Re: MOMENT OF INERTIA

[Mervin Koehlinger <tchkoehlimw@CRF.CUIS.EDU>]

On Sat, 19 Dec 1998, Jerome Epstein wrote:

> Sorry must disagree. If you lean the upper half of your body forward and
> your legs go backward then your CM will not move.
> On the rotational part. You are able to change the orientation of your
> body, though not the angular momentum. If you start at rest, stretched
> out in the "up and down" direction", you can rotate the upper half of
> your body, say clockwise, a bit by rotating your lower half the other
> way. ANgular momentum stays zero.

Angular momentum only stays zero if, when you rotate the upper half of
your body clockwise, the lower half of your body rotates counterclockwise.
But there is zero net rotation of your body either clockwise or
counterclockwise.  When you return the upper half of your body to its
original position, the lower half also returns to its original position.
The entire body is exactly the same as it was before.

 Then you can stretch out in the
> "east-west" direction, bringing both halves of your body to rest.
> ANgular momentum remains zero. One part of your body can exert a torque
> on another part, but the total angular momentum of both parts cannot be
> changed by internal forces. YOu can swing your arm, but your body will
> rotate the other way.

No way!  If you started in the north-south position, you will end in the
north-south position.  Angular momentum must be conserved.  Your
explanation has twice violated conservation of angular momentum.

Mervin Koehlinger
Physics Instructor
Concordia Lutheran High School
Fort Wayne, Indiana