There is no law of conservation of mass, and there never was. However, there was a law of conservation of matter in chemistry, and there still is. There is no conflict with relativity here. The law of conservation of matter only reflects the evident fact that all of the atoms which go into a chemical reaction, come back out. Hence, the matter in the reaction is "conserved".

It was common in Newton's day, and is probably still common in the popular view of science, to equate the "amount of matter" to the "amount of mass", but that is wrong. If you look at Newton's F = ma you can see that m is just a proportionality factor between F and a. It was assumed to be constant only because no body knew at the time how to vary m without also varying the amount of matter. But there are a number of interesting classical problems that involve a varying m, suchg as a sandbag pendulum losing mass, or the ballistic problem of a siege gun cannon ball attached to a chain.

The key to understanding m is to see its true role in classical physics. It is not a measure of how much matter you have. Rather, it is a measure of the inertia of whatever matter you do have. Once you move your concept of m from "matter" to "inertia", things get conceptually easier.

So, the increase of mass with speed only says that inertia increases with speed, reaching infinity in the limit as speed approaches that of light. The "amount of matter" remains invariant, but the "amount of inertia" does not.

The common way to express this is by using a velocity dependent mass in Newton's equations, but it is not the proper treatment. The problem is that in Newtonian equations the laboratory ("coordinate") time is used. If you recast the equations to use "proper" time, mass once again becomes invariant. The variablity of mass is just another of those illusion induced by the relativistic confusion over reference frames (see Spacetime Physics by Taylor & Wheeler).

<font size=-1>[ This Message was edited by: Tim Thompson on 2001-12-05 10:16 ]</font>