  == quote from Arons ==  Thanks for cleaning up the above discussion. By the way, I am not anonymous, my name and affiliation are on my user page.    As promised, here is the quote from A Guide to Introductory Physics Teaching by Arnold B. Arons (1990, page 263): For many years it was conventional to enter the discussion of dynamics through derivation of the relativistic mass, that is the mass-velocity relation, and this is probably still the dominant mode in textbooks.  More recently, however, it has been increasingly recognized that relativistic mass is a troublesome and dubious concept.  [See, for example, Okun (1989).] Not only does it get one into the infelicities associated with longitudinal and transverse masses, but it also tempts one to associate relativistic mass (rather than just rest mass) with gravitational effects.  The latter association is basically incorrect.  The sound and rigorous approach to relativistic dynamics is though direct development of that expression for momentum that ensures conservation of momentum in all frames: :  rather than through relativistic mass.  Unfortunately, it is more difficult to derive the momentum expression in a simple way that it is to obtain the mass-velocity relation from the collision gedanken experiments prevalent in the literature. [See Peters (1986) for a recent effort to simplify this derivation.]  I suggest the above quote be included verbatim in the article.  