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Homework Help: Relativistic CM frame Kinetic energy

  1. Apr 23, 2010 #1
    Now then, this is a past paper question for only 3 marks but its annoying me.

    2 identical spheres undergoing an elastic relativistic collision. The spheres move with a speed such that their CM frame kinetic energy is equal to their rest energy. Calculate the speed the spheres in the CM frame.
    Not entirely sure about this, i took the obvious option and calculated (root3/2)c as my speed.

    Any ideas?
  2. jcsd
  3. Apr 23, 2010 #2


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    Hi Bigfoots mum! :smile:

    (have a square-root: √ :wink:)
    Yes, that's the right answer :tongue2: …

    but how did you get it? :confused:
  4. Apr 23, 2010 #3
    I used Kinetic Energy T = E - mc^2

    where E is the total particle energy and mc^2 is the rest energy. Therefore gamma is 2, and we're away with the mixer.
    I just wasnt sure if this method would apply in the CM frame, but then im not sure why it wouldnt!
    Thanks for the response
  5. Apr 23, 2010 #4


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    All your favourite formulas apply in any inertial frame. :biggrin:
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