From: jboyce@physics.berkeley.edu
Newsgroups: rec.juggling
Subject: Challenge to Everyone (physics)
Date: 19 Nov 1992 19:54:05 GMT
Organization: University of California, Berkeley
Distribution: world
NNTP-Posting-Host: physics.berkeley.edu
Originator: jboyce@physics.Berkeley.EDU


Robert Partridge writes:
>Thought I would post one of the ways I use to learn new vastly more difficult
>tricks! Other than just trying it, how about slowing the whole thing down?
>
>If you use round balls (bags don't roll well enough) you can get used to the
>look of a trick and what your hands should be doing by rolling balls on
>an inclined board. Of course the board must be big enough so that the balls
>don't roll off the edges. By changing the angle of the board you can change the
>speed of the trick. As you get better, the board gets more vertical, until you
>can do it without the help of the wood.
>
> I've never heard of anyone else using this method so I guess I'm the 
>only one fruity enough to try it!


This has essentially been done before, by none other than Claude Shannon
(the father of information theory, he's also a juggler on the side).  His
scheme was to use an inclined air-hockey table.  The lower friction,
as well as the fact that the objects don't have to spin, presumably
results in more "realistic" trajectories.  It seems to me that the
problem with these schemes is that the thing you're training your body
to learn isn't strictly what you want to learn -- the objects are
confined to a plane automatically, so you're not exercising that part
of your brain which controls this.

Ideally, one would want some kind of a situation where you could juggle
as you do normally, but with a controllable "gravity".  You could start
with a very small gravity (say moon-equivalent) where presumably things
would be easy to do, and then slowly increase g until you're on your own.
I don't have any hard evidence, but it seems intuitively that this would
make the learning process easier, faster, and less error-prone.

My challenge to all you physics/engineering-minded jugglers out there is
to conceive of some method of doing this.  Essentially you want to
exert an (approximately) constant upward force on all the objects (call
them balls for now) throughout their paths.  A constant force effectively
decreases g.  This should of course be done in such a manner that
doesn't interfere with the juggling.

I thought about it for a while and came up with the following
possibilities (I haven't really considered practicality yet):

   1.  Juggling underwater (constant bouyancy force).  Adjust the
       ball density to achieve different effective gravities.  The
       problem here is comfort, as well as drag due to viscosity
       (which needs to be substantially less than the force of gravity)

   2.  Juggling lighter-than-air balls.  Viscous drag kills this.

   3.  Balls on long elastic lines.  Adjust tension to get different
       g's.  Here the problem is that the lines run thru the pattern,
       screwing it up.  Can anyone fix this problem?

   4.  Charged balls inside a constant electric field (stand inside a
       huge parallel plate capacitor).  Change g using a knob on the
       wall (voltage across capacitor).  The balls will also repel one
       another, however -- I think this will force you to use huge
       electric fields (impractical and dangerous).  Could be wrong.
      
   5.  Superconducting balls inside a constant-gradient magnetic field
       (Meisner effect).  Theoretical problems:  (1) B must be
       divergence-free, so there will always be horizonal component to
       force at most points inside the juggling area (balls are pushed
       to the side), (2) flux pinning and other superconducting
       effects make the force a bit difficult to predict (there is
       some hysteresis), and (3) a reasonably high juggling area
       requires magnetic fields in excess of critical fields of most
       superconductors.

   6.  Juggling inside an upward-pointed wind tunnel.  Laminar flow
       with velocity substantially larger than typical ball velocity
       implies a constant upward force.  The balls themselves cause
       turbulence (as does your body), unfortunately.  [Not to mention
       the obvious practical difficulties.]

I haven't thought these through carefully, but none seems adequate.
Can anybody out there think of a scheme that might work in theory?
Post details to the net, with "Physics" in the title so that those 
disinterested don't start frothing at the mouth...

Jack
jboyce@physics.berkeley.edu