Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/5/84; site mordor.UUCP Path: utzoo!watmath!clyde!cbosgd!cbdkc1!desoto!cord!ihnp1!ihnp4!zehntel!dual!mordor!space@mit-mc 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 CaplingerThe 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.