbles of air inside take on
interesting shapes.
This evening I was idly rolling one around a slightly bent stack of
paper as I shared a bottle of wine with my friend, until we too were
slightly bent. It seemed to me that the ball rolled in a way that
was quite wrong for its size and mass. It would roll up a hill and
change direction, and reverse quite abruptly. A solid ball would
accellerate more gently.
Finally it occurred to me that these devices are rolling spheres that
have both a very high mass, and so high linear momentum at a given
velocity, but only a very small fraction of the mass actually rotates,
so they have a very low angular momentum while rolling at a given
linear velocity, and of course a very low moment of inertia.
Other than wishing I could afford a case of these so I could toss
the whole batch down the physics department hallway, it seems to me
that these would make an interesting demonstration to frosh physics,
and maybe there could be some use for these in actually useful
devices. It's not clear what, though.
--
Mike Crawford | Author of the Word Services Apple Event Suite.
craw...@scipp.ucsc.edu | Free Mac Source Code: ftp sumex-aim.stanford.edu
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