Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site uwvax.UUCP Path: utzoo!watmath!clyde!cbosgd!ihnp4!mhuxn!mhuxj!mhuxr!ulysses!allegra!bellcore!decvax!genrad!panda!talcott!harvard!seismo!uwvax!judah From: judah@uwvax.UUCP (Judah Greenblatt) Newsgroups: net.space Subject: Re: Light Sails Message-ID: <136@uwvax.UUCP> Date: Thu, 28-Feb-85 16:24:25 EST Article-I.D.: uwvax.136 Posted: Thu Feb 28 16:24:25 1985 Date-Received: Mon, 4-Mar-85 04:16:20 EST References: <722@mordor.UUCP> <133@uwvax.UUCP> <6893@rochester.UUCP> Organization: U of Wisconsin CS Dept Lines: 25 > ... 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 The principle of light pressure is correct, but unfortunatly, it is not what drives a radiometer (as these little toys are called). If you look closely at a radiometer spinning in the sunlight, you will see that the vanes spin with the SILVER face leading and the BLACK face trailing. If light pressure drove the spinner, the BLACK face should lead and the SILVER face (which would be pushed twice as hard by the light pressure) would follow. The radiometer is actually a simple heat engine: the BLACK side of the vane heats up slightly more than the SILVER side and the vanes are driven by the difference in momemtum of gas molicules bouncing off the warmer and cooler sides. Light pressure is actually much weaker than the forces that drive a radiometer. Judah Greenblatt ARPA: judah@wisc-rsch.arpa U. of Wisconsin C.S. Dept. UUCP: {seismo ihnp4 lbl-csam}!uwvax!judah