Re: keeping those darned planets apart
- From: Glenn Knapp <kahuna@VCN.COM>
- Date: Thu, 26 Aug 1999 20:10:16 -0600
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
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