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Chemical energy into mechanical.

  1. Oct 27, 2012 #1

    Given that our body is 18-26% efficient in converting chemical energy into mechanical, does that mean that 74-82% of what I consume is not burned? Furthermore, assuming a certain task requires 1000 KJ, how many meals 2000 KJ each will I need to consume for completing it without gaining fat?
  2. jcsd
  3. Oct 28, 2012 #2


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    The efficiency is measured in terms of desired output. The Mechanical output is 18-26%, the rest is used in other bodily activities like maintaining our body temperature.

    For the second question, just find out the energy whose 18%/26% is 1000 KJ.
  4. Oct 28, 2012 #3


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    It is burnt either to serve another needs, or burnt - and lost. Think why you are sweating when running - to get rid of excess heat. Where do the excess heat comes from?
  5. Oct 28, 2012 #4
    So let me get this straight. Let's say I am to climb a mountain and most of the energy is spent on attaining a certain altitude. So chemical energy turns into mechanical energy which is in turn converted into potential energy. Supposing I would consume about 1000KJ in the process of climbing. Once I attain the desired altitude I'd like to compensate myself for the calorie loss but without gaining any weight. Does that mean that I can eat up to 1.18-1.26 of 1000KJ, hence 1180-1260KJ?
  6. Oct 28, 2012 #5


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    Nope. You need to eat that much amount whose 18% is 1000 KJ. That would be minimum of 3864.15 KJ (for 26% efficiency).
  7. Oct 28, 2012 #6
    But won't the rest 2864.15kJ be turned into fat?
  8. Oct 28, 2012 #7


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    !? It won't. As mentioned previously, the rest will be consumed in other metabolic activity. Human body is designed to live, not to do mechanical work.

    The rest of the energy is required to sustain the life in Human body. That 26% is the "efficiency" of the human body to do mechanical work only, so it accounts only mechanical energy, not other metabolics.
  9. Oct 28, 2012 #8
    Yea, don't get hung up on the "efficiency" word too much. They're referring to mechanical motion specifically, but you need to do a lot more than just move around in order to continue to exist.

    This reminds me of the "humans only use 10% of their brain" fallacy. Your brain has to keep you breathing, and keep your heart beating, etc... So, having 10% of anything in your body to do anything more than simply keep you from collapsing into a blob of (mostly carbon) goo is nothing to look down on.

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