Re: The Olympics

[Doug Craigen <dcc@ESCAPE.CA>]

Leigh Palmer wrote:
> At 5:48 AM -0700 9/29/00, Justin Parke wrote:
> > ...the law.  I thought perhaps they were merely slowing the rotation by means
> > of changing their moment of inertia but it really seemed that the rotation
> > stopped.

The commentators will talk about divers pulling out of a spin too early
leading them to overrotate.  This suggests to me that they are still
rotating in the last part of the dive and its a matter of timing to
appear to drop straight into the water.  My original post said that the
appearance of stopping the spin after a series of tight somersaults was
a combination of slowing the spin by straightening out and have a
greater vertical velocity by this point so that a great height is
traversed with little rotation.  So far I haven't seen anything that has
changed my mind about this aspect of the discussion.

Having watched a bunch of these tight somersault type dives last night I
am pretty confident in saying that they generate a large amount of
angular momentum by tucking the upper body just as the feet are about to
leave the board.  More complex twisting type dives are harder to be sure
about, and it is difficult to see how much angular momentum is imparted
even in a "vertical" jump.  (These jumps are not truly vertical or the
diver would land on the board.)  The fact that there is horizontal
velocity means that the different body parts might have different
horizontal velocities (ie - net angular momentum), but divers and
gymnasts do not allow for the fact that physicists are watching and want
to see the same manouver repeated over and over for analysis.

> There's been a lot of nonsense in this thread. Justin seems to have
> known the correct answer all along; He explains it almost completely
> in the note above. (The only thing missing, which he must also know,
> is that the diver took off with the same angular momentum which she
> exhibited in her tightest tuck.)
>
> No explanation was needed, only reinforcement. Why did this thread
> go on so long?

Why does anything go on so long in PHYS-L?  (that's rhetorical)

The original question didn't specify what aspect of diving appeared to
violate conservation of angular momentum, leaving it up to our
imaginations to invent part of the question as well as the answer.
Since there are three aspects to consider: change of angular velocity by
tucking, zero angular momentum turns, and "twisting" - and many of us
seem to have only thought much about one or two of these, it left a lot
to talk about.  Actually, I consider that a good thing since I learned
quite a bit.

\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\

Doug Craigen
          http://www.dctech.com/physics/about_dc.html