Re: The Olympics

["B. Esser" <besser@MARIAN.CREIGHTON.EDU>]

On Wed, 27 Sep 2000, Rick Tarara wrote:

> 1)  They impart some angular momentum into their motion during the jump IF
> they clearly have angular momentum ANYTIME during their flight.
>
> 2)  There IS enough air resistance to impart a net torque to produce the
> angular momentum.
>
> 3)  Our eyes get fooled because by moving different parts of the body in
> just the right way, the diver/gymnast can reorient their bodies while
> maintaining zero angular momentum throughout.
>
> I think that (3) is the solution to the demo that Dewey refers to below.  It
> is probably also the solution to SOME of the maneuvers seen done by
> divers/gymnasts.  However for other motions (several complete rotations with
> all parts of the body moving in the same way) then either (1) or (2) must
> hold.  I suspect (1) in most cases.

My oldest daughter was a gymnast and I spent many years watching
practice/meets.  She was trained to try to take off straight.... As was
mentioned before imparting a twist on takeoff results in a significant
deduction. In addition, in some moves (twists off the vault, twists on the
second somersault in a double back etc.) the twisting takes place after
a considerable time in the air.

While the "cat twist" has already been described there is another non zero
angular momentum possibility called the tilt twist.
 From an article by Hardy Fink
" The tilt mechanism
requires the gymnast to shorten one side of the body relative to the other
(by throwing the arms, one up, one down, or by side flexion or both). This
has the effect of tilting the somersaulting axis away form the "AM" vector
the direction of which (in order to be conserved) requires the body to
undertake a compensating sustained twist about the longitudinal axis as
long as the tilt exists."
It is the total angular momentum that is conserved, a nonrigid body
rotating about two axis (tumbling and twisting) can by tilt increase the
twisting.
 A fairly good summary of twisting in gymnastics can be found at:
http://www.usa-gymnastics.org/publications/technique/1997/2/twisting.html

At the lower levels of gymnastics, imparting angular momentum at takeoff
(or on blocking the vaulting horse) is common.  As gymnasts improve they
rely more on tilt twisting, particularly on twists taking place after a
first somersault.  My own daughter had just made it to the point where her
takeoffs were "square", relying on the tilt mechanisms to acquire rotation
when she crashed on a vault and broke her back, thus ending her
participation in gymnastics.

Bruce Esser
Physics Teacher                    Something witty
Marian High School                     Should go here
http://marian.creighton.edu