Gravity

Discussion in 'Physics & Math' started by timojin, Nov 5, 2017.

  1. timojin Valued Senior Member

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    Does gravity changes positively with the dept in the ocean ? as gravity gravity changes negatively claiming the mt. Everest ?
     
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  3. DaveC426913 Valued Senior Member

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    Yes.

    Water is much less dense than rock, so as far as gravity is concerned, even if you are at the bottom of an ocean, you are still above the surface f the Earth, were g is maximum.

    As you descend into the mantle, gravity will actually begin dropping off, until, at the centre it reaches zero.
     
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  5. Q-reeus Valued Senior Member

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    Wrong. That assumes a uniform density sphere, but Earth's actual density profile is very non-uniform. See e.g. https://en.wikipedia.org/wiki/Gravity_of_Earth#Depth
     
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  7. RajeshTrivedi Valued Senior Member

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    welcome back after a brief hiatus.
    The science section had almost become too dull for quite some time.
     
  8. Q-reeus Valued Senior Member

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    Thanks - nice to not have a totally hostile cold-shoulder treatment. But as confirmed by some recent admissions by admin and in keeping with my parting comments back in June this year, SF is not really a science forum. Rather a friendly and inviting home for all that express allegiance to officially decreed mainstream paradigms. Especially political correctness and officially decreed history. IMHO

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    Thus I will try and be very sparing in participating here again.
     
  9. paddoboy Valued Senior Member

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    Correct of course Dave, as taken to obviously mean, "The formula treats the Earth as a perfect sphere with a radially symmetric distribution of mass; a more accurate mathematical treatment is discussed below".
    https://en.wikipedia.org/wiki/Gravity_of_Earth#Depth
     
  10. DaveC426913 Valued Senior Member

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    I'm well aware that it is not uniform. (I was actually going to post this very graph)

    I did not realize that g continues to increase as you descend below the surface, all the way to the Core. You learn something new every day.

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  11. RajeshTrivedi Valued Senior Member

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    Nope, pl have a relook at the graph. g decreases to zero as you descend to center from the surface of the Earth (barring the exacting kink somewhere at the end of lower mantle).
     
  12. DaveC426913 Valued Senior Member

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    I did not say to the centre; I said to the core: which is at radius ~3500km.

    So, "pl have a relook at the graph" for the labels, where it shows Outer Core.
     
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  13. timojin Valued Senior Member

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    3,155
    Thank you for the graph
    Is the inner core solid and the outer core like liquid mercury ?
     
  14. DaveC426913 Valued Senior Member

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    Well, it's under tremendous pressure and temperature so rock behaves differently.
    Probably best if you just read the Wiki:
    https://en.wikipedia.org/wiki/Structure_of_the_Earth#Core
     
  15. Q-reeus Valued Senior Member

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    Interestingly it levels off for a good ~ 1000km depth. A region with no appreciable radial tidal g component. Squashed but not stretched. And definitely roasted!
     
  16. arfa brane call me arf Valued Senior Member

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    If you can handle it, there is a series of online lectures which seriously covers the subject of gravity. It looks to me like a senior or graduate level course in that subject.

    There are 28 video lectures, plus some tutorials, and it's all titled: The WE-Heraeus International Winter School on Gravity and Light (click it and see!).

    Being German (I suppose), everything is very efficiently presented. There are even blackboard wiper people, I kid you not.
     
  17. Q-reeus Valued Senior Member

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    Link doesn't work. But going to youtube.com, and a cut & paste of title "The WE-Heraeus International Winter School on Gravity and Light" into search bar there provides all the choice one needs. A bit heavy going though and huge overkill re OP query! Which is adequately handled within Newtonian gravity.
     
  18. arfa brane call me arf Valued Senior Member

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    I would say that even the first lecture on topology would be over more than a few people's heads.

    But it occurred to me that there might be one or two here who could answer any questions those with trouble understanding it all, me included, might have.
    Since this is supposed to be a science forum, maybe it is possible to learn something about the theories and something about all the mathematics that surrounds the Newtonian simplification like a fog of differential equations.

    At least it wouldn't be a bad idea if people with criticisms of the theories understood them to some extent.
    By the way, during one of Susskind's online lectures on youtube he answers your question. Again, although he goes fairly gently it isn't anywhere near the fast clip of the Winter School (in fact I find he gets distracted a lot for my taste, nonetheless if you can sit through the lectures, even if you ignore all the equations, just some of the things Susskind says along the way is good stuff).
    The Winter School lectures look like a one semester course, so there would have been assignments, tests and so on.

    Here at scifo, the "assignments" would be more like being able to explain, correctly, possibly to someone math-naive (like me, for instance), something about what the lecturer is saying, or maybe what a partial differential is and how you can describe a gradient with it, or something (anything, please!).

    That is, if moderators or other parties can assign the grades, so to speak. On the other hand, why bother?
     
  19. Q-reeus Valued Senior Member

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    My question? Maybe you are thinking of another thread? Anyway I agree Sussking has in the YT lectures an annoying propensity to often lose a train of thought. Presumably his written material is free of such foibles and a better way to study.
     
  20. RADII Registered Member

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    This to all: my impression is that g <> 0 @ the Core, but rather is more or less symmetrically distributed around the Core such that as you get closer to the Center, g is equal in all directions.
     
  21. DaveC426913 Valued Senior Member

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    g is a force, with a direction, but opposing forces cancel, and all you care about is the net force.

    Near the centre g will be small, almost zero, but still pointing to the centre.
    At the centre, it will have zero magnitude and an essentially undefined direction.
     
  22. Q-reeus Valued Senior Member

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    2,533
    Actually a proper acceleration for a static observer - but I'm sure you knew that. Maybe #17 had in mind difference between idealized concentric shells model of Earth vs actual highly complex case. With e.g. huge convective blooms between core and mantle regions, differential rotations between core(s) and mantle etc. Hence density fluctuations that are more or less 'chaotic'. But in the overall scheme very minor departures from the ideal concentric shells model. Or more strictly (rotation) - concentric geoids.
    Or maybe just confused as to what g means for spherically symmetric matter distributions.
     
  23. RajeshTrivedi Valued Senior Member

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    1,415
    oops! Sorry;
    I continued with your#2 wherein you stated centre.
     

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