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You can extract geothermal energy from the interior, but it's not an infinite source of energy. Extracting geothermal energy will make the core cool down a bit faster than it otherwise would (so, in case of a perfectly insulated planet this would be the only source of heat loss), the drop in pressure would make the planet shrink a bit more, so you then get ...


It is the "infinite source" that is giving the downvotes. As the answer by Ramchandra Apte states, gravitational compression is a one way street as far as energy goes. But of course a complicated state like a planet has energy exchanges with its sun, and one can get transformation of sun energy to gravitational energy as in hydroelectric power.. One can ...


The heating is one-time as once the planet reaches equilibrium with gravity (i.e. compression force from gravity equals normal force), no more compression with occur. Basically the planet will compress one-time and that's it. (In reality, this occurs when the planet forms and its various pieces join together)


If we assumed an "ideal fan", that had no forces slowing it at all (itself impossible), then you're right that if left alone, it would keep spinning forever. However, if you extracted energy from the fan - by whatever method - then that would cause it to slow down and stop, as the energy that you were "producing" would be the kinetic energy of the fan.


If there is friction (air resistance), that friction will extract energy from the spinning fan, thus slowing it to a stop. If you extract energy in any other way, you are also applying friction to the fan, again slowing it to a stop. In other words, yes you can extract energy from the fan, but no more than the rotational energy that the fan possesses: ...

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