If we could utilize gravitational force to create energy, could gravity, being unlimited, generate unlimited energy?
Consider electricity generation from tides. Since tides are due to gravity, aren't we creating energy from an unlimited source and thus generating unlimited energy?
This seems highly contradictory to conservation of energy. What am I missing?
Tides gradually reduce the internal kinetic energy and gravitational potential energy of the Earth-Moon system, that is, the speed and height of the Moon's orbit.
Whether or not humans extract some electricity from it along the way, the same amount of energy per tide ends up as random thermal internal energy in the materials that make up the Earth and the Moon. The Earth and Moon are doing work on each other; harvesting tidal energy lets us redirect a tiny fraction of this work to our own purposes before it ends up dissipated as heat.
The energies available are exceedingly large relative to the energies dissipated.
Our Moon is actually on a (very, very slow) outward spiral. The energy dissipated by tides on Earth and on the Moon (which lacks water, of course, but rock still deforms a little bit) acts to counteract this. If the Earth-Moon system were left alone forever, tidal energy dissipation would eventually stop the outward spiral and begin an inward spiral, eventually ending either with the Moon crashing into the Earth or perhaps developing an orbital eccentricity that eventually would result in the moon being captured by another body's gravity and leaving Earth forever. I haven't seen the calculation, but the estimate I've seen projected for time at which the outward spiral would stop and the inward spiral would begin is about fifty billion years from now, with collision or ejection at about 65 billion years. https://en.wikipedia.org/wiki/Timeline_of_the_far_future
You are missing the fact that the tidal energy you convert into electricity comes from the relative motion of the Moon and the Earth. Very gradually, that motion is slowing down as energy is lost through tidal effects. So energy is conserved, as the gain in tidal energy is offset by the loss of kinetic energy of the Earth and the Moon.
