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Slowing down the earths rotation and changing the moons orbit

  1. Jun 10, 2010 #1
    I have a friend who believes that if he spins in a circle he is changing the angular momentum of the earth and therefore is slowing down the earths rotation. Can someone explain exactly why this is not true? To me it is common sense, but I am no physics major.

    Also, the same friend is telling me that if we put too many wave generators in the ocean that use the ocean tides to make energy, we will change the orbit of the moon and cause major problems. Can someone also explain why this is not true?

    Thank you for your expertise.

    Kind regards,

    GoodQuestions
     
  2. jcsd
  3. Jun 10, 2010 #2

    D H

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    Over the short period of time that your friend can spin around in a circle, the Earth+friend system is essentially an isolated system. As angular momentum is a conserved quantity for an isolated system, your friend's spinning is changing the Earth's angular velocity. Whether that is an increase, decrease, or change in direction depends on the direction your friend is rotating and his latitude. Whether that change amounts to a hill of beans is another issue. It doesn't. For one thing, your friend can't keep spinning forever. For another, his puny angular momentum is immeasurably small compared to that of the Earth. Assuming your friend is rather bulky but can still spin rather quickly, his angular momentum is less than 10-27 of that of the Earth.

    Some people like to see problems where none exist.
     
  4. Jun 10, 2010 #3
    When you jump the earth's path around sun also changes for a moment. Gravity pulls you back together afterwards.
     
  5. Jun 10, 2010 #4

    DaveC426913

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    I think this friend is saying in principle. Which is ... well ... true.

    In principle, he is changing the Earth's rotation.


    Not quite as convinced about the other one.
     
  6. Jun 10, 2010 #5

    diazona

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    My instinct would be that all the "wave generators" humanity could possibly build would not have a noticeable effect on the moon's orbit. The moon is BIG. So is the earth. And it's easy to underestimate just how hard it is to push them around by any significant amount.

    But that's just my instinct... I don't have any calculations or anything to back that up.
     
  7. Jun 10, 2010 #6

    DaveC426913

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    Yes. But the friend isn't claiming waves are moving the Moon noticeably, just that they are moving it. Again, in principle.


    Of course, they're also moving the Sun... in principle...
     
  8. Jun 10, 2010 #7
    By even building skyscrapers we change the angular velocity of the earth by altering its rotational inertia. Of course this effect is negligible.
     
  9. Jun 10, 2010 #8

    diazona

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    Yeah, agreed.

    It seems to me that some people have trouble with the idea of an insignificant/unnoticeable influence, so I've become kind of wary of saying "yes but it's really small."
     
  10. Jun 11, 2010 #9

    D H

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    The friend is claiming that "if we put too many wave generators in the ocean that use the ocean tides to make energy, we will change the orbit of the moon and cause major problems." Causing major problems, whatever that means, is far beyond in principle.

    In principle, we are changing the Earth's rotation rate by sending space vehicles up to geosynchronous orbit. Anyone care to take a stab at how much?
     
  11. Jun 11, 2010 #10
    Some definitions might be in order.

    Call the Earth, E which constitutes everything in and on it, including friend, f. Call the Earth minus your friend, E'. E = E' + f. Everyone here has been referring to E'.

    I suppose that idea, here, is that if E' spins slower, there will be less tidal drag from the moon on the Earth. This may call for a UN general alert to ensure that the planet's population doesn't all spontaneously decide to start spinning in the same direction. Should something like this happen, the Moon may not follow its predetermined orbit but miss it by an Angstrom. Considering the shear magnitude of one Angstrom should give us all pause, considering what jumping up and down all at once can do.
     
  12. Jun 11, 2010 #11

    DaveC426913

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    Conceded. I missed that salient tidbit.

    This friend isn't being smart-***, as I thought he was; he seems to think there's a measurable effect.

    Bzzt.
     
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