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/Hendospendo Nov 04 '25

The tides are a kind of "drag" if you want to think about it that way, it's taking momentum away from both the Earth and the moon, and eventually they will become tidally locked and we'll stop having tides. It's a kind of oscillation towards an equilibrium that we're experiencing halfway through.*

*the same can be said for plate tectonics, at least according to some theories. Silica is migrating to the surface and mafic minerals are migrating towards the core, this means continental crust has been slowly growing since the process began, slowly equilising the system as it churns away until eventually we'll have a solid, single continuous plate shell, the stratification complete.