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Relativistic Momentum of a photon

  1. Sep 28, 2013 #1
    1.
    An elementary particle of mass M completely absorbs a photon, after which its mass is 1.01M. (a) What was the energy of the incoming photon? (b) Why is that energy greater than 0.01Mc2?




    2. Relevant equations
    p (photon) = E/c
    p (particle) = γmv
    p(i) = p(f)
    Ek = γmc^2 -mc^2

    3. The attempt at a solution
    I really have no idea where to start without having knowledge of the velocity of the particle after the collision
     
  2. jcsd
  3. Sep 28, 2013 #2

    mfb

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    Just use v as velocity. Momentum conservation will give you that velocity during the calculations.
     
  4. Sep 28, 2013 #3

    vela

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    Was that the complete problem statement? I don't think the situation described is physically possible.
     
  5. Sep 28, 2013 #4

    mfb

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    It is possible, it is the reverse process of a gamma decay.
     
  6. Sep 28, 2013 #5

    vela

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    But that's with a nucleus, which has a substructure. I guess it seems a little odd to me to say that an elementary particle changes its mass by absorbing a photon unless it's referring to its relativistic mass.
     
  7. Sep 29, 2013 #6

    mfb

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    It becomes a different elementary particle. There are no suitable elementary particles for that process (and it would probably need weak higher-order processes), but I don't think we have to worry about that.
    "Mass" has to be rest mass, otherwise the second question does not make sense.
    Elementary just guarantees we don't have to worry about rotations and so on.
     
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