Suppose we created a vacuum and spinned a turbine inside it with some amount of force. According to newton's second law it will keep spinning as there is no air resistance, so why can we not make electricity out of it?
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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:
$$E=I{\omega}^2/2$$ where $I$ is the moment of inertia of the fan and $\omega$ is the rotational speed of the fan.
That is the maximum energy you can extract, and it makes no difference whether the fan is spinning in air or in vacuum. All it means is that, if it is spinning in air, some of the available energy is lost to friction with the air. In vacuum all the energy is available.
hdhondt
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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.
Flyto
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