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Why doesn’t the water fall out of the Earth?

  1. Dec 2, 2017 #1
    So I was on my way to school this morning and looking at the beach made me think of something. Why doesn’t the water on earth fall out and into space? Like, if you really didn’t think too hard, you’d be like, yeah, the earth is tilted, so why, doesn’t the water fall out like a glass of water? If you are still confused about what my question is, I’ll try explain it a little better. So, the earth is tilted on its axis right? Well, why doesn’t the water from the ocean fall out? Like, if you look at a glass of water for example. When you tilt the glass, the water shifts to one side. Then, eventually it just falls out. Why doesn’t the earth do the same? I honestly think it’s because of gravity. Like when you spill water out of a cup. Where does it land? On the floor. Meaning that there is gravity affecting it. But, just like the water in the cup, why doesn’t the water on earth also fall out? Is gravity that strong to pull all that water to stay in a ball or is there something else other than gravity (and possibly the atmosphere) affecting it? I’m just curious, and sorry if you guys think that this is an imature question to ask, but I’m only a weird analytical kid who’s constantly thinking about this kind of stuff and I wanted to settle this question once and for all. If any of you could help, that would be great. Mahalo!
  2. jcsd
  3. Dec 2, 2017 #2


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    Yes! You got it right.
  4. Dec 2, 2017 #3


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    What's "underneath" the earth (in this picture) that would create gravity (or something like it) to pull the water out?
  5. Dec 2, 2017 #4


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    Not only is gravity strong enough to pull the water into a spherical shape, it is the exact reason the entire earth is spherical! It's pulling all of that rock and dirt that make up the Earth into a big sphere, and we're lucky we don't have mountains of rock on top of us (like the interior of the Earth does) otherwise we'd be in a rather unpleasant situation indeed!
  6. Dec 2, 2017 #5
    Where would water fall to?
    The Sun?
  7. Dec 2, 2017 #6


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    It does. When the water falls out of your glass it is pulled towards the centre of the planet. Exactly the same for the oceans.
  8. Dec 3, 2017 #7
    The fact that there is a glass of water is all you need to know that gravity is the dominant force acting on the water. You can not have a glass of water on the space station because gravity is no longer dominant.
  9. Dec 3, 2017 #8


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    Ah, but the force of gravity on the space station is still greater than 90% of the force of gravity on the surface of the Earth. The difference is that both the water and glass on the space station are in free fall!

    But your point about just having water in a glass is still valid. Gravity is the reason that the water stays in the glass instead of drifting off somewhere.
  10. Dec 4, 2017 #9
    Into the oblivion :wink:
  11. Dec 5, 2017 #10
    Its axis of spin is "tilted" compared to the plane of its orbit around the sun; not compared to some kind of off-the-planet absolute downward -- no such thing.
  12. Dec 5, 2017 #11
    I’m a little confused. Could you explain that a little different?
  13. Dec 5, 2017 #12


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    It doesn't matter how the earth's axis of rotation is tilted -- gravity will pull any object, including water, toward the center of the earth.
  14. Dec 5, 2017 #13
    There is nothing special about earth's tilt relative to its orbit. Imagine a set of coordinate axes whose z-axis aligns with the rotational axis of the earth. Then everything else is tilted relative to earth. The dominating force on earth is the gravity caused by earth, so it's tilt compared to other bodies in the universe is irrelevant in this context.
  15. Dec 5, 2017 #14
    When you slowly tilt a half-full glass of water, you're tilting the top of the glass away from straight up and down, while the surface of the water remains the same horizontally, so the glass is tilted to the water. The tilt of the Earth compared to the imaginary disk of its orbit includes its water. That water doesn't spill away from the Earth; it's already spilled onto and into the Earth, so it's part of the Earth, just as the atmosphere is -- the water stays here because down means toward the center of the sphere.
    Last edited by a moderator: Dec 5, 2017
  16. Dec 5, 2017 #15
    Oblivion is not generally considered to be a scientific concept.
  17. Dec 5, 2017 #16
    Not just water, how about that other fluid called AIR
  18. Dec 5, 2017 #17
    Ahhh. That makes more sense. Thanks.
  19. Dec 9, 2017 #18
    Think of the Earth as a big glass, and the seas and oceans as the water that fills it up.

    I hope I helped.
  20. Dec 9, 2017 #19
    A couple of the commentators mentioned that there was no gravitational attractor to pull the oceans away from the Earth.

    Isn't that called Tides? The tidal influence of of the Moon and Sun? Though of course the Earth's gravity is stronger due to proximity.
  21. Dec 9, 2017 #20
    Lol. I guess that’s a nice way of thinking about that...
  22. Dec 9, 2017 #21


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    In a sense, yes. Though tides are due to gravitational differential rather than direct gravitational pull. For instance, if we were to move the Earth to an orbit 3,686856 km from the Sun (center to center distance), then the gravitational pull from the Sun on a object sitting on the surface of the Earth directly under the sun would be exactly equal to the Earth's gravitational pull on that object. But that does not mean that the weight of that object relative to the Earth would be zero.

    The Earth is in orbit, which means it is in free-fall around the Sun. This means that it is freely responding to the Sun's gravity. The object is also following almost exactly the same orbit, it just happens to be 6378 km closer to the Sun than the center of the Earth. So most of the that 1g of gravitational force from the Sun is accounted for by it following the Earth around in its orbit. What isn't accounted for is the difference in gravitational acceleration on the center of the Earth due to the Sun compared to the gravitational acceleration on the object due to the Sun. This difference works out to be ~ 0.17% of a g. In other words, the object would only weigh ~ 0.17% less than it would by the Earth's gravity alone. An object on the other side of the Earth, would undergo ~0.17% less gravitational acceleration from the Sun as the center of the Earth does, meaning it would also weigh ~ 0.17% less relative to the Earth, which is why Both the Sun and Moon each produce two tidal bulges on opposite sides of the Earth.

    For the purposes of the OP's question, this small tidal effect can be ignored, and we can just consider the Earth's oceans and the Earth's solid body as following the same free-fall path around the Sun together and thus the net difference in the gravity pull on them from the Sun is 0, leaving just the Earth's gravity to consider.
  23. Dec 10, 2017 #22
    Thanks Janus. I appreciate a clear and coherent explanation of the subject. Sorry I have to mess it up. The audience for fiction expect ridiculous levels of fabulism.
  24. Dec 23, 2017 #23

    jim mcnamara

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    We are drifting off topic here. Thread locked - thanks for some well written answers!
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