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Let's say we have 100 grams of chocolate which has 571 kilocalories (so it's about 2.4 megajoules) and we have barbell which weights 100 kilograms and need to lift it to 0.5 meters height.

So how many times you can lift it eating only 100 grams of chocolate? Let's use potential formula $$E=mgh$$ n – how many times you lift $$E=nmgh$$ $$n=\frac{E}{mgh}$$ $$n=\frac{2.4\cdot 10^6J}{100kg\cdot10\frac{m}{s^2}\cdot0.5m}=4800$$ So I get 4800 times, but I doubt it's true. So where and how many (approximately) our gained energy disappears, I mean becomes other kind of energy?

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You are ignoring the energy exchanges of the body.The cells in the muscles have to eat energy and keep the temperature at 37C and there is infrared radiation in addition to chemical losses and lifting themselves up along with the barbell. This question is for a biology/physiology forum. –  anna v Jul 12 '11 at 15:38

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Yes, if you combust the chocolate to extract all chemical energy from it, and have a machine with perfect energy efficiency, then you can lift 100 kilograms 2.4 kilometers high. But your body is rather far from a machine with perfect energy efficiency. (Your heart needs to pump, your diaphragm goes up and down, in addition to other mechanical and chemical "inefficiencies".)

For example, doing a google search brings up this study which studies aerobic weight lifting exercises (so presumably the weight is much less than 100 kilograms, though distance is about the same). The measured total aerobic energy consumption (from carbon dioxide content of the exhaled breath) is around 10 kilojoules mer minute. For that exercise, your 100 grams of chocolate can sustain you for around 4 hours. (If you were to do 4800 reps in 4 hours, that would require sustaining 20 reps a minute, a truly fearsome rate...)

If you look at the basal metabolic rate for humans, say we have a typical male standing at 170 centimeters, 75 kilograms, and 30 years of age, he would burn about 1600 kilocalories per day doing absolutely nothing. So a 100g chocolate bar gets him through about a third of the day.

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Of course if you are trying to do enough exercise to counter the energy input of the chocolate bar then your body is a depressingly efficient machine –  Martin Beckett Jul 12 '11 at 16:54

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