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Energy question…

  1. Oct 19, 2004 #1
    According to Einstein’s equation “E = m c^2” an object that loses energy loses a VERY slight proportion of its mass. An object that, on the contrary, gains energy also gains a VERY slight increase in mass.

    Now, if an object (in space, or vacuum, say) moves or changes position, in general physics we say; the potential energy of the object has transformed into kinetic energy (because the object has moved). Thus it goes that the object lost part of its mass (or potential energy) as kinetic energy, where has the lost part gone or transformed into? Remember the law of conservation of energy states that energy lost = energy gained.

    Let me re-frame that: What happens to the K.E. after the object sets to rest? Is it re-transformed into potential energy so that the mass of the object returns the same?

    NOTE: I know I'm bullsh*tting, but.. what to do?
  2. jcsd
  3. Oct 19, 2004 #2
    The equation quoted is for the energy at rest. If the particle moves, thats not true.

    And remember, and object don't changes its linear momentum spontaneously, it does when something happens for example when a disipative force makes the object loose speed and this energy goes god knows where (internal work, heat, ...). So we can't imagine a isolated particle with a linear momentum p which spontaneusly reduces its speed and finally keeps in rest. Thats magic, not physics :)
  4. Oct 20, 2004 #3
    So you're saying when a particle moves (in vacuum and without gravity or any other force applying on it) it will just keep on moving straight forward?

    Let's say that a particle has a K.E. of 10 Joules; is this embodied by the continuous motion of the particle or does the the particle slows down as the K.E. is used up?
  5. Oct 20, 2004 #4

    Kane O'Donnell

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    That's precisely what will happen! (Newton's first law)


  6. Oct 20, 2004 #5
    I am not sure I understand your question. Are you asking how does the KE of the mass m that releases a photon change? If so, the answer lies in the momentum imparted by the photon to the mass. It imparts E/c momentum to the mass so the mass experiences a change in speed of v=E/mc
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