If the moon creates the tides, why don’t any other objects get lifted by its gravitational pull?
In: Planetary Science
Like what it has the power to move oceans, why not you?
Gravity pulls on every kilogram equally. Water moves pretty freely. So, if a 70kg person is pulled a bit by the moon, then the 730000000000kg of water in the pacific is going to be pulled on a *lot more* by the moon. Not because any one bit of water experiences more pull, but because there’s so much more water getting pulled on.
Oh, and unlike water, a big pile of people or dort does not move freely. Water sloshes even with weak forces pushing on it. Dirt just sits there.
And it turns out that “a lot more” is around the scale of a dozen feet, depending on where you are and how much water is there.
Edit: Oh, and if this gets you wondering about our atmosphere as well, then congratulations! You’ve got a keen eye! The atmosphere does experience lunar tides from gravity! They are just relatively small compared to other forces. [Link](https://en.m.wikipedia.org/wiki/Atmospheric_tide).
Gravity’s effect depends on how large area it’s being applied to, and how easily an object being pulled on can move.
Since water is a liquid, it moves much easier than other (solid) objects.
Larger bodies of water are more effected by gravity because it becomes stronger with more area.
Those two factors combined are why water has tides but why something like land doesn’t.
Tidal effects are the culmination of billions of years of forces at play. The moon only exerts a tiny pull. But it has been a tiny pull for so long that it has grown into what it is now. If there were no tides and no moon, and then suddenly the moon appeared, it wouldn’t immediately have the same effect.
Also, it does pull on other objects. You can jump higher if the moon is directly overhead versus if it’s on the other side of the planet. It’s just such a miniscule amount that it’s almost immeasurable.
They do get lifted in that they get a little lighter.
But they don’t get lifted ENOUGH to completely overcome Earth’s pull.
Gravity is a force that’s very very strongly affected by distance. The further away something is, the less gravity you will be subjected to from it.
The Earth is very massive and very close. Its gravity is immense.
The Moon is much less massive and not nearly as close. Its gravity is honestly pretty tiny.
But that gravity is pulling on EVERY SINGLE molecule of water in the ocean, and that’s enough to make ALL those molecules want to shift the tiniest tiniest bit toward the moon. And because the ocean is a liquid, these molecules can move around individually. So one pushes on another and that pushes on a third and that pushes on a fourth and… and so on and so on. Gigantic stupidly big numbers of molecules of water all want to move toward the moon, they CAN because they’re a liquid, so they create pressure, and the built-up pressure creates tides.
But you won’t see that in a puddle or a pond or a small lake…. there’s just not enough individual water molecules to add up to any sort of shifting pressure. And anything solid? Not affected, really, because the individual molecules in it can’t move individually. They’re locked in place.
There’s other factors. Air itself also gets pulled toward the moon a tiny bit and that removes some of the pressure on the ocean, for example. But in general it’s because the ocean is just so so so vast and is a liquid that it can create visible changes in water levels near coasts.
It does, but there is SO. MUCH. WATER. Images like this one are what really got me to understand the effect the moon has on the tides:
[https://1.bp.blogspot.com/–A70-ySFXNY/X3clzgCpgPI/AAAAAAAAJsA/v30SSDrh2sAnp2PYmEmHV7UaubqxUj-mgCLcBGAsYHQ/s2048/bigstock-Lunar-And-Solar-Tides-Vector-I-234308305.jpg](https://1.bp.blogspot.com/–A70-ySFXNY/X3clzgCpgPI/AAAAAAAAJsA/v30SSDrh2sAnp2PYmEmHV7UaubqxUj-mgCLcBGAsYHQ/s2048/bigstock-Lunar-And-Solar-Tides-Vector-I-234308305.jpg)
The moon creates tides because of the large distance between the water on the side of the planet facing the moon and the water on the opposite side. Gravity gets weaker as distance increases, so it pulls on the water closest to it stronger than it does on water that’s farther away.
This creates a “tidal bulge” that the earth rotates inside of. Local geography and other physics stuff can change the exact timing of when the tides happen.
It’s not just gravity, but the difference in gravity. That’s what Tides/Tidal force means.
It’s because the oceans are freer to move.
Your premise is correct in that every gram of matter near to sea level experiences the same gravitational force, be it water or dirt or air – the gravitational force is primarily made up of the moon’s pull and the earth’s pull (the sun also contributes nontrivially but we can ignore for the purpose of this answer).
Given that, if gravity were the only force and all matter is free to move, we’d expect the earth to be roughly shaped like an ellipsoid pointed in the direction of the moon – the closer points experience more pull and the farther points less.
However the other matter on the planet are less able to move. Land is mostly solid and harder to deform. There’s more forces holding land together. So for the most part we think of the landmass on earth as a solid shape moving through the bulging water mass.
For an ELI5 visualization, think of spinning a water balloon at high speed vs spinning a bowling ball at high speed. The water balloon is more ready to deform and thus follow the direction of the centrifugal spin. The bowling ball has the same tendency but is held back internally by molecular forces.
If I run my finger over the surface of the chair I’m sitting in, I can create a moving dimple about 1mm high. The height is limited by the fact that a solid like a chair is resistant to being deformed. If I exert the same force on a liquid, I can get a much larger wave, especially if I move at the right speed, or if it sloshes up against the side of a tank. Similarly the moon lifts the land by about 50 cm at the equator. It just lifts the ocean more.
OP, did you get an answer that you could understand here?
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