When talking about forces, you often assume the mass to be as if it were all acting at one point, so you'd measure the gravitational attraction between the Sun and the Earth from the centre of the Sun to the centre of the Earth. For spherical objects, this is usually a fairly accurate approximation, but it is less so for non-spherical objects. You may learn in school that mass is the amount of matter, but this is not strictly true (the mole is used to measure the amount of substance), mass is the resistance to motion of a body.

But whether you're assuming a black hole to be a sphere or not, it isn't going to change the actual magnitude of the mass. A black hole cannot have infinite mass. If it had infinite mass, it would have infinite gravitational attraction, and the entire Universe would be pulled into it instantly. No, a black hole may have infinite density. It is a large star that has collapsed in on itself. The mass will be more or less the same, but it will be acting upon a very small area, and so will have infinite density. But this is only at the singularity.

Ah, yes, my mistake, I meant to put infinite density then. And I think you've thoroughly confused me--the resistance of a motion on a body is inertia...I thought...or...gah.

You broke me

Gravity is like a harsh mistress.

The differences between inertia and mass are very subtle. Newtonian physics say the two are exactly the same.
m = F / a means resistance to motion exactly. They're both measured in the same units, though it's often said that mass exhibits inertia.

There is a significant difference between speed and velocity (the latter is a vector unit), though the two are measured in the same units. However, with mass and inertia I don't think you'll meet too many frowns if you use the two synonymously.

I take it you don't get this in your first physics class.

Interesting...yeah. Mass=inertia (for all intensive purposes). I must say, that's quite a blow to everything I though (inertia is a property that mass has)

Another thing--concering black holes--wouldn't they be at absolute 0? The big problem with getting to there is you always have volume which teh mass takes up--considering that it has practically 0 volume, couldn't you say it's at Absolute 0? I dunno. Just a though.

"for all intents and purposes"

I always make that mistake >_<

It doesn't have practically 0 volume, it has practically infinite volume. We covered this before.


Also, the reason I say mass=inertia is that any definition you can put to 'inertia' you can apply to 'mass' just as well. You cannot have mass without inertia and you cannot have inertia without mass.

Well theoretically, the volume is how much space it occupies as well as how much space is contains, neh? So it's both huge and tiny... BRWOR!

Not for a black hole, sorry. :-)

When you enter a black hole, time becomes a spatial dimension. The volume of a black hole is the surface area of the event horizon multiplied by the length of time the hole exists.

About the inertia thing...

Inertia is not a property of matter. The relevent property is mass. Matter does tend to resist acceleration; this is described by Newton's first law. How much it resists acceleration is called mass. To explain the effects of Newton's first law, scientists once thought that matter had some actual physical substance, which they called 'inertia', that caused it to resist acceleration.

Thus, inertia is akin to ether (the substance that was once postulated to permeate space and be the medium that transmitted light waves, since all other known waves had to have a medium (eg. water waves, etc.), or phlogiston, the substance that was thought to flow out of combustable objects before Lavoisier demonstrated that the process actually involved oxygen.

So inertia is neither a property of matter nor a description of matter's tendencies. It is an explanation for phenomena that is a least a century out of date.

Why it's still taught in schools baffles me.

I know you are but what am I?

I'll keep that in mind the next time I get pulled into one.