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Homework Help: Relativistic speed of a particle

  1. Aug 13, 2016 #1
    1. The problem statement, all variables and given/known data
    Find the speed of a particle whose total energy is twice its rest energy.

    2. Relevant equations
    E2 = p2c2 +m2c4

    3. The attempt at a solution
    4m2c4 = p2c2 +m2c4
    3m2c4 = p2c2
    SQRT(3)mc2 = pc
    SQRT(3)mc = p
    SQRT(3)mc = mv
    SQRT(3)c = v
    I know I can use E = gamma*mc2 to get the answer, but what have I done wrong in my approach? Thank you.
  2. jcsd
  3. Aug 13, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Your mistake was thinking that momentum equals mv. That's not true in relativity. In relativity, p = gamma*mv.
  4. Aug 13, 2016 #3
    Thank you. I appreciate the help.
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