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What is mass ?

  1. May 10, 2006 #1
    What is mass ??

    Text books define it as the "amount" of substance...
    You can also use some equations and toss Mass out, giving other definitions.

    But my question is about the wording "amount of substance"
    > What is this "amount of substance" ?

    > Because "amount" refers to counting..
    How can you count this "substance" ?

    > I know that mass of a cube of butter is not the amount of sub-atomic particles that butter has... then what is it ???

    > Some humans have spread the thought that the number & unit 1Kg, refers to the idea of the stuff kept here
    [​IMG]

    This means
    the amount of matter present in 1KG of butter is same as the amount of matter present in the reference Platinum-Iridium Alloy. ???
    How should I digest the concept ?
    How can I compare Butter & the Pl-Ir Alloy


    > I am fed up of circular definitions where
    for eg. Mass can be defined in terms of Force, accel, etc. ..and Force in turn will be defined interms of Mass, accel etc.

    :blushing:
     
    Last edited: May 10, 2006
  2. jcsd
  3. May 10, 2006 #2

    russ_watters

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    Why not?
    Have you taken chemistry yet? Consider this: the mass of an atom is the mass of the protons, neutrons, and electrons added together and the mass of a glob of those atoms is the mass of a single atom times the number of atoms. Why should butter be any different? Its just more types of atoms - but still, the fundamental building blocks are the same and have the same mass as the fundamental building blocks in that hunk of platinum in Paris(?).
     
  4. May 10, 2006 #3
    I kind of think of it this way, mass is to energy as area is to length.....

    But im prob wrong...
     
  5. May 10, 2006 #4

    Astronuc

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    Length is one dimension spatially, where as area is 2-dimensional spatially.

    Mass is defined as the property of a substance (matter) to resist acceleration, and hence represents a proportionality between force and acceleration.

    http://whatis.techtarget.com/definition/0,,sid9_gci549408,00.html

    It is dimensionless, i.e. it is not a dimension in the spatial or temporal sense.

    Energy is the capacity of a physical system to do work.
    http://searchsmb.techtarget.com/sDefinition/0,290660,sid44_gci213564,00.html

    Energy may be proporational to mass, e.g. kinetic energy KE = 1/2 mv2.

    And then there is potential energy.
     
  6. May 10, 2006 #5

    DaveC426913

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    Mass is the drag that an object experiences by its resistance to the Higgs field.
     
  7. May 10, 2006 #6
    Why is that the concept of Force & Accel brought in ? :confused:
    Are they not concepts that require time & happen over time OR requires time as a component ?? :uhh: {For eg. a push or pull happens over time and without bringing in time, the concepts of Force & Acceleration might go absurd..doesn't it ??:eek: }

    Can't the idea about mass be brought up without any other basic units ???
    [offtopic] I am a Satch fan too :) [/offtopic]

    Do you mean to say that the amount of protons/neutrons etc on 1Kg of butter is equal to what is present in 1Kg of that alloy ?
    OR
    Do you mean to say that the total mass of the protons/neutrons etc on 1Kg of butter is equal to what is present in 1Kg of that alloy ?
     
    Last edited: May 10, 2006
  8. May 10, 2006 #7
  9. May 10, 2006 #8

    ZapperZ

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    The problem with questions such as this has always been defining the bondary of knowledge to which the answer can be given satisfactorily. At some point, if you want the "full-blown" answer, you have to be capable of understanding such an answer, i.e. your boundary of knowledge must be the same as those people who are actually doing research in such a field.

    When you start with only wanting to ride a tricycle, then your ability doesn't require that much sophistication since it is realtively easy to learn how to ride such a thing. But if you then say "OK, I now want to be able to drive a car", then you just simply cannot get behind the wheel and drive around blindly. You have to learn how to drive, acquire sufficient skills to drive, learn all the traffic rules, and pass some tests to be allowed to drive.

    When you ask "what is mass", you can be given a pedestrian answer that are often sufficient for most purposes. But if you go beyond that and start to extend the boundary to include energy conversion, field theory, gravitational versus inertial mass, etc.. etc., then it gets very involved and requires a solid understanding to be able to comprehend the type of answer being given. For example, would it suffice to say that something with a mass will interact with the Higgs field, while those that don't have any mass will not? This would be the identical answer one would be given when asked for the definition of a "hardron" (something that will interact with the gluon field).

    In the end, and this is often true in physics, one needs to figure out if a question actually has any importance to it. Remember, in science, something that is interesting isn't necessary also be important (when one is in this field for a long time, one tends to know how to pick and choose which is which). One needs to figure out where such a definition would be meaningful and important. Was it that important to figure out if the electron neutrino has a mass, or was it more important to discover the neutrino oscillation that implicated that it has a mass? It was the former, not the latter. We simply have waaaay too many stuff on our plates to worry about something that has very little impact.

    Zz.
     
