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I found 5 different names for

  1. Jun 16, 2011 #1
    I found 5 different names for "mass", and I can't clearly translate them into my native language:
    1) Rest mass
    2) Inertial mass
    3) Invariant mass
    4) Relativistic mass
    5) Gravitational mass

    In Italian we have "inertial" and "gravitational", but due to "false friends" I can't be sure of their exact translation...

    F= m*a

    F= GMm/r^2

    Are masses from 1 to 4 the "m" in first formula, and the last one the "m" in second formula?
  2. jcsd
  3. Jun 16, 2011 #2
    Re: Mass

    In special relativity, we have the energy relation E=mc². The m is called the relativistic mass and it is given by

    where m_0 is the rest mass and v is the velocity of the object.

    So this covers 1 and 4.

    2 is basically the mass in your first formula, while 5 is the mass in your second.

    Invariant mass (3) is basically a term for the Lorentz invariant norm of the relativistic four-momentum, given by [itex]m_0^2[/itex].
  4. Jun 16, 2011 #3
    Re: Mass

    Thanks, but I still don't get how 2, 3 and 5 are related together and to 1 and 4 .
  5. Jun 16, 2011 #4
    Re: Mass

    You should look those up in Wikipedia online....there is even an Italian version.

    In very simple terms:
    rest mass and invarient mass are the same thing....and inertial mass and gravitational mass are also the same. But people did not always know this!!

    Relativistic mass relates to the total energy (increase) and gravitational change that takes place at speeds anywhere near "c", the speed of light....a fast moving mass appears to have increased energy and gravitational attraction.

    More here:
  6. Jun 16, 2011 #5


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    Re: Mass

    I believe this is false. From any objects frame, no matter how fast or slow it is from another frame, it never has increased gravity. This means that moving at 99.999999999999.....% the speed of light will NOT cause you to collapse into a black hole, contrary to some peoples beliefs.
  7. Jun 16, 2011 #6
    Re: Mass

    If you're talking about passive gravitational mass then I agree with you. However, I do not agree that active gravitational mass and inertial mass are the same thing, even though they may be proportionally equivalent.

    If the OP wants to split gravitational mass into it's two different types then he can increase his count to six. Yes, mass is a mess. :) I think I'm quoting someone there, but I can't remember who.
  8. Jun 16, 2011 #7


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    Re: Mass

    That's a good suggestion: there are links between the English and Italian wikipedias, listed under "In other languages" / "Altre lingue" in the left hand column.

    For example:

    Invariant_mass (="rest mass, intrinsic mass, proper mass or just mass") links to Massa_a_riposo (="massa invariante o massa intrinseca"), and it's pretty clear that both articles are talking about the same thing because they use the same equations.

    And I worked that out despite not speaking a word of Italian!
  9. Jun 18, 2011 #8
    Re: Mass

    I know for sure the second part is true; you may be right about gravity not increasing....but if so, then I don't understand how observed energy increases but gravity doesn't.

    In the frame of the fast moving mass itself, everything local appears as at rest...so there is no issue about black holes forming.
  10. Jun 18, 2011 #9
    Re: Mass

    I don't trust Wikipedia at all for scientific matters: most of times Italian page are just translations from English, sometimes bad translations... And I can't even trust orignal English source, being it written just by uncontrolled amateurs!
    Wikipedia is good for all but for science... :rolleyes:
  11. Jun 18, 2011 #10


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    Re: Mass

    You are right to be suspicious, but I find Wikipedia is often a useful starting point, then you can check with other sources whether it is correct or not. In my experience, it is often correct but sometimes wrong.
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