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Mass to energy conversion in living beings

  1. May 19, 2010 #1
    I have a doubt regarding mass to energy conversion. Suppose I consume m1 kg of food at beginning of a day. Will it be equal to the mass of solid and liquid wastes out of my body on that day (say m2) and increase in mass of my body (say m3) and added to energy gained by me from that food (Eg) according to famous relation [itex] E=mc^{2}[/itex] (Let us assume initial conditions of my body to be zero). That is
    [tex] m1=m2+m3 + \frac{Eg}{c^2} [/tex].
    Since a small difference in mass (Mdiff=m1-(m2+m3)) is equivalent to enormous amount of energy, and by my common sense (which may be wrong) I feel Mdiff will be significant leading to energy gained Eg to be very high, I doubt the mass-energy equation. :confused:
    Last edited: May 19, 2010
  2. jcsd
  3. May 19, 2010 #2


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    That's the problem with E=mc2, conversion factor (c2) is so large it makes is almost impossible to observe mass changes in the surrounding reality. But it doesn't make E=mc2 wrong, just impossible to observe in everydays life.

    That's not the only thing that is hard to believe using just a common sense, time dilation or tunneling effect also don't make sense when you try to understand them using intuition trained to deal with low speed macroscopic objects. Does it make these effects wrong? No, just more interesting :biggrin:
  4. May 19, 2010 #3


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    Because of this, that equation really only works well for nuclear reactions. Not for chemical reactions.
  5. May 19, 2010 #4


    Staff: Mentor

    Also, in your equation you missed mass lost due to breathing. Exercise doesn't really have much impact on urine or feces production. So when you exercise to lose weight you lose it chiefly in two ways:
    1) sweating - water and electrolytes, replaced upon drinking
    2) breathing - water and CO2, the water is replaced upon drinking and the CO2 represents the real weight loss
  6. May 19, 2010 #5
    Are you sure about that? A water molecule does not have the same mass as 2 H's + 1 O, does it?
  7. May 19, 2010 #6


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    Russ stated "it only works well" - by which I suppose he meant that this equations can be easily used to calculate amount of energy liberated in nuclear reactions, but in other cases using it is a waste of time (even if technically correct).
  8. May 19, 2010 #7
    Sure it does, not including relativistic effects.
  9. May 19, 2010 #8
    By 'working well' I think we mean, 'we can measure the calculated change in mass.' Since c^2 is so big, the changes in mass associated with chemical reactions are tiny, where 'tiny' means they can't be measured with normal equipment.

    Yes, of course the H2O molecule weighs less than the individual atoms - but I doubt you have a scale that can make that determination. And that's why the relationship (mass to energy) remained undiscovered for so long. And also why it was discovered by a guy with a pencil, rather than by someone in a lab coat.
  10. May 19, 2010 #9

    Andy Resnick

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    This is a totally inappropriate approach. Humans consume about 2 kCal/day- figure out the mass equivalent from that.
  11. May 19, 2010 #10
    See my post #3 in


    A Snickers bar has about 19,900 joules per gram of metabolic (oxidation) energy, or about 2 x 107 joules per kilogram. Dividing by c2 = (3 x 108)2 yields 2.2 x 10-10 kilograms of pure energy-equivalent-mass per kilogram of food. (I think U235 fission is ~202 MeV per 220,000 MeV, or 9 x 10-4 kilograms per kilogram).

    During the day, you will probably inhale about 0.7 kilograms of oxygen, and exhale about 1 kilogram of CO2 and 0.4 kilograms of water vapor. So you won't be able to detect the added M = E/c2 energy from a Snickers bar..

    You can also use the energy-mass conversion: 1 fructose molecule (C6H12O6; 180 grams/GMW) = 29 eV of metabolic energy per molecule.

    Bob S
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