Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site rochester.UUCP Path: utzoo!watmath!clyde!cbosgd!ihnp4!mhuxn!mhuxj!mhuxr!ulysses!allegra!bellcore!decvax!genrad!panda!talcott!harvard!seismo!rochester!nemo 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