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Homework Help: Zwiebach quick calculation 8.7

  1. Sep 28, 2007 #1
    1. The problem statement, all variables and given/known data
    http://books.google.com/books?id=Xm...ts=cV3Uhs4Tnr&sig=C_bLwW8j_Nyc4sM0fJWooLkGLDY
    I am trying to do QC 8.7. If I use the equation given on page 127, l = 2/pi E/T0, I do not think that produces right answer. Is there another equation I should use
    ?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Sep 29, 2007 #2
    You should use equations (3.88) on page 54 but replace the equation for G with the equation for [itex]\alpha'[/itex]. Then follow the technique on that page.
     
  4. Sep 29, 2007 #3
    Sorry, can you explain what you mean replace the equation for G with the equation for alpha' (8.63 I assume)? G and J/hbar do not even have the same units.
     
  5. Sep 30, 2007 #4
    There are three equations labeled (3.88). Replace all of the P subscripts with S. Toss out the equation that defines G and use equation (8.63) in its place. Express [itex]\sqrt{\alpha'}[/itex] in terms of [itex]{l}_S, {m}_S,[/itex] and [itex]{t}_S[/itex].
     
  6. Sep 30, 2007 #5
    OK. I see why that works. My only concern is how do you know that you can replace the p subscripts with s subscripts. Those relations are only true in the Planckian system of units, right?
     
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