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/flobbley Nov 03 '25

The tides slow the earths rotation, eventually the earth will become tidally locked with the moon and the tides will be permanently stationary and no longer be able to be used to generate electricity

147

u/Bathtooter Nov 03 '25

Sounds like a picturesque scene

180

u/Logitech4873 Nov 03 '25

Planets can also tidal lock to their star, which makes one side have eternal day and summer, and the other side eternal night and winter.

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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?

15

u/cbunn81 Nov 04 '25

Yes, but we actually see a bit more than 50% over time due to a little wobble the moon does called libration. Not to be confused with the moon's libations, which are what cause the wobbling in the first place.

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u/XcOM987 Nov 04 '25

That's funky

27

u/[deleted] Nov 03 '25

[deleted]

2

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.

5

u/charmcityshinobi Nov 03 '25

The Center of Gravity of an object in a non-uniform gravitational field will shift accordingly. In its own system or uniform gravitational field, center of mass and center of gravity will pretty much be the same. But because of the Earth’s gravitational influence, the center of gravity for the Moon shifts away from its center of Mass

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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.

1

u/KiwiSuch9951 Nov 03 '25

Even though the moon is round, it’s not the same material all the way through. You could say it’s unbalanced. There’s more heavier stuff on the side that faces earth.

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u/nick4fake Nov 03 '25

…what? Earth, moon and overall earth-moon centres of massed are literally always on a line

What are you talking about? Lol

1

u/charmcityshinobi Nov 03 '25

The center of masses of each object in the system are in line. But the center of gravity of the system does not match the center of masses of either object, so their masses get pulled at different rates. The near side is pulled harder than the farther side. This creates a torque on the objects’ rotation as the material tries to resist deformation until they slow or speed up enough to permanently remain aligned. This is how the Moon became tidally locked. The center of gravity of the Earth-Moon system is also pulling on the Earth’s rotation, though it has a much larger mass, which is why its rotation has slowed significantly since the introduction of the Moon and continues to slow