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From: matt@oddjob.UChicago.UUCP (Matt Crawford)
Newsgroups: net.physics
Subject: Re: Conservation of momentum
Message-ID: <375@oddjob.UChicago.UUCP>
Date: Tue, 14-Aug-84 22:09:43 EDT
Article-I.D.: oddjob.375
Posted: Tue Aug 14 22:09:43 1984
Date-Received: Wed, 15-Aug-84 07:03:16 EDT
References: <268@heurikon.UUCP>
Organization: U. Chicago: Astronomy & Astrophysics
Lines: 27

Actually, you are talking about the law of conservation of ANGULAR
momentum, angular momentum being conserved separately from linear
momentum.  Here is an experiment you can try to demonstrate to your-
self what is happening to cats and divers when they are falling:

    Take a wheel from a bicycle and attach some sort of axle by
    which you can hold it.  Now sit in a reasonably well-lubricated
    swivel chair, holding the wheel's axis vertical.  Now grab a
    spoke and give the wheel a spin.  You will begin to turn in the
    other direction.

    If you can't manage the bike whhel, do this instead:  sit back
    in the swivel chair with your legs up and with a heavy book in
    your hands.  (I am using Misner, Thorne and Wheeler's GRAVITA-
    TION.)  Slowly hold the book out to one side, then smoothly
    swing it in front of you and around to the other side.  Bring it
    back in to your lap and extend it out to the original side
    again.  Repeat until enlightenment is acheived.

All the while, your total angular momentum is zero (ignoring
friction), yet the angle your are facing changes.  If you want to
get some net angular momentum you can hold the book out and fling it
to one side, with a velocity perpendicular to the direction from you
to it.  (This will hurt your shoulder, I think.)
___________________________________________________________
Matt		University	ARPA: crawford@anl-mcs.arpa
Crawford	of Chicago	UUCP: ihnp4!oddjob!matt