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Re: keeping those darned planets apart



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


--Boundary_(ID_wxxfb+Nr/iTtuL4k2PmlaQ)
Content-type: text/plain; charset=us-ascii


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