    Last edited: May 10, 2006
  10. May 10, 2006 #9

    Gokul43201

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    The first implies the second, and hence, both are true.

    The idea of calling mass "the amount of stuff" is a form of passing the buck. What then is "stuff", and what is an "amount" ? But in a sense (ie : if cleaned up some), it does serve as a pretty good approximation. And this is the purpose that it needs to serve for anyone that is not actively involved in research in the field.

    Where it doesn't work so well is in answering a question like : Does a "box" with 2 neutrinos have more mass than a "box" with one neutrino ? Then you're back to the more basic question about a specific property of a fundamental particle.

    On the other hand, it's perfectly sound to say that mass is nothing but the proportionality constant in some specific relation, so long as all the appearances of the term mass can be derived from that same relation. If you are okay with accepting what a spring constant is, why not mass ?
     
    Last edited: May 10, 2006
  11. May 10, 2006 #10
    Mass is that property of matter which gives it momentum. I.e. it is the m in p = mv. Mass resists changes to momentum. Mass gives a body inertia.

    Pete
     
  12. May 11, 2006 #11
    mass is the resistence to acceleration or a measurement of inertia
     
  13. May 11, 2006 #12
    This statement, while it works somewhat in non-relativistic mechanics, is better stated as "mass is the resistance to changes in momentum." Otherwise you'll get people writing the wrong definition of force, i.e. F = ma, rather than the correct definition of F = dp/dt. Hence my phrasing above.

    Pete
     
  14. May 11, 2006 #13

    Astronuc

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    Just some of my thoughts

    I think that given the definition of momentum as the product of mass and velocity, that a correct definition of mass is that property which resists change in velocity, i.e. acceleration. I suppose its a matter then of semantics, or rather how one gets one's mind around the concept of mass.

    Then one has to look at rest mass vs relativistic mass. In which case, mass resists not only change in velocity, but change in itself, since m is dependent on v. In that sense, mass is that property which resists change to momentum.

    Face it classical mechanics is seemingly more tangible than relativity and quantum mechanics, and I think that is where the problem lies for some people. Relativity and QM, and related areas require an ability to abstract beyond direct experience (observation).

    With regard to
    Well, mass exists in space-time, and as far as we know, there are three fundamental spatial dimensions and one temporal dimension, and they coexist. For [itex]\Delta{t}[/itex] = 0, mass and existence would be meaningless.
     
    Last edited: May 11, 2006
  15. May 11, 2006 #14

    Gokul43201

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    Am I missing something here ?

    If I double the mass, it does not take twice the force to produce the same change in momentum (it still takes the same force).
     
    Last edited: May 11, 2006
  16. May 11, 2006 #15

    Astronuc

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    Last edited: May 11, 2006
  17. May 11, 2006 #16

    Gokul43201

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    I didn't disagree with that. I disagreed with the definition of mass as that which resists change in momentum.
     
  18. May 11, 2006 #17
    I try to avoid saying that mass is what resists changes in momentum. If I said that here then I retract it. The most precise definition of mass that works best is that of Weyl and that's m = p/v. The definition of mass as that which resists changes in velocity can be attributed to Mach as I recall.

    Pete
     
  19. May 11, 2006 #18
    Not sure if there is any way you can get the discription more dumbed down then that. Mass is how much "stuff" there is there. There that is my best effort if you don't understand it then I don't know what to say.

    Obviously, you count it, and the amount of it is reprisented as Mass.

    That is basically what it is, it is actually a bit more complicated then that, but at a basic level that is exactly what mass is.
     
  20. May 12, 2006 #19

    pervect

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    In many situations, the equation

    m = p/v

    will not be what the author is talking about, so assuming that this is a universal defintion of mass will be quite, quite, wrong.

    For instance, the mass of a photon is zero, however p/v for a photon is not zero.

    When we say the photon has no mass, we are talking about the SR notion of invariant mass:

    m = sqrt(E^2 - p^2c^2)/c^2

    (Note that this question about the "mass" of a photon comes up about once a week, so see any of the numerous threads for more information.)

    There are many other defintions of mass that are used in GR, three of the most common are ADM, Bondi, and Komar mass. This is not a complete list by any means, which should server as a warning that the subject starts to become complicated.

    The bottom line is that "mass" is an ambiguous concept. While for most pedestrian non-relativistic purposes one can use m = p/v, this defintion is hardly universal.
     
    Last edited: May 12, 2006
  21. May 12, 2006 #20
    The question "What is mass?" is not an easy one to answer. Modern textbooks never really get into this question very deeply so students go on to think that they've mastered the concept with a simple answer that they've satisfied themselves with. But the question is hardly simple. As Max Jammer comments in Concepts of Mass in Contemporary Physics and Philosophy, page ix
    If anyone truly wants to get into this question deeply then this book is a must read.

    Pete
     
    Last edited: May 12, 2006
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