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Let's say I am using a machine to lift weight by pulling a bar. When I do that, the state of the machine is exactly the same at the beginning and the end of the workout; no heat has been generated since it's just weight lifting.

If I replaced the weight by a mechanism to generate electricity, I will do the exact same workout and it will have the exact same effect on my body, but instead of ending up with a machine in the same state as the beginning, I end up with electricity!

In this case I understand that the electricity is the result of my effort but in the case of the weights, where is the energy going?

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  • $\begingroup$ Did you sweat from the excess heat you produced by burning carbohydrates and doing the work? Like an engine does that burns hydrocarbons? See what I did there? Hydrocarbon/carbohydrate each react with O2 and produce CO2 and H2O. You breathe out (hot) water vapor and CO2, and car uses exhaust pipe. Also the joints and pulleys of the machine might get hot if not lubricated and well made. You have to work harder with weights than with a generator. Your muscles are not springs and you use fuel lowering as well as lifting. $\endgroup$
    – C. Towne Springer
    Commented Jan 23, 2015 at 8:15
  • $\begingroup$ I really don't get your point. We can safely ignore default of the machine or the resistance of weights vs generator. The point is one generate electricity, the other not... $\endgroup$
    – tibo
    Commented Jan 23, 2015 at 14:06
  • $\begingroup$ The problem you describe is one of work, not energy. You are saying you have done work (force times distance) and in one case you get nothing and in the other you get electric power. If your muscles were conservative, like ideal springs, lifting and lowering weights would not use any net power. In the end you are back where you started. But muscles do not work that way. Lifting up takes work and lowering down takes work. You can imagine a generator as a device in which you only "lift up", like stacking bricks, and in the end you have done work on a mass of bricks. $\endgroup$
    – C. Towne Springer
    Commented Jan 23, 2015 at 20:55

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Ooh tricky question, I like those! Let me simplify the question a bit because I think it will become much more clear where the difference is.

Simplified proxy question:

Imagine you have lifted a bucket of water from the ground up to waist height. You now have the option of pouring the water out of the bucket onto the ground, or, pouring the bucket onto a water wheel which will turn and generate electricity. In one case, you get nothing from pouring the water on the ground, and in the second case, you get useful electricity. How is pouring water onto the water wheel better than pouring water onto the ground?

The answer to the simplified question is that when you pour water onto the ground, the water strikes the ground with a high velocity and generates heat due to the inelastic collision with the ground and the friction (viscosity) of the water. When you pour water onto the water wheel, the wheel slows the water down and extracts the potential energy from it so that when the water finally reaches the ground it has much less energy to give up.

So, pouring the water on the ground wastefully release all the energy at once when it hits the ground. Pouring it on the water wheel extracts some useful energy from it reducing the amount wasted on the ground. The total amount of energy spent by your body and the total amount of energy released is the same in both cases.

Your exercise question is just a slightly more complicated version of this. If you setup the exercise with weights versus exorcise with a generator so that the workout to your body is the same, then you will be able to find where the energy with the weights is being released as waste.

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  • $\begingroup$ That thing I don't get is what is this wasted energy. According to the law of conservation of energy, it's not lost... So where does this energy is? $\endgroup$
    – tibo
    Commented Jan 23, 2015 at 14:00
  • $\begingroup$ @tibo the wasted energy in my simplified example gets lost as heat. The same is true in your workout example. Say you lift a big weight. If you place that weight on a generator that creates electricity as the weight falls then the potential energy is turned into electricity. If you drop that weight on the floor instead the energy is lost as heat (from the sound and vibration) when it impacts the floor. $\endgroup$
    – Brandon Enright
    Commented Jan 23, 2015 at 18:17
  • $\begingroup$ Congrats on the recent gold ;) $\endgroup$
    – Kyle Kanos
    Commented Jul 2, 2015 at 20:00
  • $\begingroup$ And yet another congrats for another gold & getting all the Steward Badges! $\endgroup$
    – Kyle Kanos
    Commented Apr 7, 2016 at 11:43
  • $\begingroup$ @KyleKanos thank you :-) Boy that last one takes a ton of persistence... Now there are two of us with them all! $\endgroup$
    – Brandon Enright
    Commented Apr 8, 2016 at 19:19

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