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# Work done/Energy burnt using weight lifting

2.) Your new exercise machine involves lifting weights a standard height of 52.0 cm.

a) How much work do you do if you lift 37.8 kg? (in J)

Exercise requires a food input about four times your mechanical work output (in other words, about 25% efficiency). A "food calorie" is actually 1000 standard calories and a standard calorie is 4.185 J. Each pound of fat represents 3,500 "food calories".
NOTE: For the purposes of this problem, assume that you drop the mass each time rather than slowly lowering it back down. Otherwise things get too complicated. From a physics point of view, you do negative work on the mass when you lower it (as evidenced by the mass' lowered potential energy). However, your body does not gain this energy back. Instead, due to the above mentioned inefficiency, you must actually expend energy to lower the mass slowly.

b) How many time must you lift the weight to lose one pound of fat [Note: Since muscle is denser than fat, you would probably gain weight doing this.]?

#### Solution Preview

2.) Your new exercise machine involves lifting weights a standard height of 52.0 cm.

a) How much work do you do if you lift 37.8 kg? (in J)
Work = Increase in Potential energy = mgh = 37.8*9.81*0.52 = 192.825 J

Exercise requires a food input about four times your mechanical work output (in other words, about 25% efficiency). A "food calorie" is actually 1000 standard ...

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