Hi Everyone! I'm back! :)

A while back we discussed relativistic mass (RM), m. RM is defined as the m in p = mv. I keep seeing people support their anti-RM position in several ways. One of those was is by saying that relativistic mass m is just another name for energy since E = mc

^{2}, I the paper I wrote I give a simple example of where that notion doesn't hold true. The paper is at

http://arxiv.org/abs/0709.0687In cases where stress contributes to inertia you will find that this assertion isn't true. There are few aricles on the subject in the

*American Journal of Physics*. One of the articles is

**The inertia of stress**, Rodrigo Medina,

*Am. J. Phys. 74(11)*, November 2006

**Abstract** - We present a simple example in which the importance of the inertial effects is evident. The system is an insulating solid narrow disk whose faces are uniformly charged with equal charges of equal magnitude and opposite signs. The motion of the system in two different directions is considered. It is shown how the contribution of energy and momentum of the stress that develops inside the solid to balance the electromagnetic forces have to be added to the electromagnetic contributions to obtain the results predicted by the relativistic equivalence of mass and energy.

This means that if you have a rod lying on the x-axis which is under stress but for which the forces on the body are in equalibrium the relation E = mc

^{2} does not hold.

This was someting Einstein knew too so this isn't new by any stretch of the inagination. In 1907 Einstein wrote a paper called

**On the Inertia of Energy Required by the Relativity Principle** Annalen der Physik 23 (1907). Section 1 is called

*On the kinetic energy of a rigid body in uniform translation subject to external forces*. This kind of thing is rarely, if ever, mentioned in modern relativity texts. Its implicit in the stress-energy-momentum tensor but you have to know what to look for. I found it because I did a deep search into Einstein's papers. This idea can be used to show that E = mc

^{2} doesn't hold for stressed bodies. The artilcle on the inertial properties of stress notes this and states that for E = mc

^{2} to hold for the energy stored in the capacitor the stress in the capacitor must be taken into account.

I raised that notion in my article because it seems that nobody else who wrote articles on the subject knows about it. In the article Einstein starts out by saying

We consider a rigid body that is moving in uniform tranlation (velocity v) in the direcction of increasing x-coordinate of a coordinate system (x, y, z) that is assumed to be at rest. If external forces do not act upon it, then, according to the theory of relaivity, its kinetic energy is given by the equation

[math]K_0 = \muV^2[\frac{1}{\sqrt(1 - (v/V)^2)} - 1][/math]

where [math]\mu[/math] denotes mass (in the conventional sense) and V the velocity of light in vacuum. We now want to show that according to the theory of relativity thi expression does not hold any longer if the body is acted upon by external forces that balance each other.[

I then use this same scenario to show that relativistc mass isn't just another name for energy since the normal relationship E = mc

^{2} does hold under these circumstances. That's a very important point that has been over looked in this debate on relativistic mass vs rest mass.