Undergrad "relativistic mass" still a no-no?

[PeterDonis]

I am closing the thread for moderation.

 

[PeterDonis]

A number of off topic posts have been deleted and the thread is reopened.

 

[rude man]

The whole thing could have been avoided had the old way of calling rest mass m₀ and the relativistic mass m = γm₀. Then E=mc² would have survived, also centripetal force mv²/r. Like Regietheater opera, dumbing-down prevailed however. I was happy to see that even in the new millennial edition of Dr. Feynman's Lectures on Physics the editors decided to stick with what did not need fixin' 'cause it wasn't broke.

 

[Herman Trivilino]

The whole thing could have been avoided had the old way of calling rest mass m₀ and the relativistic mass m = γm₀. Then E=mc² would have survived, also centripetal force mv²/r.

You cannot make the Newtonian expression $\frac{mv^2}{r}$ valid by replacing $m$ with the relativistic mass.

Like Regietheater opera, dumbing-down prevailed however. I was happy to see that even in the new millennial edition of Dr. Feynman's Lectures on Physics the editors decided to stick with what did not need fixin' 'cause it wasn't broke.

Feynman states that you can replace $m$ in the expressions of Newtonian physics with the relativistic mass and create relations that are valid. It's been discussed in the literature that such a notion is in general wrong. There are a few important and often-used relations where that can be done, but in the general case it's not valid.

In other words, it's an oversimplification.

The fact is, high energy physicists have never changed the practice of referring to only one kind of mass in their work and in their professional publications. Some of them, when authoring books and articles for the public, have used the concept of relativistic mass.

I'll leave it to you to decide which arrangement is a "dumbing down".

The main argument for doing away with it was indeed that it allowed survival of $E=mc^2$. To people trying to learn physics it was obscuring the true meaning of Einstein's mass-energy relation. A misconception that often persisted into the professional phase of a physicist's life.

 

[SiennaTheGr8]

The whole thing could have been avoided had the old way of calling rest mass m₀ and the relativistic mass m = γm₀. Then E=mc² would have survived, also centripetal force mv²/r. Like Regietheater opera, dumbing-down prevailed however. I was happy to see that even in the new millennial edition of Dr. Feynman's Lectures on Physics the editors decided to stick with what did not need fixin' 'cause it wasn't broke.

The whole thing could have been avoided if Einstein and his contemporaries had stuck with either "mass" or "energy" for all mass/energy terms. Instead they adopted a confusing mishmash of various $m$'s and $E$'s.

There's nothing that $m$ quantifies that $E$ doesn't. They're the same quantity in different units.

There's nothing that $m_0$ quantifies that $E_0$ doesn't. They're the same quantity in different units.

Why we've ended up with $E$ and $m_0$ (now usually just called $m$) is beyond me. I use $E$ and $E_0$. If an answer is needed in mass units, I convert.

 

[DrStupid]

You cannot make the Newtonian expression $\frac{mv^2}{r}$ valid by replacing $m$ with the relativistic mass.

You can. It's one of the special cases where it works. If you should it's another question.