I suppose you're right, assuming that E=M(C^2) is a relativistic equation, the questions would be in relativistic terms?

Though, as ER pointed out I'm a slight bit of a beginner (understatement, I know, no worries), and perhaps can't FULLY distinguih between classical/Newtonian and relativistic/Einsteinian equations.

Again my basic questions were what is mass in real terms, and how does it relate to energy in real terms.

IE, energy and mass seem intimately related, according to their interchangeability via the formula E=M(C^2). And I'm wondering what exactly "mass" is. Since it's NOT the same thing as weight.

Basic answer so far seems to be that mass is "resistance to acceleration." IE, it's a relationship between energy input into a system and how much the system is accelerated (or not). Yeah?

So, what does that mean, exactly? That's my main question. My initial query was based on an similarity between the equation for Intensity |I|=P/4Pi(r^2) and a rearranged M=E/(C^2) or M/(4*Pi)=E/(4*Pi*(C^2))

Though I now see that C^2 is actually (D^2)/(T^2) or velocity squared, and not strictly a distance squared.

So actually, the rearranged equation could more acurately be expressed as:

M/(4*Pi) = E/( 4*Pi * ((D^2)/(T^2)) )
or
M/(4*Pi) = ( E * (T^2) ) / ( 4*Pi*(D^2) )

The denominator of the last bit is more akin to the intensity/power relationship, since it just involves 4Pi and a distance (in this case, I'm assuming the distance light travels in a second?).

Was the above mathematically correct? If so, it looks like we can alter it slightly again to say: M/(4*Pi*(T^2)) = E/(4*Pi*(D^2)) Which simplifies, I suppose, to M/(T^2) = E/(D^2), though I prefer it with the 4Pi, since it more closely approximates the int/pwr relationship.

If one wanted to make the right side look exactly like the Power side of the int/pwr relationship ( P/(4Pi*(r^2)) ), one could theoretically alter it slightly to say:

(E/T)/(4*Pi*(D^2))

of course one would also have to likewise alter the other side of the equation to say:

(M/T)/(4*Pi*(T^2))

So the whole thing looks like:

(M/T)/(4*Pi*(T^2)) = (E/T)/(4*Pi*(D^2))
or
(M/T)/(4*Pi*(T^2)) = P/(4*Pi*(D^2))
or
M/(4*Pi*(T^3)) = P/(4*Pi*(D^2))

So, we've got Mass/(4Pi*(Time^3)) = Power/4Pi*(Distance^2)

This seems like a slightly better approximation of my original question now that I know the somewhat more correct equations for velocity, momentum, acceleration, force, power, etc.

So, more accurately (mathematically, assuming I understand it right and didn't fudge the math; please tell me I didn't fudge!):

Power/(4*Pi*(r^2)) = |I| = Mass/(4*Pi*(Time^3))

So, I suppose the question becomes whether M/(4*Pi*(T^3)) has any meaning...? Anyone care to wager a guess?

It's funny, but I don't quite know how to look up that formula online... I tried 4PiR3, but it came up with weird stuff (one of them being the dipole definition on Wikipedia, interestingly enough), let me try again with 4Pi(R^3).

Trippy, if you enter that into google you get:

4 * Pi * (R^3) = 2.15472747 K^3 and then a link to look for it in documents. But it seems to be very clear that (4 * Pi * (R^3)) is equivalent google-ly to (2.15472747 (K^3)), but with no more description of what that actually means...??

On trying to search for an article I got I think a spanish article off wikipedia, which I think translated to sphere. A sphere's surface area is 4Pi(R^2), its volume is (4*Pi*(r^3))/3

Granted, the T in the mass equivalent of intensity is "time" or seconds. So, not entirely sure what 4*Pi*(T^3) would MEAN... Especially in the denominator...?

Of course we could break it down into T*(4*Pi*(T^2)). Which would be the surface area of a sphere multiplied by time. OF course the sphere itself seems like it would be a sphere that uses time as a measure? Like a time bubble? Or something? Okay, this is getting weird, and the brain can't handle any more. ;o]

So, if anyone knows what Mass/(4*Pi*(Time^3)) means... Kudos! Brava or bravo... Mind bender.

Or I suppose M/(T^3) = P/(D^2) = |I|*4Pi would be a (mostly) simpler version? Any thoughts on either the more or less complex version? I know, heady, trippy stuff, right?