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This may seem like a far fetched idea, but consider this system.

Imagine a simple system of just the Moon orbiting around the Earth, creating low tides and high tides with its gravity.

Now imagine we add a paddle wheel that produces work based on the flow of water when it is changing tides.

If the tidal cycle repeats itself many times we could expect a lot of work, but where does this energy come from?

The gravitational potential energy of the moon would be converted into what I believe is internal energy of the water and work of the paddle.

It seems to me that as long the moon orbits the Earth, then "free energy is created".

This leads me to think that if mass would be destroyed by gravity, then conservation of energy still applies. However, I have never heard about such a thing being done by gravity. Is there anything that I did not consider in my analysis of this situation? How is energy conserved?

Qmechanic
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Cedric Martens
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1 Answers1

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If the tidal cycle repeats itself many times we could expect a lot of work, but where does this energy come from?

The energy comes from the rotational kinetic energy of the Earth. As more work is extracted the rotation slows down and the day becomes longer.

The gravitational potential energy of the moon would be converted into what I believe is internal energy of the water and work of the paddle.

Actually, the loss in the KE of the Earth also results in an increase in the gravitational PE of the moon. The moon goes to a slightly higher orbit.

This leads me to think that if mass would be destroyed by gravity, then conservation of energy still applies

There is no need for that. Energy is conserved without that.

Dale
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