Re: keeping those darned planets apart

[Bob Sciamanda <trebor@VELOCITY.NET>]

The optimal solution is nature's: orbital motion of both around their CM.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Glenn Knapp <kahuna@VCN.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, August 26, 1999 10:10 PM
Subject: Re: keeping those darned planets apart


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> At 07:37 PM 25/08/99 -0700, you wrote:
> > Here's an idea which I'd never encountered before.  Suppose we have two
> > masses (planets) far out in space away from other matter.  They will
> > obviously fall together via gravitational attraction.  If we wished to
> > keep them apart, you would have to create a "contact force" between
them,
> > right?  Create some sort of repulsion, or maybe erect a tower to hold
them
> > apart?
> >
> > There's another way.
> >
> > Mount a machine-gun on one of the masses.  Fire bits of rock as
bullets.
> > fire them at the other planet so that they collide inelastically.
Adjust
> > the firing rate and velocity of the bullets until the planets hover at
the
> > desired spacing.
> >
> > Is there any repulsive contact-force between the planets?  No.  NO?!!!!
> >
> > WEEEEEEEIRD!!!!!!
> >
> > Firing of the gun creates a "recoil" force-pair, and when the bullet
hits
> > the other planet, it creates an "impact" force-pair, but there is no
kind
> > of force repelling the planets apart at all.  The gun accelerates a
> > bullet, and the other planet decellerates it by just the same amount.
> > Mass leaks continuously from one planet to the other via the
> > bullet-stream.  KE is injected into the bullets and extracted at the
> > target, so there is a net flow of energy as well.
>
> This looks like a conservation of momentum deal to me. Their momentum in
the
> one direction would have to "cancel out" the momentum of the planet   To
make
> this work, you'd need to fire one heck of a lot of bullets really fast.
This
> means that the mass of your planet is going to be decreasing.  At the
same
> time, the mass of the other planet is going to be increasing.
Eventually you'd
> run out of planet - you made it all into bullets that you fired into the
other
> world.
> That would be weird.
> Glenn