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B Gravity facts, or close to fact

  1. Jul 9, 2017 #1

    Hopefully I am not asking to simple questions, but I am just confused from all internet information, and could need some help to shed light on a few things related to gravity, just to make it an undoubtfull fact to work with.

    When I am correct, gravity is a consequence ?
    What I have read is that Einstein said gravity is a consequence of a lot of matter that distorts space-time arround it, which makes other objects being pulled into that distorted space-time curvure, towards earth. This makes it look like gravity is a force, but it is not.
    Is this the true complete picture, fact ?

    Also, why does venus not get sucked into the sun ? The gravitational pull of the sun is huge, especially at that short distance towards venus. Yet it does not get closer to the sun, how ?
    I read that planets keep their orbit due to the speed they have, but that does not make sense to me, they close in and increase distance all the time, this would make the speed obsolete to explain the not being pulled in due to high gravitatinal force. Can anyone help me out on this ?

  2. jcsd
  3. Jul 9, 2017 #2


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    Staff: Mentor

    Your second question about Venus and planetary orbits belongs over in the Classical Physics Section. Please start a new thread there for that question.
  4. Jul 9, 2017 #3
    Hi Nugatory,

    I am sorry, but that I do not understand. How can my question about suns gravity on venus not belong to the special + general relativity section ?
    I mean, I aint looking for the orbit or movement, but an answer in relation to gravity, does that not belong here (also) ?

    PS: Very possible would be that an answer comes from space-time curvure...
  5. Jul 9, 2017 #4


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    Staff: Mentor

    It is completely covered by Newton's classical theory of gravity, no general relativity involved.
    Just as you must learn to stand up before you can run, you cannot understand general relativity and Einstein's theory of gravity until you have a solid understanding of classical mechanics and Newton's theory of gravity.
    Last edited: Jul 10, 2017
  6. Jul 9, 2017 #5


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    Staff: Mentor

    Now this part is a relativity question....
    It's not just matter that curves spacetime. The curvature is determined by a mathematical object called the "stress-energy tensor" , which includes energy and pressure as well as matter. However, for many interesting and important problems it turns out that the contribution from matter is the only one that is large enough to influence the results significantly, so we often can ignore everything else.

    To understand how spacetime curvature produces effects that look like forces but aren't, search this forum for two short videos made by our member @A.T. They are waaayyy better (and shorter) than most of the stuff out on the internet.
  7. Jul 10, 2017 #6


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    Science Advisor

    To understand the local picture have a look at these videos:

    For the global picture look at the links in the video description of the first clip (at youtube).

    This sounds like you have to learn Newtonian laws of motion first, and how centripetal acceleration is related to radius and tangential speed. Your misunderstanding here is not specific to gravity:


    Unfortunately explaining orbits in GR requires too many dimensions, so you cannot visualize it as easily as the radial falls above.
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