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I Does the rotation speed of a planet affect its gravitational pull

  1. Mar 29, 2017 #1
    For example lets a satellite was orbiting the moon which has no atmosphere. If the moon suddenly started spinning twice as fast would it effect the satellite's orbit even though the satellite is separated by the vacuum of space from the moon?

    Easier way to put it, if the Earth suddenly spun 1000 times as fast, would the moon float away? I know that things on the planet may be thrown off due to centrifugal force, but would the Earth's gravitational pull become weaker?
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  3. Mar 29, 2017 #2
    I believe it would, since angular momentum needs to be conserved in such a system.
  4. Mar 29, 2017 #3


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    The force we identify as gravity here in Earth is, in fact, slightly less than it would be if the Earth were not spinning. The full force of gravity is offset to a small amount by the centrifugal force. Or, more precisely, some of the gravity is used as a centripetal force to keep us rotating with the Earth.

    This, however, does not affect objects in orbit. The moon's orbit is essentially unaffected by the Earth's spin.
  5. Mar 29, 2017 #4
    Why does angular momentum need to be conserved? Centrifugal force is fictitious force when means its not mediated by a particle. If an object is floating a in circular space station in a vacuum and the space station is spinning creating a centrifugal pulling everything down at 9.8 m/s^2, but the floating object will float forever unless it is attached to the station and grabbed by its spin.
  6. Mar 29, 2017 #5
    So a spinning black hole should not lose its event horizon no matter how fast it spins?

    Is this article wrong?
  7. Mar 29, 2017 #6


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    Spinning black holes are unique animals. For example, they have two "surfaces", the event horizon and the "ergosurface". Anything that enters the region between these two is forced to orbit with the black hole's rortation due to frame dragging( its impossible for an object to orbit in a direction opposite to the rotation).
    The article says nothing about the gravity of the black hole weakening, but instead mentions a theoretical method of "disrupting" the event horizon. (A weakening of the gravity would just lead to the event horizon shrinking.)

    The Earth also does some frame dragging, however it is very very weak, and even spinning the Earth up by 1000 times wouldn't cause a significant increase in it.
  8. Mar 29, 2017 #7


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    You started this thread asking about the rotational speed of a planet, not a black hole. The gravitational effects of rotation are altogether negligible for a planet, which we can properly analyze using ordinary Newtonian theory.
  9. Mar 29, 2017 #8


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  10. Mar 29, 2017 #9
    How about the poles?
  11. Mar 29, 2017 #10
    Law of Conservation of Angular Momentum
  12. Mar 29, 2017 #11
    Conservation of angular momentum has nothing to do with the question that the OP is asking.
  13. Mar 29, 2017 #12
    What, is Conservation of Angular Momentum not a thing anymore? Or has George Gamow been teaching me false information...
  14. Mar 29, 2017 #13
    With Newtonian gravity, if the planet around which the satellite is orbiting is spherically symmetric then rotation doesn't matter as the satellite "sees" the same gravitational source at all times. If it's not spherically symmetric, then there would be perturbations to the satellite's orbit and those would be different for different rotation speeds.
  15. Mar 29, 2017 #14


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    Conservation of momentum tells us that in order to spin twice as fast, some one or some thing would need to exert an external torque on the moon. So take that as a given. Someone or something has reached in and applied an external torque on the moon, thereby doubling its spin rate. That has little or nothing to do with the orbital velocity of a satellite going around the now faster-spinning moon.
  16. Mar 29, 2017 #15


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    Conservation of angular momentum is a real thing. It just doesn't have anything to do with the question in this thread.
  17. Mar 29, 2017 #16
    Okay I see now. I guess I misconceived what the OP was proposing. Thanks for clearing that up.
  18. Mar 30, 2017 #17


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    I've never studied relativity but I think the fact that the earth is rotating increases it's mass very slightly and hence increases gravity very slightly. The effect must be very very small.
  19. Mar 30, 2017 #18


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    it's actually slowing .... day's used to be somewhat shorter long ago

    from wiki ....

    Last edited by a moderator: May 8, 2017
  20. Mar 30, 2017 #19
    Centrifugal force does not only affect the value of gravity on a planet, but the direction of its vector.
    If a weight is suspended by a rope, we all assume it points in a vertical line, but thats not right. Lets assume the planet as a sphere .... If the line drawn by the rope were extended downwards, it would not cross through the center of the planet.
    Hence ... that vertical line, is not a geometric vertical, but only an apparent one.
    This divertion of the vertical line has, for example, very strong effects on atmospheric circulation and winds.
  21. Mar 30, 2017 #20
    If you add angular momentum to a planet does it not increase its energy and therefor modify its energy-momentum tensor, even if it is negligible for the situation in the OP?
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