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How many times will the energy in a choclate bar lift 100kg?

  1. Feb 4, 2010 #1
    Just munching away on my choclate and notice it has energy 1000kj on the packet. To stop me getting fat, how many times would i have to lift 100kg. Bench pressing it.
    Probably a few more variables needed i guess. Or is there a formula you can start me off with?
  2. jcsd
  3. Feb 4, 2010 #2


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    Well, the work you do lifting a mass m a height h in the earth's gravitational field would be given by mgh. The listance you lift the mass would be the length of your arm, I dunno, 3/4 of a meter of so. You could generalize this to lifting the mass N times and say the total work you do would be N(mgh). Note, however, that this is an upper limit to how much work you would have to do. In reality, anyone who has ever bench-pressed weight knows that it would be exhausting to simply hold the weight above them, even though physically no work is being done. Basically, humans are imperfect machines, but it's a nice estimate you can do. :)
  4. Feb 5, 2010 #3


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    Hi budd. If you calculate this purely on the output energy in lifting 100kg then you're likely to get disillusioned with exercise. But like Nabeshin said, this is an upper limit, and actually not all that realistic. Anyway lets do the calculations.

    Each press is only about 50cm (0.5m) so the energy output per lift 0.5*9.8*100 = 490 J per rep. Now 1000kJ is 1000,000 J, so theoretically you need to do over 2000 reps!

    In reality however your body uses far more energy in this process than that given as theoretical output power. Your heart and lungs are working harder and this also uses a lot of energy. Also your basal metabolic rate rises and in fact generally remains higher for several hours after you complete your workout. So even if you're chillin' on the couch watching telly an hour or more later you're still burning off more kJ than would have been the case if you hadn't done the workout at all.
    Last edited: Feb 5, 2010
  5. Feb 5, 2010 #4
    Great answers guys. thank you.
  6. Feb 5, 2010 #5


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    Another little wrinkle, budd: powerlifters in the gym train themselves to hoist as much mass as possible with the least effort on their part. They are super-efficient, and sometimes are overweight by normal standards. Body-builders, on the other hand, tend to make their exercises inefficient in order to target the muscles that they want to stress and develop. Those groups are extremes. Normal folks generally lie somewhere in between.
  7. Feb 5, 2010 #6
    Ok. the 2000 doesn't count for the energy used when the weight is on the way down. I'm guessing this would maybe about half as much as on the way up. Is there a way of working the down bit of the rep out also?

  8. Feb 5, 2010 #7
    The work you do on the bar on the way down is the same if we assume that you lower it slowly (constant speed) and don't let it drop directly on your neck .:biggrin:
  9. Feb 5, 2010 #8
    ahh.. so taking into account it's the same energy to lower the bar as raise it. we are still going to get just over 1000 reps per bar. excluding all other variables?
    That seems a lot of energy from a little choclate bar. Good stuff.
  10. Feb 5, 2010 #9


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    No, theoretically you are doing negative work on the bar on the way down so the total work done in one full rep(etition) is zero.

    So, as before, the whole thing really boils down to just how biomechanically inefficient the whole process is.
  11. Feb 5, 2010 #10
    Theoretically the work done by your muscles is not a conservative force.
  12. Feb 5, 2010 #11


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    Theoretically and in reality!
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