Re: The Olympics

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

Perhaps one of the confusions here (at least for me) is that looking at
Bruce's link (below) it is now clear that the claim is NOT that the
gymnast/diver imparts NO angular momentum on takeoff, only that they often
impart no angular momentum around the vertical axis--that is, they do impart
AM around a horizontal axis perpendicular to their direction of motion.
That this can be converted to AM around other axes without changing the
overall vector AM is what is apparently not well understood but seems to
involve techniques like the 'tilt twist'.  These maneuvers would then be a
combination of imparted AM from the jump and reorientation of the body by
twisting different parts--just right.

Rick

----- Original Message -----
From: "B. Esser"
> 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