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Re: one question
On Tue, 28 Nov 1995, Steve VanDevender wrote:
> Kevin C. Houston writes:
>
> > Out in intergalactic space (far far away from all outside gravitational
> > influences) you have the following setup:
>
> [ Figure turned sideways for brevity -- top -> ]
Fine with me.
>
>
> |--------------------------------------|
> |######~~~~~~~~~~~~~~~~~~~~~~~~ |
> p = -E/c |######~~~~~L~A~S~E~R~~~~~~~~~~ | p = E/c
> |######~~~~~~~~~~~~~~~~~~~~~~~~ |
> |--------------------------------------|
> black plate
> absorbs laser beam
> # = laser
> ~ = laser beam
> - = support wires
>
> > The top plate (the absorber) is in thermal equilibrium with a large
> > amount of solid Unobtainium m.p. 2.7 K, atomic mass = sqrt(-1) g/cm^3
> > so that there is no radiation from heat effects. power is provided by a
> > small on-board power source
>
> You must shop at the same physics experiment supply store that I
> do. That unobtainium sure is useful :-).
>
> > What happens when you turn on the laser?
> >
> > I'd say that it would just sit there, although you can clearly see that
> > without the wires to hold them together, the laser source and the top
> > plate would to move away from each other.
>
> You are correct.
>
> Although an actual device like this would actually move, but not
> with any permanent velocity. As the laser heats the absorption
that's the reson for the unobtainium, the melting never shifts above
cosmic backround radiation level
> plate, mass is transferred from the laser to the plate. As the
> center of gravity shifts to the right (in my diagram) the
> assembly would slide to the left. When the laser is turned off,
> it would stop, then begin to slide slowly rightward as the plate
> reheats the laser by radiation.
>
> > The wires are clearly dissipating a momentum equal to 2 E/C.
> > that is what i was trying to say with my bb analogy.
>
> I'm not sure "dissipating momentum" is quite the term for it.
> After all, the laser with momentum -E/c is tugging through the
> wires against the plate with momentum E/c. The momenta cancel.
>
YES! That's what I've been saying all along! that a physical structure
can transmit (through tugging) opposite momenta that can then cancel!
> Like I said before, momentum means motion. A non-moving object
> has no momentum. If the laser and the plate are held together
> with wires and don't move relative to each other or your
> observer, then you can't say that they have momentum.
The laser imparts momentum when it leaves, and when it strikes the black
plate.
>
> The wires are indeed under tension, because there is a force
> between the laser and the plate. This tension was created in the
> first instant the laser was turned on, and a small amount of its
> energy went into stretching the wires before it was all spent on
> heating the plate.
>
so you're saying that the plate heats more if it is held in place than if
it was free to move?
Kevin