From: kjmiller@mitre.org (Kevin Miller)
Subject: Re: How small rotation required for effective float serve?
Followup-To: rec.sport.volleyball
Organization: MITRE
Distribution: usa
Date: Mon, 11 Jan 1993 17:35:39 GMT

In article <1993Jan8.225548.29105@jarvis.csri.toronto.edu>,
ematias@csri.toronto.edu (Ummm... me!) wrote:
> 
> In article <1ik96u$mla@agate.berkeley.edu> sag@hera.Berkeley.EDU (Steve Goldfield) writes:
> 
> [Stuff deleted...]
> 
> >But a volleyball is roughly spherical, so the fluid path of
> >a ball that isn't spinning is the same on top as below. So,
> >what is the aerodynamic force on a nonspinning ball? If the
> >ball is spinning, there should be small drag effects that
> >slow the air in one direction and speed it up in the other.
> >
> >Is there something wrong with the above reasoning, or is
> >there another aerodynamic principle involved?
> 
>    What you people seem to be overlooking is "force" -- the force of
> friction, net force on the ball in its component axes, etc.
>    If the ball is sailing through the air with no spin and a (relatively)
> straight path, the net force applied to the ball (besides gravity) is via
> friction of the air; however, this air friction only affects the force
> vector along the axis of the ball's path.  Since the ball is a free-floating
> object, there are NO forces acting on its other axes.
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Incorrect.  There are aerodynamic forces, other than friction, at work on a
non-spinning volleyball.  The airflow around a non-spinning, spherical
object is inherently unstable.  As the air flows around the ball, small
vortices are formed where the air cannot perfectly follow the shape of the
ball.  These vortices form in a basically chaotic manner and exert force on
the ball as they break away.  The panels, seams, lettering, and valve stem
all have some minor effect on formation of the vortices.  The rate of
formation and strength of these vortices increases as the speed of the ball
through the air increases, because it is more difficult for the air to
follow the shape of the ball.

>    Now, any gym will have lots of little air currents caused by the opening
> of doors, the movement of players, etc.  Even though these currents may be
> small, they still have a net effect on those "unforced" axes of the ball
> which I mentioned earlier.  That's why a float serve is always so
> unpredictable -- nobody can see those little air currents, so there's no way
> to tell if they'll make the ball drop suddenly, curve a foot to the left, or
> sail way out.

I would argue air currents play a minor role in the erratic flight of a
float serve (see above).  I strongly suspect that a float serve in
perfectly still air would move just as erratically as one in a gym with
"normal" air currents.

> 
>      - Mark Rosteck -
> (ematias@csri.toronto.edu)
> (g1marker@cdf.toronto.edu)

No back to original question.  It's been my observation that serves with a
little spin (<2 revs) still "float" effectively, although zero-spin serves
are the most wicked.  The other big factor is the speed of the serve.  The
changes in direction on a hard-hit float serve are stronger and happen more
quickly than on a slow serve.  Sounds like a (semi-)controllable floating
jump serve would be a real killer to receive.

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Kevin Miller       | MITRE's lawyers can't moan,    |
MITRE Corporation  | 'Cause what's stated up there, |
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kjmiller@mitre.org |                                |
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