- #1

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Yet what is mass?

Mass is the quantity of matter.

This is a roundabout way of saying that you do not know what is mass.

Mass is not weight.

What is mass?

- Thread starter quantum123
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- #1

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Yet what is mass?

Mass is the quantity of matter.

This is a roundabout way of saying that you do not know what is mass.

Mass is not weight.

What is mass?

- #2

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- #3

HallsofIvy

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Mass is a measure of inertia.

- #4

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I have a problem with this dynamical definition of mass. It implies that in a static universe, there is no mass. And yet we know that all chemical substance has mass. For eg, we can say I want 4 grams of salt or 3 grams of coal, without having the salt or the coal moving. That is why mass is usually stated as the quantity of matter. It is also the reason why we don't say mass is weight because we can still have 4 grams of salt or 3 grams of coal even in the absence of gravity. Also, inertia is the resistance to force, and yet force is usually defined in terms of mass, so if mass is inertia, this will be yet another roundabout circle.

What is mass?

What is mass?

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- #6

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Then in classical mechanics, force is the fundamental quantity.

Force is defined as a push or a pull.

:)

Then mass is defined in terms of force.

And matter is defined in terms of mass.

Rather strange indeed that everything is defined in terms of a push and a pull.

In fact mass can also be defined thermally. A bigger mass of water is able to absorb more heat energy before having one degree rise in temperature. No push or pull is needed, just hot or cold! This is the definition that the chemists and other branches of science will be happier with. The dynamical properties might just be - properties only.

Force is defined as a push or a pull.

:)

Then mass is defined in terms of force.

And matter is defined in terms of mass.

Rather strange indeed that everything is defined in terms of a push and a pull.

In fact mass can also be defined thermally. A bigger mass of water is able to absorb more heat energy before having one degree rise in temperature. No push or pull is needed, just hot or cold! This is the definition that the chemists and other branches of science will be happier with. The dynamical properties might just be - properties only.

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Not necessarily. If you look at Lagrangian/Hamiltonian mechanics, there isn't even a "force". So no. It isn't a "fundamental" quantity.Then in classical mechanics, force is the fundamental quantity.

What a "mass" is, if you look at the Standard Model, is more of a "interaction" of objects that couple with a background field. If the Higgs bosons are detected, then, then we know that this way of looking at it is valid.

Zz.

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Sorry, I meant to make this clear by sayingNot necessarily. If you look at Lagrangian/Hamiltonian mechanics, there isn't even a "force". So no. It isn't a "fundamental" quantity.

To imply pre-lagrangian areas of study - that which you study in high school and the first couple of undergrad years.basicclassical mechanics

- #9

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I noticed that. However, the OP took what you wrote and generalized it to all of classical mechanics, which isn't correct.Sorry, I meant to make this clear by saying

To imply pre-lagrangian areas of study - that which you study in high school and the first couple of undergrad years.

Zz.

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Lagrangian or Hamiltonian or Newtonian is the same. They described the motions of masses, but do not tell you what a mass is , and its relationship with matter - that is the heart of the problem.

- #11

jtbell

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It doesn't have to be that way, conceptually. Think of "force" as "that which makes an object accelerate." A simple example is the force produced by a stretched or compressed spring. Hypothetically, we could define our unit of force as the force exerted by a standard spring (made in a specified way out of a specified material) compressed by a standard distance. Then we would have to introduce a proportionality constant into Newton's Second Law: F = kma.Also, inertia is the resistance to force, and yet force is usually defined in terms of mass, so if mass is inertia, this will be yet another roundabout circle.

Now the Second Law becomes something to verify experimentally, rather than a definition of force or mass. If we take our standard spring, compress it by the standard amount, and apply it to the standard mass, we observe that the standard mass accelerates by a certain amount. If we take two identical copies of the standard spring, compress them by the standard amount, and apply them simultaneously to the standard mass, we observe that the standard mass accelerates by twice the original amount. And so forth.

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This presupposes certain properties of the mass-gravity-field entity, whatever form it may have, does it not? We make theI think Newton's definition of "mass" is the best we can do, conceptually: "quantity of matter" or more colloquially, "how much stuff" an object contains.

- #13

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No one said there's no "forces". It is just that the concept isn't "fundamental" in Lagrangian/Hamiltonian mechanics, because a "force" is simply another "level" of operations.

Lagrangian or Hamiltonian or Newtonian is the same. They described the motions of masses, but do not tell you what a mass is , and its relationship with matter - that is the heart of the problem.

Furthermore, these "four fundamental forces" deals more with the

If you think the Hamiltonian/Lagrangian mechanis is no different than the Newtonian mechanics, then would you like to show me where in the Lagrangian of, let's say, a simple pendulum, is there a "force"?

Zz.

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