X-Message-Number: 19291 From: "Brett Bellmore" <> Subject: Re: CryoNet #19285 Date: Sun, 16 Jun 2002 15:18:37 -0400 I'm not terribly suprised that your physicist friend wants to remain unnamed, George; He or she isn't a very good physicist. Or maybe was just having an off day. Simply put, momentum is a function of mass times velocity. Energy is a function of (One half of)mass times velocity SQUARED. Ergo the ratio of momentum delivered to energy delivered is inversely proportional to the velocity of a projectile. The faster a projectile is traveling, the less influence it's direction of travel has in the shape of the crater it generates. Consider for a moment the limiting case, a projectile traveling at C. A packet of photons, in other words. It strikes the moon at a grazing angle. Compare it to a projectile traveling at 5x10e4 meters per second, a not unreasonable speed for a meteor, and carrying the same ENERGY. The meteor masses 100 kg, carries a momentum of 5x10e6, and 1.25x10e11 in energy. The photon packet, on the other hand, having 1.25x10e11 worth of energy, will have an effective mass of m=e/c2, or 1.39x10e-6, and a momentum of 416! Carrying the same energy, contributing the same explosion component, it carries the momentum of an 8 gram meteor. Now, granted, the difference between a bullet and a meteor is not so great as that between a meteor and a photon packet, but the fact remains that, once a projectile is traveling so fast that it vaporizes on impact, the crater will be mostly round, as the symetrically distributed momentum of the explosion will far exceed the directional component due to it's original speed. In fact, there are non-round meteor craters on the moon, but they occur because a grazing impact caused the energy to be delivered over a length of lunar surface, causing the explosion to originate from a line segment rather than a point. Brett Bellmore Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=19291