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*From*: John Denker <jsd@av8n.com>*Date*: Wed, 20 Oct 2010 13:12:04 -0400

On 10/20/2010 12:25 PM, Espinosa, James wrote:

For years, and even recently, the term "rest mass" has been used.

Well, I hope that everyone knows that the magnitude of a 3-vector is

invariant under Galilean transformations between coordinate systems

(or inertial frames). Under Poincare transformations between

inertial frames (coordinate systems) the magnitude of a 4-vectpr is

invariant. This means that the value does not change from one system

to the another. The magnitude of the 4-momentum is the mass. What

part of "invariant" do some physicists not get?

I'm not sure that's the right question.

Most physicists get this, and have gotten it for many decades.

Textbooks for upper-division students and graduate students

mostly get this right. Recommended reference:

Gary Oas

``On the abuse and use of relativistic mass''

http://arxiv.org/PS_cache/physics/pdf/0504/0504110v2.pdf

The real question is, why do the folks who write introductory

textbooks and "popularizations" of physics continue to get

this wrong, generation after generation?

Part of the problem is that some of these authors started

out as cartoonists and took up textbook-writing as a second

career. Their knowledge of modern physics is, shall we say,

sketchy.

Constructive suggestion / reminder: The place where the rubber

meets the road is the famous formula E=mc^2. It must be emphasized

that mc^2 is the _rest energy_ not the total energy.

The notion of rest energy is useful.

Mass is invariant. Calling it the "rest" mass is mostly harmless.

The idea of non-invariant "relativistic mass" aka "velocity-dependent

mass" is bad news for a number of reasons. Ditto for "velocity-

dependent rulers" and "velocity-dependent clocks". See e.g.

http://www.av8n.com/physics/odometer.htm

and references therein.

**Follow-Ups**:**Re: [Phys-l] Rest mass again?***From:*Derek McKenzie <derek_s_mckenzie@hotmail.com>

**References**:**[Phys-l] Rest mass again?***From:*"Espinosa, James" <JEspinosa@mail.twu.edu>

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