Re: MOMENT OF INERTIA

[Jerome Epstein <jerepst@WORLDNET.ATT.NET>]

brian whatcott wrote:
> At 13:22 12/19/98 -0500, you wrote:
> > > > Suppose you were standing on a frictionless surface.  Could you lean
> > > > forward?  Could you change the position of your center of mass?
> > > >
> > > > Mervin Koehlinger
> > >
> > > Yes for the following conditions:
> >
> > No. Sorry. You cannot move your center of mass without a force from
> > outside the body. But you could lean forward. All that would happen is
> > that your legs would go backward. Ask an astronaut.
> > J. Epstein
> Jerry's assertion defeats itself. A spaceman would say it is the easiest
>  thing in the world to change his C of M in weightless, frictionless
> conditions.
>    By launching upwards. Ask an astronaut.
>
> Brian

Obviously we are talking past each other. What do you mean "by launching
upwards"? I imagine you mean pushing against something, such as a
platform. Then (by the 3rd law) there is a force on your body from
outside your body. It is this force that allows your center of mass to
move. If you are free floating in space, you CANNOT move your center of
mass. This is surely elementary.

When you "launch upwards" by pushing against something, that something
"launches downwards", the CM of the whole system does not move.
J. EPstein