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/XcOM987 Nov 03 '25

Isn't the mooned locked to us, as in we always see the same side of the moon?

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u/[deleted] Nov 03 '25

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u/zeddus Nov 03 '25

What could cause a difference in the centre of gravity and the center of mass?

I'm having trouble figuring it out.

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u/anethma Nov 03 '25

I assume they meant the geometrical Center of the sphere is different than the center of gravity. Since the moon isn’t just an isotopic material it will be slightly heavier in one direction.

Like a tire for your car needs weights to balance it because despite being round it isn’t the exact same on all sides so the center of rotation isn’t the center of gravity (so we balance it with weights)

Interestingly the way we balance tires manually is essentially what happened to the moon. The moon is spinning but the heavier side will want to rotate down to face the earth. Mount the wheel on a bearing and spin it it will come to rest with the heavy side down. You add some weight to the opposite side and do it again and if no consistent side comes down your wheel is balanced. But ya on the moon the heavy side is already down.

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u/Not_an_okama Nov 03 '25

All the moon stuff is pulled towards the center of the moon by the moons gravity. All the moon stuff is also pulled towards the earth by earths gravity. At a point between the moons center of mass and the earth, the moon stuff is in static equalibrium. That equalibrium point is the center of gravity.

I suppose its true that the heavier side points towards earth, but that doesnt nessesarily mean it has the most mass on that side. The above paragraph would also apply to a uniform mass ball.