"It can attract something with an electric field, if it has a charge. It can attract or repel something magnetically if it has magnetic moment. It *must* attract things gravitationally, if it has mass. It *must* resist being pushed, if it has mass. It can glow in IR, if it warm at all. It can whistle Dixie if it has the right equipment. It can do all of these things at once without there being any contradiction at all.

As for why something with mass must resist acceleration, a puzzle: Remember the equation, F=MA (one more time). What happens to A, if you apply some F to an object that has zero M? (solve for A, with M=0)"

Okay, granted, there may be different factors in play, such as gravitation vs electromagnetism vs whistling Dixie...

One can't solve for A if M=0. If M=0 F=0 (rather, 0 * A is still 0). But, with F/M setting M=0 doesn't work 'cause you divide by 0. And that's generally not allowed.

So, you'd have to set M as a limit and say A = F/M with the understanding that as M -> zero, A -> infinity.

And vice versa, as M -> infinity, a -> zero.

Of course that depends on F, too (the above two statements rely on F being the same throughout). If F and M are changing at the same rate, IE, if somehow you're upping the force as you're upping the mass by the same ratio, A won't change, since you're just multiplying essentially by 1/1 which does nothing to A. If F is increasing, and M is decreasing (somehow) then a will take on a very sharp upward curve. If F is decreasing and M is increasing (perhaps F is being converted into internal energy or mass?), then acceleration will be decreasing... Etc.

Anyway, I assume that you were pointing to the limit of as mass -> 0 acceleration -> infinity and the opposite that as mass-> infinity, acceleration -> 0. Yup, that's the math definition of F=MA alright. But it still doesn't directly answer what "mass" is. It shows the effect of mass, as observed (more mass mean less acceleration; simple algebra). But it doesn't address the issue of what exactly mass is or HOW the mass resists acceleration (which was the whole point of the thread to begin with, to understand what mass *is*, what it *does*, and *how that relates to energy* gained or lost). I get what it *does* now (more mass = less acceleration; got it). Still working on the HOW it does it and what exactly mass IS part.

It seems to be a circular argument we're going in. On the one hand, I'm asking what's mass. On the other hand you're saying here's this equation. The equation explanation is circular looking at the equation tells us the definition of what the equation does for it in terms of calculating by USING mass. But it tells us nothing much about the structure of mass such that it exhibits the behavior described in the equation.

Anyway, I think SDAI-tech1 kind of answered my question in another spot in the thread, and it was actually along the lines I was figuring, in terms of mass being how effectively something absorbs and re-transmits energy. IE, if absorbtion and transmission are equal at any given point in time, then total energy remains constant (thus, in the world of M=E/(C^2) if total energy doesn't change neither does total mass). If, however, something absorbs more energy than it can re-transmit, then the internal energy (and mass) goes up, until the energy can be dissipated again. And the energy, according to the equations, factors in velocity which includes a directional vector component, thus conferring directionality to the energy (which seems to make sense in terms of imparted energy changing the object's velocity/acceleration). It's a pretty simple concept. Makes sense, methinks. So, kudos to SDAI-tech1 for weighing in with an attempt at a real-world definition.

SDAI-tech1's comments elsewhere...

Cheers,
~Michael