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Mass: Matter or Inertia?

by Gib Z
Tags: inertia, mass, matter
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May13-08, 10:50 AM
P: 87
Quote Quote by dmt740 View Post
Ok, I'm confused now. I thought inertia was the energy of a mass in motion. If the mass is not in motion its only energy is that of the mass itself. If a force is applied to the mass then the inertia is the mass plus the energy that was applied to it. Am I missing something?
Inertia is the property whereby an object in motion tends to remain in motion and an object at rest tends to remain at rest. In other words, inertia = mass. See Newton's Laws of Motion.

The energy of motion is called "kinetic energy."

The relationship between force and energy depends on the properties of the force. Specifically, it depends on the potential energy corresponding to the force.

It is important to keep in mind that Newtonian physics and relativistic physics are different theories based on somewhat different assumptions.

In Newtonian physics, mass and energy are completely different things. They are not equivalent.

In relativity, mass and energy are equivalent. Mass is a form of energy, and energy possesses inertia.
May13-08, 01:12 PM
P: 63
I think Gib Z was onto something that is important at a fundamental level. The terms matter, inertia, mass, and momentum bring different things to the table, so I wouldn’t sweep the differences under the rug.
  • The term matter helps us distinguish between physical objects and empty space or imaginary objects.
  • The term inertia helps us understand that a physical object doesn’t spontaneously and independently change velocity (accelerate) without interaction with an outside force.
  • The principle of inertia allows us to reliably credit 100% of acceleration to interaction with an outside force which we couldn’t very well do if the physical object were not 100% inert to self-acceleration.
  • The reliable relationship between force on an object and the acceleration of an object is called mass and can be measured in kilograms.

In the very important fundamental sense above, inertia has no units. An object, regardless of size, mass, weight, etc., can either independently and spontaneously accelerate or it can’t. Real, physical objects (matter) can’t. I'm trying to draw out in the series of statements above, that these terms are building blocks for the next idea and not all the same idea.

After speaking of the fundamentals, we move to other more day-to-day usages of the term inertia which can cause confusion:
In another important sense of the word, inertia has been taken to mean resistance to acceleration which then correlates to mass and can be measured in kilograms.
In yet another more informal sense, inertia can informally mean momentum: the product of mass and velocity.
May14-08, 05:44 PM
P: 119
well for my two cents which dont really address the op question

i recall reading that mass can be thought of in two ways

inertial mass> defining the mass by its inertia, or resistance to acceleration

gravitational mass> defining mass by the amount of gravity it creates

and interestingly if you determine the mass of an object by method 1, it will not be the same as method 2. i dont remember which one is ever so slightly larger.
May14-08, 10:43 PM
P: 32
Quote Quote by Gib Z View Post
Well my trouble stems from that I learned Inertia to be a property of all masses, the property that all masses will not accelerate unless a force is applied. To me, this was always just a property, like a square having adjacent sides at right angles.

Today in my Physics class I was told : Mass is a measure how much inertia as object has, when previously I had replaced Inertia with Matter. I was somewhat confused and even right now, I think that was wrong - its like having 2 cubes with one with edges 2 units and the other 5 units, and then asking which cubes edges were "more equal" to each other. I asked my teacher and he said just because it was a property didn't mean it wasn't quantifiable, like density. But his example of density doesn't seem to do it for me, the fact that an object has a density merely states all objects that have mass take up a finite volume.

So my question is: Is inertia quantifiable? If so, what are its SI Units? I asked my teacher that as well and he seemed to ignore that question =[

Thanks for any replies guys, greatly appreciated.
inertia is quantified by the object's mass.
May16-08, 04:14 PM
P: 99
to post #21:

i think it was eotvos that showed gravitational and inertial mass are equivlent.

einstien used some of that info for the principle of equivlence.

in other news,

my prof once said "if you are ever having a hard time sleeping try to define mass and you'll pass right out"
Count Iblis
May16-08, 04:55 PM
P: 2,157
The mass of an object is given by the total energy content (divided by c^2). So, an empty box with conducting walls has a mass that is slightly larger than the mass of walls alone due to the Casimir energy. If I remember correctly, the Casimir energy of a cube is positive unlike the case of two infinite plates.

