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Homework Help: Is this a misprint in my SR homework?

  1. Dec 7, 2013 #1
    1. The problem statement, all variables and given/known data
    "In frame of reference S, an electron moving along the x-axis has
    energy 3mc2 and momentum magnitude √(8)mc
    Use the transformations of energy and momentum to find the energy and momentum magnitude observed in
    frame S′ moving with speed 4c/5 relative to S in the positive x-direction."

    2. Relevant equations
    How can the electron have a proper velocity of c?

    If relativistic velocity is dx/dτ = γdx/dt, and relativistic momentum is simply the mass time the relativistic velocity, how is m(γ(dx/dt)) = √8mc?

    3. The attempt at a solution
    I solved the problem correctly, I am just unsure of a detail in the wording of the problem.
  2. jcsd
  3. Dec 7, 2013 #2


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    How do you get that the electron has a proper velocity of c?

    What is it about m(γ(dx/dt)) = √8mc that you find disconcerting?
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