# Re: one question

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On Tue, 28 Nov 1995, Steve VanDevender wrote:
> What I was really objecting to in your parasail design was not
> the idea that you had cancelling momenta, but that you thought
> that creating sideways momentum meant a decrease in forward
> momentum.  For example, a device like this:
>
>
>    E/c --v                         :::: p = [ 0 E/(2*c) ]
> |----------------------------------:::/|
> |######~~~~~~~~~~~~~~~~~~~~~~~~~~~~::/ |
> |######~~~~~L~A~S~E~R~~~~~~~~~~~~~~:/  | beam splitter
> |######~~~~~~~~~~~~~~~~~~~~~~~~~~~~:\  |
> |######~~~~~~~~~~~~~~~~~~~~~~~~~~~~::\ |
> |----------------------------------:::\|
>                                    :::: p = [ 0 -E/(2*c) ]
>
> # = laser  ~ = laser light  -- = wires
>
> also won't move.  The beam splitter "absorbs momentum" just like
> the black absorbing plate did, even though it splits the beam
> into two beams traveling in the +y and -y directions.  This setup
> also has the advantage of not requiring an unobtainium heat sink,
> as long as you get a Perfect Mirror (tm) from Acme Physics
> Warehouse.

So you're saying that the beam splitter above would move at the same
speed as a absorbtion plate of the same mass?

>  > > 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.
>  >
I've always been taught to view atomic bonds as tiny springs which obey
hookes law F=kx  where k is some constant.  this is the force that
balances the force of the laser / absorber plate.  Is this view correct?
Doesn't a spring provide a constant force as long as it's stretched from
it's initial position?

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