So, it is not correct to say that mass is matter in the form of particles as the vacuum also contributes to mass.
Gib Z
May17-08, 05:54 AM
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Quote Quote by Count Iblis View Post
The mass of an object is given by the total energy content (divided by c^2). So, an empty box with conducting walls has a mass that is slightly larger than the mass of walls alone due to the Casimir energy. If I remember correctly, the Casimir energy of a cube is positive unlike the case of two infinite plates.

So, it is not correct to say that mass is matter in the form of particles as the vacuum also contributes to mass.
While I don't deny the truth of this statement, It doesn't quite cut it for me as a definition unless you can tell me a definition of energy that doesn't involve mass in any way.
May17-08, 08:58 PM
P: 99
energy is the ability to do work, if we let work be PdV instead of Fds you may be pleased. i dont have a problem with circular definitions
May18-08, 10:23 AM
P: 267
Gib Z, you're not alone. The confusion between inertia, mass, matter and momentum is still common today in text books and physics papers.
As kwestion said you are "onto something that is important at a fundamental level."
In quantifying one or the other we often negate or assume a certain relative quantity.
What is important, is to understand the fundamental conceptual differences first, so the math does not become ambiguous.

A body does not move of its own accord. This seems incredibly obvious from classical physics to today.
It essentially says, unless an external force is applied, a body will remain at "rest".
Why? Because in classical terms, inanimate bodies, are inert. (unable to move or act)
So matter is inert, force is the antithesis of matter, the two together are action. (not to be confused with motion)
The idea that a body in motion will remain in motion is just as obvious but is more easily understood as
a relative property of rest. A body at rest remains at rest even it that state of rest is "motion" with respect to another observer. So now a body is both at rest and in motion depending on the frame of reference taken.
But in either case, it still has inertia - it will not change of its own accord.
Now we want to quantify the force required to change the state of motion of this body.
When its at rest, with respect to one observer, no matter how fast it is moving with respect to any other, the force required is always the same. This force is a direct measure of the quantity of matter of the body and since motion does not change the total number of particles in a body, rest mass is constant.
As a measure of energy, that quantity of matter is called mass.
When the same body is measured by an other observer that measures the body to be in motion, the force required to bring it to rest depends on the speed of the body with respect to that observer. This force is called momentum. The momentum is a measure of the bodies mass and speed.
To recap:
Inertia is a property of Matter (the property that it remains inert, regardless its state of motion)
Mass is the constant, quantifiable energy of matter
Momentum is a relative measure of Mass.

Then along came Einstein and everyone started mixing metaphors.
As soon as E=mc^2 appeared everyone began to talk of mass as energy , which is fine as long as nobody
confuses a quantity of energy as a statement of a quantity of matter.
The most common confusion was energy expressed as relativistic mass.
This was analogous to saying: 2+2 = 4 unless you start with 3.
The remaining expressions are much less so, but still prone to confusion.
Gravitational mass, attractive gravitational mass, passive gravitational mass, "energy" of momentum, inertial "energy"...
May18-08, 09:30 PM
P: 1,534
Mass is a quantitative measure of inertia.
Loren Booda
May18-08, 11:15 PM
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Apparently, a photon imparting momentum to a mass (or vice versa) allows inertia for the mass but not for the photon.
May20-08, 09:11 AM
P: 13
Quote Quote by Loren Booda View Post
Apparently, a photon imparting momentum to a mass (or vice versa) allows inertia for the mass but not for the photon.
Forgive me, but... what?
Loren Booda
May20-08, 10:58 AM
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P: 3,408
If the definition of inertia includes changes in momentum, by its definition how do interactions of photons (non-inertial) or masses (inertial) differ otherwise?
May29-08, 10:32 PM
P: 37
Photons and mass are two different subjects. They both 'obey' the geometric properties of spacetime though. That is, both will 'bend' to gravity. Photons have what's called an instant acceleration, relative or rest mass do not. Otherwise I agree in that it is a 'muddy' question :)

We can slow down light and even stop it, when we do that it disappear, as far as I understand, only to show itself when we accelerate it again. Then again, I might be wrong here. On the other hand, electrons are not defined particles either, they just have a probability focus, right:)

And inertia is a property of relative / rest mass which differs from photons instant acceleration. But I have problems with exchanging mass for inertia as inertia is a intangible property concurring from mass, whereas mass does not, as far as I know, create itself by experience inertia.

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