From: markc@smsc.sony.com (Mark Corscadden)
Newsgroups: sci.physics
Subject: Re: Converting the masses
Date: 28 Jul 92 00:42:59 GMT
Organization: Sony Microsystems Corp, San Jose, CA

Matt McIrvin and Matthew Austern both pointed out the well known
(except to some laypeople like me) fact that the ratio of force to
acceleration does not produce a usable concept of relativistic mass.
From Matt McIrvin: if you're going to mess with relativistic mass
(I'm paraphrasing here!) use F = dp/dt and p = mv; don't use F = ma.
Defining relativistic mass won't be an issue in what follows, though.

The original thread has always been about the problems associated
with using the notion of relativistic mass.  The general recommendation
was that relativistic mass is an idea who's time has come and gone and
that you should use the word "mass" to mean "rest mass" or the intrinsic
mass of a particle or a system.

Here's a brief description of part of what I thought people were saying.
I still think this is the position that's being held by "rest mass only"
people like David Knapp, but since it's *their* position only they can
say whether or not I am expressing it correctly:

	Let's say that a cannon ball with a rest mass of
	1 kg is moving past you at 0.99 lightspeed.  Then
	people who want to talk about the cannon ball's
	"relativistic mass" would say that its mass is
	10 kg in your reference frame.  This is a bad idea!
	Don't use this concept of mass and don't use the
	word "mass" this way.

	We say that the mass of the cannon ball is 1 kg,
	period.  The mass of a cannon ball is an intrinsic
	property which doesn't depend upon your frame of
	reference.  If it is moving at 0.99 lightspeed in
	your reference frame, then we say that you have a
	1 kg cannon ball which happens to possess a very
	large amount of kinetic energy in your frame of
	reference.  This kinetic energy is not mass, its
	energy!  The word "energy" is perfectly fine, we
	don't need to change the word "mass" to mean the
	same thing as "energy".

	The amount of kinetic energy possessed by the cannon
	ball is dependent on the frame of reference you are
	using.  The exact same cannon ball may be stationary
	in a different reference frame, and in that frame
	its kinetic energy would be zero.  However its mass
	in all reference frames is the same: 1 kg.

That's how I took the comments being made.  I hope that's an accurate
representation of the position.

Anyway at first I took this farther and felt that what was being said was
that kinetic energy is *never* to be called mass, period.  With that in mind
I didn't think that this notion of "mass is rest mass, period" could be used
in practice when talking about a composite system who's parts were in
relative motion.  After reading the replies I think I was wrong.  I think
that you *can* give a clear unambiguous meaning to the concept of the one
and only intrinsic rest mass of a given system, based upon nothing more
than identifying the objects which comprise the system.  However I'm still
puzzled because (as far as I can tell) it requires you, among other things,
to treat the kinetic energy of the parts of the system, measured relative
to the system as a whole, as mass.  I'm puzzled because at first I though
that treating kinetic energy as mass was not kosher, period.

Here are two questions.  I'll apply the notion of mass, as in rest mass
or intrinsic mass, below in a simple situation.

question 1:  Am I using the word "mass" correctly according to the
folks like David Knapp who want to get away from the notion of
"relativistic mass"?

question 2:  Are physicists in general who use "mass" to mean "rest mass"
or "intrinsic mass" really all in agreement as to how the concept works?
Specifically, would the vast majority of such physicists reach the same
answers that I reach below?

Since both questions refer to people, only those people can give the
definitive answers.  Here's the situation:

	You have two cannon balls (again).  Each has a mass
	(always read "rest mass" or "intrinsic mass" here) of
	1 kg.  They are separated by one million kilometers,
	so for all practical purposes they are independent
	of each other.

	They are moving directly away from each other at 0.995
	lightspeed.  Then the mass of this system, consisting of
	the two cannon balls together, is 20 kg.  It is not 2 kg.

	On the other hand, if the center of mass of this pair of
	cannon balls is moving at 0.9999 lightspeed in your frame
	of reference, then their mass is still 20 kg.  Their mass
	is an intrinsic property of the system and does not depend
	in any way upon how the two-cannon ball system is moving
	with respect to you.

Is that right?

Mark Corscadden
markc@smsc.sony.com
work: (408)944-4086
Newsgroups: sci.physics