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Missing v?

  1. Oct 7, 2005 #1
    for the following question:
    dynamite liberates about 5.4*10^6 J/kg when it explodes. What fraction of its total energy content is this?

    my problem:
    the total energy formula=(gamma)mc^2
    but the gamma factor has v, but the question doesn't have v...
    any suggestions?
  2. jcsd
  3. Oct 7, 2005 #2

    Tom Mattson

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    Assume that the dynamite is at rest.
  4. Oct 7, 2005 #3
    if it explodes, it shouldn't be at rest, right?
  5. Oct 7, 2005 #4

    Tom Mattson

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    If the stick of dynamite is at rest prior to the explosion, then its center of mass will surely be at rest afterwards. Also, the total energy is the same before and after the explosion.

    So what would you rather do? Would you rather calculate the energy content after the explosion, including the kinetic energy of each individual fragment? Or would you rather calculate the total energy of a stationary stick of dynamite whose mass is known?
  6. Oct 7, 2005 #5


    Why not?
  7. Oct 7, 2005 #6


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    Staff: Mentor

    Initially they dynamite is at rest, and so the total available energy is only the rest energy. The energy released in the explosion is the stored chemical energy.

    Simply use E = mc2 as Tom mentioned, where m is the rest mass.

    Hopefully one should discover that the specific chemical energy <<< rest energy.
  8. Oct 8, 2005 #7
    i see... thanks!!! :)
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