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I just started doing some very light weight lifting and decided to do a rough calculation of how many calories I might be burning during each of my workouts, however the figure I arrive at seems too high by comparison with the numbers one regularly see bandied about. What am I doing wrong?

My calculation goes as follows:

I'll use conservation of energy and only account for the upward movement (when you're letting the weights down again in the second half of each repetition you're also exerting force in the direction opposite to movement so as not to let them fall at the full acceleration due to gravity).

I just started so I'm using a pair of 9 lbs dumbbells. This gives us ~4Kg for each dumbbell, 8Kg for the two, and 78.4N. Let's say the average length that you extend each dumbbell upwards during a single repetition is 0.3m. This gives us 23.52J spent in each repetition according to conservation of energy principles, mgh=W.

Now, I do 12 repetitions of each of 8 different exercises, and repeat the whole cycle 3 times. This gives us 23.52(12)(8)(3) = 6774J, which converted to calories result in ~1600 calories, not taking into account the work done during the downward movement of the weights nor leg work, which is perhaps more strenuous.

By all accounts though, 1600 cal burned during a single workout session seems a lot, even more so for such light weights. Is my physics wrong?