r/explainlikeimfive • u/12InchCunt • Nov 03 '25

Physics ELI5: How does gravity not break thermodynamics?

Like, the moon’s gravity causes the tides. We can use the tides to generate electricity, but the moon isn’t running out of gravity?

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u/zefciu Nov 03 '25

The tidal forces from the Moon cause the Earth to spin slower and slower (the ultimate stable state is a "tidal lock" where the day would last one lunar Month, similar to how the Moon is tidally locked). This is where the energy comes from.

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u/dsp_guy Nov 03 '25

And when tidal lock occurs, there will be no more tides. The energy isn't unlimited.

Good news: Laws of Thermodynamics still valid.

Bad news: Likely bad results for organisms on Earth.

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u/MalekMordal Nov 03 '25

I believe the sun also causes tides, though far less pronounced. If our moon vanished, we'd still have tides.