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Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!ihnp4!zehntel!dual!decwrl!dec-rhea!dec-dvinci!fisher
From: fisher@dvinci.DEC
Newsgroups: net.space
Subject: Solar sail energy transfer mechanism
Message-ID: <823@decwrl.UUCP>
Date: Fri, 1-Mar-85 10:40:28 EST
Article-I.D.: decwrl.823
Posted: Fri Mar  1 10:40:28 1985
Date-Received: Sun, 3-Mar-85 03:04:39 EST
Sender: daemon@decwrl.UUCP
Organization: DEC Engineering Network
Lines: 40



uwvax!derek asks
>~"how does energy get transferred from the photon to the sail..."

The problem with your explanation is that it does not consider the vector
nature of momentum.  Rather than deal with vector equations, let's consider
a one-dimensional system so I can deal only with + and -.

The photon is plowing along with momentum  +mv (m=mass of photon, v=velocity of
photon).  It smashes into a stationary sail (momentum 0) in a totally inelastic
collision and bounces back, now with momentum -mv.  Since momentum is
conserved, the total {photon, sail} system must still have a total momentum of
+mv.   The momentum difference between the photon after the collision and the
total system before the collision is (-mv)-(+mv) or -2mv.  Thus the solar sail
(the only other component of the system in this simple model) must now picked
up a momentum of +2mv, and thus a velocity of +2mv/M (M is the mass of the
sail). 

Note that if the photon "sticks" to the sail (ignoring kinetic->heat
conversion), we have a much different situation. The total sail/photon system
still has a momentum of +mv, but now, since they are travelling together, the
velocity change of the sail is only mv/(M+m), (slightly) less than half as
much!

Now (wave hands) consider a two or three dimensional system, and you see that
if the photon is absorbed, the sail can only change velocity in the direction
that the photon was moving.  If the photon is reflected, you can "tack" by
tilting the sail and forcing the photon to bounce off at a non-pi angle, thus
generating a velocity change in the sail at an angle. 

Ta da!  Not bad for having not dealt with this stuff (except in sci-fi books)
since freshman physics!

Burns


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