Answer to Question #4558 in Mechanics | Relativity for Jean-Pierre Twagirayezu

Question #4558
I have the following question:

Why is it supposed to be impossible to generate energy from gravity? I see the force of gravity as a loose force that could be harnessed to generate useful mechanical energy. Why is this supposed to be impossible? I have struggled with this problem for many years and I would like an in depth analysis that would help me understand the scope of this problem.


Expert's answer
Not sure where you heard it was impossible. Gravity is routinely used to produce energy! The best example is in a hydroelectric dam, where gravity creates the water pressure to turn the turbines that generate power. Another, very small-scale example is in the design of some old clocks, which were powered by very-slowly descending weights.
But the conversion of gravitational energy into other, useful forms is not very practical in many situations. In every case, It requires some mass which descends from a high place to a low place. The amount of energy produced is proportional to the amount of material that descends, and proportional to the distance it falls, which means you need to use something very heavy and/or very high in order to produce much energy.
In addition, the energy isn't "free"; some other energy source has to initially move the mass up to the high place, and replace it again after it descends. In the case of hydroelectric dams, that energy source is the sun, which lifts the fallen water by means of evaporation, after which it gets deposited again onto some mountaintop. In the case of the above mentioned clocks, every once in a while (usually every few days) somebody has to pull a chain to lift the weights back up to the top.

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