Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site decwrl.UUCP 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: 40uwvax!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 UUCP: ... {decvax|allegra|ucbvax}!decwrl!rhea!dvinci!fisher ARPA: decwrl!rhea!dvinci!fisher@{Berkeley | SU-Shasta}