[Phys-L] mass, energy, and spacetime

[Brian Blais <bblais@bryant.edu>]

I was just reading John Denker's lovely spacetime intro, http://www.av8n.com/physics/spacetime-welcome.htm, and came across this quote which I have a slight problem with:

"
Do not let anybody tell you that mass is “equivalent” to energy. The rest energy is only part of the overall energy.
"

The two sentences are not mutually exclusive, and mass is, in a sense, “equivalent” to energy.  I'd prefer to use the phrase that mass is a form of energy, that in this way there is an equivalence between mass and energy - not the total energy, of course.  just as a hamburger is a form of energy, but not the entire energy. 

I think this is word-play, but still important because so many of these concepts can be confusing. 

On another note, I saw this:

"The idea of plotting x versus t as we have done here is completely standard. It’s something you should have seen in 8th grade (if not before), and seen many times since then."

I would hasten to add that I have never seen in 8th grade or earlier any plot of x vs t which looked like Figure 4 (http://www.av8n.com/physics/spacetime-welcome.htm#fig-ruler-cart-t-const-x-red-transp).  It is this non-orthogonal axis plotting which is the tough part for students, not the fact that we are plotting x vs t. 

Sometimes I think the guide assumes something is simpler (for the reader) than it really is.

Still, a very nice introduction!  I'm just putting this out to see what others think.

	thanks!
			bb

-- 
Brian Blais
bblais@bryant.edu
http://web.bryant.edu/~bblais
http://brianblais.wordpress.com/