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Does an A mass always give the same amount of energy?

  1. May 6, 2008 #1
    hello people,

    i am new at physics and i have a question that you will be able to answer !!

    supposedly we have a subject with 5kg mass...
    if we make it travel at the speed of light we will get E = 5 x 300.000 x 300.000 = 450.000.000.000 J is this the same amount of energy it will give off if we mix it with anti-matter???

    P.S (soory for the noobie question i just would like to learn more and more ! )
  2. jcsd
  3. May 6, 2008 #2
    Learning is always good; great question.

    A 5kg mass has (E=mc^2) energy in the form of its rest mass. If it were to perfectly annihilate into energy (from anti-matter presumably) then all of this energy would be released (as photons).

    You can't accelerate any massive body TO the speed of light - you can only get them close to the speed of light. If you do that however, you drastically increase the energy of the body to significantly more than the rest mass. At relativistic speeds, E=mc^2 isn't the total energy, just the rest mass energy.
    If you add the gamma (lorentz) factor, E='gamma'*mc^s -> that represents the energy stored in the net mass (the relativistic mass); but still does not include the kinetic energy.
  4. May 6, 2008 #3

    Does this mean that mixing matter with anti-matter can only give off electromagnetic energy?


    When we say "at rest" (invariant mass) we mean when the aren't any forces on the mass?(i read it on wiki and understood nothing)


    What's net mass??
  5. May 6, 2008 #4
    rest mass, is the mass in a reference frame in which the object is stationary. if you see an object moving past you, it has a mass (which i was calling "net mass") that is larger than its rest mass. Does that make sense?
    (If you put forces on the object - then it makes it even more complex - but don't worry about that).

    If you mix matter with anti-matter, if they collide / annihilate, they will give off only and purely E&M energy. Two things are noteworthy:
    first, often the E&M energy will reform into mass; second, matter and anti-matter don't always annihilate.
  6. May 6, 2008 #5
    I think you are thinking any object posesses mass moving at speed of light will have E=mc^2. E=mc^2 have nothing to do with object moving at the speed of light. It just saying if one convert pure mass into pure energy, that's what you should get. I think if an object with near c speed will still have conventional kinetic energy 1/2mc^2. Particle accerlator graph show that energy spike up to ifinity near speed of light. Of course that is due to scientists keep adding energy to the particle.
    Last edited: May 6, 2008
  7. May 7, 2008 #6
    so the mass of an object is different when moving and different when its stationary??

    and what kind of mass will E&M energy create??

    thanx for you assisstance friend
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