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From: nemo@rochester.UUCP (Wolfe)
Newsgroups: net.space
Subject: Re: Light Sails
Message-ID: <6893@rochester.UUCP>
Date: Thu, 28-Feb-85 11:28:40 EST
Article-I.D.: rocheste.6893
Posted: Thu Feb 28 11:28:40 1985
Date-Received: Sun, 3-Mar-85 07:36:28 EST
References: <722@mordor.UUCP> <133@uwvax.UUCP>
Organization: U. of Rochester, CS Dept.
Lines: 18

> Got time for a dumb question?  Too bad.  I can't figure how the energy of
> photons from the sun is going to be converted into motion of the light sail.
> I jus' don't get it.  If the photon is reflected, there can be no change in
> momentum of the sail (momentum being conserved), unless the photon loses
> derek
Remember that the momentum is directed.  The mv of the photon becomes -mv
when it is reflected, so there is a net momentum change of 2mv, which is
imparted to the reflecting surface.  There are these little globes with
vacuum and a paddle/pivot mechanism inside which work on this principle.
The four paddles are arranged so that the intersection of the two planes
is the axis of rotation of the pivot.  Each paddle has a black side and a
silver side with radial symmetry.  Placed in light, the paddle(s) on one
side of the pivot will reflect the light, the one(s) on the other will absorb
the light.  The absorbing surface gets only the inelastic momentum change,
or mv, while the reflecting surface gets the 2mv momentum change.  This
creates a torque on the paddle structure, and the little bugger rotates.
Available at novelty stores near you!
Nemo