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E=mc^2 and its use

  1. May 4, 2012 #1
    1. The problem statement, all variables and given/known data
    see attached question


    2. Relevant equations



    3. The attempt at a solution

    I chose A but this is wrong :S - why?

    The answer is C but I cannot see why B/D are wrong.
    I literally guess A as this was the only time I had used the fomula
     

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  2. jcsd
  3. May 4, 2012 #2
    A is wrong because, for example, a proton can have kinetic energy separate from its rest-mass energy.
     
  4. May 4, 2012 #3
    so why is C correct and B/D wrong?
     
  5. May 4, 2012 #4

    Andrew Mason

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    Science Advisor
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    First of all, I think A is wrong because there is no change in mass. B is wrong because you cannot determine the original mass of coal from just the rest mass of a proton and rest mass of an electron. You would have to have other information. Similarly with D you would cannot determine the energy from the rest masses of a proton or electron. You would have to know how fast the protons were moving before collision.

    AM
     
  6. May 5, 2012 #5
    thanks
     
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