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From: space@mit-mc
Newsgroups: net.space
Subject: Re: Light Sails
Message-ID: <798@mordor.UUCP>
Date: Fri, 1-Mar-85 18:54:01 EST
Article-I.D.: mordor.798
Posted: Fri Mar  1 18:54:01 1985
Date-Received: Sun, 3-Mar-85 04:26:49 EST
Sender: daemon@mordor.UUCP
Lines: 23

From: Mike Caplinger 

The original question was more about conservation of ENERGY than
conservation of momentum.  The momentum of a reflected photon is
reversed, but its energy, a scalar, is the same (assuming perfect
reflection = no wavelength shift.  The velocity, hence kinetic energy,
of a photon can't change.)

So if the sail starts moving from the impulse, where did the kinetic
energy of its motion come from?  Remember that in a collision both
momentum AND energy are conserved.

I would really like to know the answer.  My physics seems to be too
rusty to generate it, but I know there's something funny...

	- Mike

ps.  Those bulbs with the vanes ("radiometers") invariably spin in the
WRONG direction.  That effect is caused by bad vacuum in the bulb
causing convection currents off the black surface.  If light pressure
were doing it, they would rotate black surface first, as the white
surfaces are reflecting, not absorbing, and get twice the momentum
exchange.