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Circular motion of earth and gravity

  1. Oct 23, 2006 #1
    Just to tease your minds, I propose this question. The earth is rotating, right? Centripital motion is provided by gravity. Objects undergoing circular motion tend to travel in a tangental trajectory relative to the circle they are released from. If we say the earth is still, and the universe is moving around it(using frames of reference), then how come if gravity suddenly went away, we would still fly off? This not only pertains to the earth but any other object, where the example of gravity which i used can be replaced by any other center-seeking force, such as tension.

    PS, I already know the answer, i just want to see who else knows it too.
  2. jcsd
  3. Oct 23, 2006 #2
    But we can't just say the earth is still, because that is an accelerating (non-inertial) frame of reference... on the other hand, if you physically stopped the earth's rotation and then spun the rest of the entire universe around, "Mach's principle" (which is basically that you would still fly off except for centripetal forces) is as yet only conjecture.
  4. Oct 23, 2006 #3
    we would still fly off?

  5. Oct 25, 2006 #4
    The universe as a whole cannot spin, spinning involves frames of reference and reference is with respect to the total mass of the universe
  6. Oct 25, 2006 #5
    You cannot say that, since acceleration is not relative but absolute. You can measure it when you are accelerating.
  7. Oct 25, 2006 #6
    when you say we would still fly off do you mean us as people, or the planet as a whole?
  8. Oct 27, 2006 #7
    Fg = ma
    a = 9.8ms-2
    if gravity suddenly went away then a would be 0, thus force would also be 0. However if we flew off then a would be negative because we would accelerating away from earth. However in terms of graitational force there would be none as gravity is no longer in force.

    Earth is rotating around the sun, which is subject to the sun gravitational force. Therefore earth would continue in its orbit. I would also assume the moon would no longer rotate earth and be under the pull of the sun.

    I think im wrong. I had a wild guess.
  9. Oct 27, 2006 #8
    I suppose if you want to look at it like that you could say we don't fly off, but everything around us does.
  10. Oct 28, 2006 #9
    general relativity does teach us that the universe as a whole cannot spin.

    inertia is a function of the rest of the mass in the universe:

    "If, in a material spatial system, there are masses with different velocities, which can enter into mutual relations with one another, these masses present to us forces. We can only decide how great these forces are when we know the velocities to which those masses are to be brought. Resting masses too are forces if all the masses do not rest. ... All masses and all velocities, and consequently all forces, are relative. There is no decision about relative and absolute which we can possibly meet, to which we are forced, or from which we can obtain any intellectual or other advantage. (Mach, The Science of Mechanics, ch.2, vi-3, Open Court, 1960, 279) "

    In other words: interia here is related to mass there...
  11. Oct 28, 2006 #10
    This is excellent. A lot of interesting responses, and a lot of correct ones too! very good.....am I the only one who found this fun?
  12. Oct 29, 2006 #11
    Yes you probably are.

    Just for clarity, what is the "correct" answer?
  13. Oct 29, 2006 #12


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    Since the laws of physics are only assumed to be equivalent for all inertial frames, the same thing doesn't have to happen for the two cases you describe
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