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Conservation of Energy

  1. Sep 16, 2007 #1
    [SOLVED] Conservation of Energy

    From 'Superstrings and Other Things' by Carlos Calle

    I understand and agree with everything up to where he says
    , shouldn't increase be decrease. I cant see how the bird can gain mass, only loose it by converting it into it kinetic energy? which would make sense. I might be missing the point completely or its simply just a printing error?

    This then lead me to thinking about us, when we move about, does the energy come from particles in our body undergoing reactions giving off energy, resulting in an overall decrease in our mass and increase in kinetic energy? and using E=MC^2 calculates the energy given off?
  2. jcsd
  3. Sep 16, 2007 #2


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    I think he is invoking the increase in "mass" as in the "relativistic mass".

  4. Sep 16, 2007 #3
    I sort of understand relativistic mass, but it has not been mentioned prior in the book, E = MC^2 has only been mentioned so far, but i do appreciate that as the
    birds speed increases so does its mass due to the relativistic mass idea.

    So say the bird at rest has a mass M1, therefore using E=MC^2 gives the energy equivalent of its mass which we shall call E1.

    The flying bird has gained kinetic energy, say e2. So e2 = MC^2 , so e2 has an equivalent mass of say m2.

    The bird must have lost a mass of m2 in order for it to have gained the kinetic energy (e2)?

    Is this true? if not where does e2 come from?

    and is this idea totally independent from relativistic mass?

    So this idea decreases its mass but relativistic mass increases its mass? Ahhh! I am totally making a mess of this,
  5. Sep 16, 2007 #4
    I think you're mixing two concepts.

    If you count the bird's metabolism in the equation, then yes, there would be a minuscule mass loss as respiration takes place and the compounds the bird uses for energy are chemically changed. This energy allows it to fly.

    However, I think in the problem you're describing, you don't care how the bird got that energy, you're just using its movement as an example for relativistic mass, which purely from that assumption, increases.
  6. Sep 16, 2007 #5
    Great, so i have confirmed there are 2 processes going on here,

    1. being the conversion of the birds mass to allow it to fly (decrease in mass)

    2. being the increase in the birds mass because of its increase in speed.

    As to which one he was referring to, i guess i will never know for certain but if you read what came before the original quote...

    ...followed by...

    ...because there is no mention of relativistic mass and that he has only just introduced the concept of E=MC^2 i think he must be referring to the conversion of the birds mass into energy. So he has made a mistake by using increase instead of decrease in the birds mass?
  7. Sep 16, 2007 #6

    Doc Al

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    No, looks to me he's talking about relativistic mass. Note that he says "Conversely, energy is a form of mass" in the first passage you quote, from which I deduce that he means that the kinetic energy of the bird gives it more "mass". I think he's doing you a disservice by not being clear.

    Lot's of luck trying to learn physics from a popular book!
  8. Sep 16, 2007 #7
    Thanks for that, your most probably right. i thought that this book would give me a nice refresher guide to most physics topics, instead it seems to creating lots of problems and waisting lots of time! On the plus side, its making me question things more and probably in the long run understanding things better.
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