Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Planetary gravity?

  1. Mar 9, 2009 #1
    If an object rotates in zero gravity, it generates a small gravity outwards, right?
    Now, howcome planets attract objects?

    It doesn´t make any sense!

    Last edited: Mar 9, 2009
  2. jcsd
  3. Mar 9, 2009 #2


    User Avatar

    Staff: Mentor

    Gravity and centrifugal force aren't the same thing. Objects have gravity whether they rotate or not and it is a function of their mass.
  4. Mar 9, 2009 #3


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Andy! Welcome to PF!:smile:
    No … gravity is always inwards

    i think you're thinking of "centrifugal force", which only exists in a rotating frame anyway, and isn't gravity …

    for example, someone floating inside a rotating space station feels no "outward" gravity … the space station won't affect him at all until one of the walls hits him :smile:
  5. Mar 10, 2009 #4
    Thank you tiny-tim!:smile:

    Allright, i guess this is about the core of it, and pretty much what i´m asking for but couldn´t put words on..

    What makes mass attract other objects in space?
  6. Mar 10, 2009 #5


    User Avatar
    Science Advisor
    Homework Helper

    The standard general relativity answer is that mass doesn't attract other masses, but the curvature of space causes mass to move :wink:
  7. Mar 10, 2009 #6
    Guess that´s a nice way of putting that not even NASA got any idea.. right?:wink:
  8. Mar 10, 2009 #7


    User Avatar
    Science Advisor

    You could say that. It's probably better to say that it's outside of the realm of what we can know.
  9. Mar 10, 2009 #8
    I don´t think so.
    Unless you are more of a believer than a scientist, you can´t draw that conclusion without proper reserch..

    I can´t let go of the resemblence to microscopical physics.. everything in this word is built by cores, and satelites of diffrent types cirkeling them..perhaps we should start there, and really understand, not only exploiting what we know they can do.. (Electricety for example).

    I won´t buy that it´s out of our grasp, i just feel that we should not only ask what, but also why?
  10. Mar 10, 2009 #9


    User Avatar
    Science Advisor

    In physics you have to accept some things as a set of axioms in order to prove everything else. And sometimes you just can't prove these axioms. This is probably the case with something like gravity (mass just warps spacetime, it's just what it does). I don't think it's a premature conclusion to draw, and I think it's necessary to state because a lot of people think science can explain the why for everything and I think that's a very dangerous notion.

    The resemblance you're talking about of an atom to a solar system is really a very poor analogy, because it makes no sense from a quantum mechanical point of view. The scales are all wrong, too.
  11. Mar 10, 2009 #10
    Ofcourse it´s a poor comparison!
    But it´s the best i can give to make you understand what i mean..

    If we only accept things for what they are, we can never truly understand WHY they are..

    I´m a car mecanic.. i know **** about the tec aspects about it all, i just say that in our lust for discovery, we might have missed some crusial parts..
  12. Mar 11, 2009 #11
    Re: Welcome to PF!

    The essence of GR is that they are the same thing and is the reason why under GR every imaginary coordinate frame is a valid coordinate frame.

    That's not a reason, that's a description. Gravity have been a hot topic for a while since its quite puzzling how all forms of energy have that property. And the main reason GR is accepted is because it can describe observations indescribable by the normal Newtonian laws.
    Last edited: Mar 11, 2009
  13. Mar 13, 2009 #12
    Perhaps it´s time to think out of the box and realize that Newtons law only is a theory based on one mans observations a very, very long time ago..
    It´s really not the fact that an object behave in a certain way that´s interesting, but why..

    As i said, to really understand why things work, we must know how it works, otherwhise it´s useless in alternate appliences.
  14. Mar 13, 2009 #13

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    There are many, many things wrong with this post.
    • "Perhaps it´s time to think out of the box"
      That is exactly what scientists strive to do. Some, such as Albert Einstein, exceed at doing so. General relativity supplanted Newtonian gravity because it does a better (more accurate) job of explaining what we se.

    • "Newtons law only is a theory ..."
      That term, it's only a theory, indicates a complete misunderstanding of science. The word theory is used in the lay sense to indicate a haphazard guess. Scientific theories are anything but haphazard guesses. Scientific theories are the pinnacle of scientific thinking.

    • "... based on one mans observations a very, very long time ago"
      The reason scientific theories are the pinnacle of science is because they are tested, retested, and re-retested by scientists from all over the world and over a long span of time. Newton's theory of gravitation withstood that test, and still withstands that test today. Newton's theory is no longer viewed as universal, but within a limited, but extremely useful, domain it does just as good a job of explaining things as does general relativity.

    • "It´s really not the fact that an object behave in a certain way that´s interesting, but why"
      Science is concerned much more modeling how an object behaves rather than why. The standard model of physics is much more about modeling what rather than why. Why do quarks exist? Philosophy. How do quarks bind to form protons, neutrons, etc? How do they interact? Physics.

      Eventually all science falls apart under a never ending stream of why questions. At some point, science has to react much as parents do with kids: "because that's the way it is." Science leaves those why questions to religion and philosophy.

    • "As i said, to really understand why things work, we must know how it works, otherwhise it´s useless in alternate appliences."
      Engineers much more than scientists are the ones who build things. Engineers don't particularly care about the deep, underlying reasons for why things work the way they do. Deep understanding of the standard model of physics or general relativity will not help a civil engineer build a better bridge. There are many, many levels of abstraction that separate theoretical physics from bridge building. Even electrical engineers who build computational devices that explicitly rely on quantum interactions operate at a different level of abstraction than do the theoretical physicists who are pushing the bounds of the standard model.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook