I think the reason for the word "relativity" should be explained on this page. What is relative, and why?
- appears to be done.
I thought E=mc
2 also applied when the mass was moving - no longer is kinetic energy separate, but bundled in with the increased mass of the object? Or is it - as I am thinking - that you're only using E=mc
2 to get the rest energy, then tacking on KE afterwards for that particular application? Perhaps this needs clarifying.
Dave McKee
Regarding a recent change to this article--I'm no physicist, but I always thought that relativity states that mass increases as velocity approaches the speed of light. Am I wrong on this?
Rest mass is a constant, but mass itself indeed increases in such a way that one can never accelerate an object beyond c.
I'd like very much to hear about the [twins paradox]?. Never been much comfortable with that one.
It's a convention, not an absolute. In the way that mass is used currently in physics, it's an invariant between reference frames, i.e. it doesn't change with velocity. Mass = "rest mass", and "relativistic mass" is not used. There is an alternate formulation of relativity that uses the concept of "relativistic mass" because using it lets you keep using some familiar Newtonian mechanics (e.g. F=m
r a). But the invariant mass approach turns out to be somewhat easier to generalise into GR, so that's what basically everyone uses now.
You'll still find "relativistic mass" in some textbooks (e.g., Feynman's Lectures on Physics) and in a lot of popularisations (I think it's in "A Brief History of Time"), but not in, say, graduate level textbooks and research papers.
For more on this, see [
http://www2.corepower.com:8080/~relfaq/mass.html]. Come to think of it, this should probably be written up in mass. -- DrBob
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