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Re: keeping those darned planets apart



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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