Physics Help Forum What is mass?

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Apr 20th 2017, 02:10 PM   #21
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 Originally Posted by leesajohnson ...(Matter is anything you can touch physically) ...
Not according to the modern definition which is anything which has a non-vanishing stress-energy-momentum tensor. That means that even an EM field is considered matter.

 Originally Posted by leesajohnson Things that have a big mass are harder to move, or harder to stop than objects with little mass.
Since even light has inertial mass but is not considered to be able to start and stop its mass is said to determine its momentum. That means that it can make objects with non-zero proper mass change velocity.

 Originally Posted by leesajohnson Mass (also known as inertia) is a property of energy.
That is incorrect. It's not what $\displaystyle E = mc^2$ means if that's what you were thinking.

 Originally Posted by leesajohnson Mass (F = ma).
That's not how mass is defined in modern physics. Its defined by the relation p = mv

Last edited by topsquark; Apr 20th 2017 at 02:45 PM. Reason: Tweeked the LaTeX

 Apr 21st 2017, 05:28 AM #22 Senior Member     Join Date: Jun 2016 Location: England Posts: 163 Stoke the ashes of an old thread. Mass is a measure of the amount of "stuff" Generally it is applied to matter, but it can also apply to energy. If a large amount of energy is concentrated into a small region of space, then matter is created. I have the (rather vague) idea of matter as a dense form of energy and energy as a thin and diffuse form of matter, or rather they are different manifestations of the same underlying structure of the universe. What that structure actually "is" is very much open to conjecture.
Apr 21st 2017, 10:21 AM   #23
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 Originally Posted by Woody Mass is a measure of the amount of "stuff" Generally it is applied to matter, but it can also apply to energy.
That's an old definition of mass. It applies more to areas of science like chemistry than it does relativivity.
If a large amount of energy is concentrated into a small region of space, then matter is created.

 Originally Posted by Woody I have the (rather vague) idea of matter as a dense form of energy and energy as a thin and diffuse form of matter, or rather they are different manifestations of the same underlying structure of the universe.
The relationship between energy and mass/matter is often mistaken. Energy is not a physical concept but rather a "bookkeeping" system. That means that energy is really a constant which applies to a closed system. For details see:
What is Energy?

Mass is a characteristic of matter and should not be confused with being the same thing as matter. E = mc[sup]2[/sup] only means that a body which has mass m also "has" (in a loose sense) energy E. Nothing more.

 Apr 22nd 2017, 04:42 AM #24 Junior Member   Join Date: Nov 2016 Posts: 13 The quantity of matter present inside our body is called mass. SI unit is kg.
Apr 22nd 2017, 08:03 PM   #25
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 Originally Posted by sukalp The quantity of matter present inside our body is called mass. SI unit is kg.
So what? That in no way defines the term mass. In fact the term "quantity" as you used it is not well defined since quantity can also refer to volume.

 Apr 22nd 2017, 09:00 PM #26 Junior Member   Join Date: Nov 2016 Posts: 13 we can say quantity is just a amount or number. mass is a quantity of matter present inside our body. quantity are atoms only. as matter is made up of atoms.
Apr 23rd 2017, 03:54 AM   #27

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 Originally Posted by sukalp we can say quantity is just a amount or number. mass is a quantity of matter present inside our body. quantity are atoms only. as matter is made up of atoms.
There are a couple of issues with this statement.

For starters there is the question if the m in F = ma and the m in the gravitation equation are the same number. They are at least close to each other, ie $\displaystyle \frac{m_{F}}{m_{grav}} \approx 1$ to a ridiculous amount of precision, but so far there is nothing that says that they have to be the same.

The second problem is more easy to demonstrate. Muons are matter just as much as electrons and protons are, but there are no muons inside an atom.

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Apr 23rd 2017, 12:07 PM   #28
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 Originally Posted by sukalp we can say quantity is just a amount or number. mass is a quantity of matter present inside our body. quantity are atoms only. as matter is made up of atoms.
To be honest I get very disappointed when what I've already posted is ignored with no reason. If you claim its wrong then please explain why!

I explained that in modern physics, which includes relativity, non-gravitational fields are also considered to be matter. This definition was given by Einstein when he published general relativity in which he wrote in his 1916 GR review paper
 We make a distinction between "gravitational field" and "matter" in this way, that we denote everything but the gravitational field as "matter." Our use of the word includes not only matter in the ordinary sense, but the electromagnetic field as well.
The use of the term "mass" to denote the EM field dates back to early special relativity. It was published in Einstein's article

The Principle of Conservation of the Center of Gravity and the Inertia of Energy, Albert Einstein, Annalen der Physik, 20 (1906): 626-633.

 If we assign the electromagnetic field too a mass density ...
This use of mass to refer to fields and even photons is quite prominent in a great many modern relativity textbooks.

If can prove that wrong, please do so. Otherwise please stop ignoring it and implying or claiming otherwise. Thank you.

Apr 23rd 2017, 12:10 PM   #29
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 Originally Posted by topsquark For starters there is the question if the m in F = ma ...
Actually, even on non-relativistic mechanics F is defined as F = dp/dt, not F = ma.

 Originally Posted by topsquark They are at least close to each other, ie $\displaystyle \frac{m_{F}}{m_{grav}} \approx 1$ to a ridiculous amount of precision, but so far there is nothing that says that they have to be the same.
While true its a matter of both hypothesis, theory and experimental observation. If they weren't strictly proportional then the equivalence principle would be wrong.

 Apr 23rd 2017, 07:36 PM #30 Junior Member   Join Date: Nov 2016 Posts: 13 well frankly to say PMB i right. standard definition of mass is large body of matter having definite shape. this is just a basic idea.

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