Re: [Phys-l] A ball at the center of a planet

["LaMontagne, Bob" <RLAMONT@providence.edu>]

OK - I'll bite.

If the ball is a perfect sphere of finite size when at the center of the earth, then the center of the ball has zero net force acting on it - so it does nothing. A point on the surface of the ball has a weak but finite force on it pointing to the center of the ball and earth. Therefore, shouldn't a deformable ball therefore decrease in radius relative to deep space?

Bob at PC

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Dr. Richard Tarara
> Sent: Monday, October 04, 2010 1:06 PM
> To: betwys1@sbcglobal.net; Forum for Physics Educators
> Subject: Re: [Phys-l] A ball at the center of a planet
>
> I suspect that the intent of the original question is much more simple
> than
> we are making it, but the original condition should have been that the
> ball
> is a perfect sphere in the vacuum of deep outer space.  Then taking it
> to
> the center of a planet should increase the radius--at least in theory
> (sort
> of like the earth moves up to meet the dropped ball somewhere in
> between.
> ;-)  As a question for students, this is not so easy as many will
> assume the
> zero net gravitational force at the center of the planet is equivalent
> to no
> force  in outer space.  Others (at least my students) get totally
> confused
> between pushes and pulls and might well assume the planet is pushing
> from
> all sides.
>
> If I am mistaken, and the intent is really to include the deformation
> forces
> at the surface of the planet, then previous posts have covered the fact
> that
> there is insufficient information here.
>
> Rick
>
> ***************************
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, IN
> rtarara@saintmarys.edu
> ******************************
> Free Physics Software
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> *******************************
> ----- Original Message -----
> From: "brian whatcott" <betwys1@sbcglobal.net>
> To: <phys-l@carnot.physics.buffalo.edu>
> Sent: Monday, October 04, 2010 12:48 PM
> Subject: Re: [Phys-l] A ball at the center of a planet
>
>
> >  On 10/4/2010 9:43 AM, Fakhruddin, Hasan wrote:
> > > Greetings folks!
> > >
> > > Here is a question for your intellectual entertainment:
> > >
> > > A solid rubber ball has a radius of r in vacuum at the surface of a
> > > planet that is a solid uniform sphere.  The ball is now placed in
> vacuum
> > > at the center of the planet.  Will the radius of the rubber ball
> > > (a) Increase
> > > (B) Decrease
> > > (C) Stay the same?
> > >
> > > Thanks
> > >
> > > ~ Hasan Fakhruddin
> > > Instructor of Physics
> > I believe I can respond to a question that is the physics counterpart
> of
> > angels dancing on pinheads
> > :-)
> >
> > If a highly elastic ball forms a perfect sphere at the surface of
> some
> > planet with appreciable gravity,it would be asymmetrical when
> > transported to some location where the gravity gradient is reduced,
> > taking a slightly  oblate form.
> >
> > Brian W
> > _______________________________________________
> > Forum for Physics Educators
> > Phys-l@carnot.physics.buffalo.edu
> > https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
>
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