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Einstein's energy equation and kinetic energy

  1. Nov 19, 2004 #1
    Why is the kinetic energy given by [tex] E=\frac{1}{2}mv^2 [/tex]
    yet Einstein's energy equation is [tex] E=mc^2 [/tex]?
    Why is there a different constant (ie [tex] \frac{1}{2}[/tex] and [tex]1[/tex])?
  2. jcsd
  3. Nov 19, 2004 #2


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    Er... because they are NOT the same beast, so why should they be identical? One is the energy of motion, the other is the REST mass energy.

  4. Nov 19, 2004 #3


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    You are confusing relativistic energy with classical kinetic energy.
    If I remember correctly, the relativistic energy of a particle with rest mass m is given by:
    (where v is the measured velocity of the particle)

    In the low speed limit, we see that the relativistic energy can be written as the sum of the rest mass energy ([tex]mc^{2}[/tex]) and the classical kinetic energy.
    Last edited: Nov 19, 2004
  5. Nov 20, 2004 #4
    hi dirtydog,
    as zapperz says, thery are not the same thing. kinetic energy (KE) is a form of energy that a body has due to its motion, while the other eqn describes the total amt of inherent energy that a body has due to its mass.

    therefore for the KE, a body's KE can be increased by increasing its speed. a drop in speed would therefore cause the KE to drop. an obj at rest would then hav 0 KE.

    as for E = mc^2, no matter the body is moving or not, the E here remains the same if the body's mass does not change. u can onli vary this E by varying the mass of that body. therefore this eqn gives u the total amt of energy that is 'associated' with a certain mass.

    hope that helps to clarify things.
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