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KE and relativity

  1. May 27, 2009 #1
    The wikipedia article on Kinetic Energy: http://en.wikipedia.org/wiki/Kinetic_energy

    "Kinetic energy for single objects is completely frame-dependent (relative). For example, a bullet racing by a non-moving observer has kinetic energy in the reference frame of this observer, but the same bullet has zero kinetic energy in the reference frame which moves with the bullet."

    If two people in separate reference frames disagree on the kinetic energy of a bullet, do they also disagree on the mass? (I'm thinking of E=mc^2 here)
  2. jcsd
  3. May 27, 2009 #2


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    Well, yes, of course. If an object has rest mass m0 (the mass in a frame relative to which it has speed 0), then an observer moving at speed v relative to the object it has mass [itex]m_0/\sqrt{1- v^2/c^2}[/itex]- the faster it moves (relatively, of course) the greater the mass.
  4. May 27, 2009 #3


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    So the answer to your question is "yes" if you mean relativistic mass, but "no" if you mean invariant mass.
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