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

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