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Homework Help: Invariant mass

  1. Mar 4, 2008 #1

    I'm working on this problem and I'd like to know how to find the invariant mass using just the lab-frame momentum and rest mass.

    I've found a lot of equations that deal with E, and i'm not completely sure what that is either.

  2. jcsd
  3. Mar 4, 2008 #2


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    You have to tell us "this problem".
  4. Mar 4, 2008 #3
    I have a proton with momentum = 1GeV hitting a neutron at rest, and I'd like to find the CM-momentum before collision.

  5. Mar 4, 2008 #4


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  6. Mar 4, 2008 #5
    More work...

    So here's what I have so far...

    E* = (Ep* + En*),

    Ep* = Mp + Pe
    En* = Mn

    Pe = momentum of electron in lab frame
    Ep* = energy of proton in CM frame
    En* = energy of neutron in CM frame
    Mn/Mp = mass of neutron/proton

    Is E* = Invariant mass? If so, I've got this problem done.
  7. Mar 5, 2008 #6
    As measured in an inertial frame of reference - If m0 = invariant mass of system, p = total momentum of system and E = total inertial energy of the system then

    E^2 = m02c4+(pc)2. Simply solve for the invariant mass m0 of the system and you have you're answer.

    Last edited: Mar 5, 2008
  8. Mar 5, 2008 #7
    Thanks for your response. I found that exact equation in many texts and I haven't a clue what the total inertial energy of the system is.

    In my particular problem, would it be E = KE(proton) + Mass(proton) + Mass(neutron)?
  9. Mar 6, 2008 #8
    The total inertial energy, E, of a particle is the sum of the particle's rest mass and its kinetic energy. The total energy, W, of a particle is the inertial energy + potential energy. That is to say that

    E = K + E0

    W = E + V

    Best wishes

  10. Mar 9, 2008 #9
    Thanks much.
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