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Why c^2 in e=mc^2 ?

  1. Aug 14, 2010 #1
    Why c^2 in "e=mc^2"?

    Ok, i'v got that e=mc^2 meens some mass at max possible speed will equal max possible energy.
    But max possible speed is c, not c^2. Nothing can have speed greater than speed of light, isnt it?
     
  2. jcsd
  3. Aug 14, 2010 #2

    cjl

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    Re: Why c^2 in "e=mc^2"?

    You're kind of misinterpreting the equation. It doesn't mean that the mass has the maximum possible energy at maximum speed. It means that the mass itself has an inherent energy, even when it isn't moving, that is equal to m*c2. The full equation is actually E2 = (m*c2)2 + (p*c)2, which accounts for any motion that the object has in addition to the rest energy.

    Oh, and part of the reason that the c needs to be squared is simply for the units to work out - mass times velocity gives units of kg*m/s, which is momentum. Energy has units of kg*m2/s2 (J).
     
    Last edited: Aug 14, 2010
  4. Aug 14, 2010 #3
    Re: Why c^2 in "e=mc^2"?

    Exactly. In my view this is like do some precise mechanism with duct tape.
    There is no objectivity reasons for c^2 in this formula, only for the units to work :/
    Imho, the formula must be
    E=mc
    what do you think? :)
     
    Last edited: Aug 14, 2010
  5. Aug 14, 2010 #4

    cjl

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    Re: Why c^2 in "e=mc^2"?

    You'll notice I said that was part of the reason, not the whole reason. In order for the equation to be valid, the units must work out, but that alone isn't sufficient.

    I remember that there's a fairly good explanation for why E=mc2 is correct, but I don't remember it off the top of my head at the moment. I might have to glance through one of my textbooks to refresh my memory.
     
  6. Aug 14, 2010 #5
    Re: Why c^2 in "e=mc^2"?

    I think your units don't work.

    You could give mass the units of energy and have E=m.
     
    Last edited: Aug 14, 2010
  7. Aug 14, 2010 #6
    Re: Why c^2 in "e=mc^2"?

    Yes it would not work. But squaring с isnt correct cause light cannot exist on the speed of c^2.
     
  8. Aug 14, 2010 #7
    Re: Why c^2 in "e=mc^2"?

    The equation is not suggesting light can reach speeds of c^2 at all.
     
  9. Aug 14, 2010 #8
    Re: Why c^2 in "e=mc^2"?

    The rest mass of a photon is zero, so its energy is E=pc.
     
    Last edited: Aug 14, 2010
  10. Aug 14, 2010 #9
    Re: Why c^2 in "e=mc^2"?

    how? when? where? 0_o
     
  11. Aug 14, 2010 #10
    Re: Why c^2 in "e=mc^2"?

    What are you talking about?

    Take for example the classical kinetic energy which is K=1/2 m u^2. This doesn't mean that the speed of particle is u^2.
     
  12. Aug 14, 2010 #11

    russ_watters

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    Staff: Mentor

    Re: Why c^2 in "e=mc^2"?

    Again, C^2 is not a speed so what you are saying is meaningless. Check the units!

    ....and consider that this equation is very similar to the kinetic energy equation: e=.5mv^2
     
  13. Aug 14, 2010 #12

    Dale

    Staff: Mentor

    I agree with Russ, c² isn't a speed. The units are different. I think you need to learn to be more careful with units. That should have been the first thing you were taught in physics.

    Let's say that we begin with the supposition that mass has some intrinsic amount of energy, even when it is at rest, and we want to find the formula that determines how much energy is in a given amount of mass. First, we know that the units of mass and energy differ by a speed² so we can immediately write E=kmv².

    So, what v can we use? The mass is at rest (v=0). So we cannot use its speed; it must be some sort of universal constant with units of speed. The only such constant is the speed of light, so we know E=kmc² just from consideration of units.

    Determining that k=1 requires some actual derivation, so if anything you should ask why k=1 instead of k=1/2. But c² should be obvious: what else could it possibly be?
     
    Last edited: Aug 14, 2010
  14. Aug 14, 2010 #13
    Re: Why c^2 in "e=mc^2"?

    In Einstein's original paper, it said E = mc with a reference to footnote 2. He intended to write in the footnote: E = ma and E = mb are rejected for orthographical reasons. However, he forgot to add the footnote and thus his fame was assured.
     
  15. Aug 14, 2010 #14

    DrGreg

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    Gold Member

    Re: Why c^2 in "e=mc^2"?

    Nice joke, but of course totally untrue.
     
  16. Aug 14, 2010 #15
    Re: Why c^2 in "e=mc^2"?

    You ignored the rest of the sentence entirely...
     
  17. Aug 14, 2010 #16
    Re: Why c^2 in "e=mc^2"?

    i like kinetic energy equation. because some speed v' may be grater than v (if only v<<<c), so its allowed to be squared
     
  18. Aug 14, 2010 #17

    Doc Al

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    Re: Why c^2 in "e=mc^2"?

    Huh?
     
  19. Aug 14, 2010 #18
    Re: Why c^2 in "e=mc^2"?

    my English is bad, i dont know how to say my thought more clearly, sorry :(
     
  20. Aug 14, 2010 #19

    Mute

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    Homework Helper

    Re: Why c^2 in "e=mc^2"?

    You are seriously confused about what these formulas mean. As has already been said, [itex]E = mc^2[/itex] is the amount of energy in an object of mass m in its rest frame, it is NOT "the energy of a mass m moving at maximum speed". i.e., it is the object's internal energy due to the fact that it has mass. The [itex]c^2[/itex] is the conversion factor between mass and rest energy. It's no different that how the Boltzmann constant is a conversion factor between energy and temperature, e.g. [itex]E = k_B T/2[/itex].
     
  21. Aug 14, 2010 #20

    russ_watters

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    Re: Why c^2 in "e=mc^2"?

    There is no v' in any of the equations we've discussed so far...

    ....you're not trying to say that v'=v^2, are you? If you are, you need to reread the entire thread (actually read it this time!) and then repeat the following over and over again until it sinks in:

    v^2 is not a speed.
    v^2 is not a speed.
    v^2 is not a speed.
    v^2 is not a speed.
    v^2 is not a speed.
    v^2 is not a speed.
     
    Last edited: Aug 14, 2010
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