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Rest energy and Kinetic Energy of a Photon - velocity?

  1. May 15, 2009 #1
    1. The problem statement, all variables and given/known data

    Thanks for everyone that helped me with the physics questions! I had my physics 2 final today and got a 94!!! There was a question that I could not figure out for a long time. It goes like this.

    What would the velocity of the photon be if the Kinetic energy of the photon equals the rest energy of the photon? Express v in terms of c and anything else. Luckily, the question was a multiple choice, and the correct answer (which I guessed) was sqrt(3)/2 x c

    I still cannot figure out why this is true.


    2. Relevant equations

    K=1/2mv^2 --> I assume this is the right equation to use...
    Er=mc^2

    3. The attempt at a solution

    Equaling the two would give you:

    1/2mv^2=mc^2

    v^2=2c^2
    v=sqrt(2) x c

    This is obviously not true since you can't go faster than the speed of light. Can someone explain to me how the answer is sqrt(3)/2 x c and why I am getting this thing wrong? Am I using the right Kinetic Energy equation??

    Thanks!
     
  2. jcsd
  3. May 15, 2009 #2

    tiny-tim

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    congratulations!

    Hi max8404! Many congratulations!! :biggrin:
    oooh, I wish examiners wouldn't use "kinetic energy" in relativity :cry:

    they just mean total energy minus rest energy :rolleyes:

    so 2m = m/√(1 - v2/c2), so 1 - v2/c2 = 1/4 :wink:
     
  4. May 15, 2009 #3
    Re: congratulations!

    Hey Tiny Tim, thanks for your response, but I doon't quite get it. I understand the equation,
    E=KE+mc^2, but in this case, what would E be? Or am I once again looking at it the wrong way?

    Thanks!
     
  5. May 15, 2009 #4

    tiny-tim

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    E is the total energy (or relativistic energy), mc2/√(1 - v2/c2) :smile:
     
  6. May 15, 2009 #5
    duh. thanks a lot! I got it now! :)
     
  7. May 15, 2009 #6

    Redbelly98

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    I have a big problem with a photon's velocity being less than c.
     
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