>He says that "mass is mass". This doesn't take into account that actually two different masses have to be considered - the rest mass, and the mass while the object is moving. The statement that "mass is mass" is one guy's interpretation, with which I don't happen to agree.
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>The wikipedia, at http://en.wikipedia.org/wiki/Special_relativity#Mass.2C_momentum.2C_and_energy
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>gives a summary of the relevant aspects of the Theory of Relativity. "It is often stated that in special relativity the mass of a body increases as its velocity increases. However, this statement depends on one's definition of mass, and in SR there are actually two different notions of mass."
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>In summary, the rest mass is invariant.
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>And the relativistic mass depends on the speed of an object. It is not invariant. This is one of the basic conclusions of the Special Theory of Relativity.

I just checked the definition of "relativistic mass" in the same wikipedia, and it seems that the notion is not very popular amongst modern scientists. But the article also explains that it is more a matter of semantics.

What I understand from the article is that the mass (relativistic mass) of an object will, indeed, increase with speed. But scientists tend to take the rest mass as a "reference point".