1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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...

    3. The attempt at a solution

    Equaling the two would give you:


    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??

  2. jcsd
  3. May 15, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper


    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?

  5. May 15, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I have a big problem with a photon's velocity being less than c.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook