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Homework Help: Determining momentum from energy?

  1. Nov 17, 2013 #1
    1. The problem statement, all variables and given/known data

    What is the momentum of a proton with 800MeV of kinetic energy?

    2. Relevant equations


    3. The attempt at a solution

    I know that E should be kinetic energy + rest energy, so I calculate rest energy first.

    rest energy = mc^2= (1.67*10^-27)*(3*10^8)^2 = 939.57 MeV. total E=939.57+800=1739.57 MeV

    Plug and chug and solve for p. m^2c^4=.1414MeV (this seems wrong)

    (1739.57)^2-.1414 = 3026103.644 MeV, divide that by (3*10^8)^2 and take the root to get p, which comes out to be 5.7985*10^-6. My answer needs to be in GeV, so it seems that I'm off by a factor of 40. I've been doing physics homework all day so my brain is fried and I've been beating my head against a wall for the last hour trying to solve this problem. I know that I'm missing something stupid, but I just can't see what it is. Help!
  2. jcsd
  3. Nov 17, 2013 #2
    Scratch this one, I got it!
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