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How can tidal friction on Earth affect the Moon?

  1. May 1, 2007 #1
    I know the following statement to be true, being proven both mathematically, and verified by astronomical observations. What I need help understanding is HOW. . .

    "The law of Conservation of Angular Momentum requires that the slowing down of the rotation of the Earth around its axis caused by tidal friction must result in an equal increase of angular momentum of the Moon in its orbital motion around the Earth. . . [this] must result in the increase of its distance from the Earth and the decrease of its linear velocity." George Gammow, Gravity.

    Here's the question: How is the impact of tidal friction on the Earth COMMUNICATED to the Moon? By what force is the change in angular momentum of the Earth communiated to the Moon, thus affecting it's angular momentum? Gravity? If so, how?

    If by some supernatural occurance, the Earth suddenly stopped spinning entirely, can I assume that the Moon would immediately move rapidly farther away in its orbit? If so, why? how? what would "push" it away? Would the gravitational attraction to the Earth somehow lessen ?

    Thanks for any explanations in as laymen of terms as possible to a former student of first year physics only.
  2. jcsd
  3. May 2, 2007 #2
    The ocean tides also exert a gravitational force on the moon.

    In the supernatural case, moon would not be affected, although the .. supernature.. would start spinning. It's basically Newton's third law.
  4. May 2, 2007 #3


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    Friction between the rotating Earth causes the Earth to drag the tidal bulges along with it. As a result, they do not line up exactly with the moon but lead it a little. It is the gravitational attraction between these non-aligned tidal bulges and the Moon that pulls forward on the moon, increasing its angular momentum.

    If the Moon orbited the Earth faster than or in the opposite direction of the Earth's rotation the tidal bulges would lag behind the moon and you would get the opposite effect, with the moon losing angular momentum and getting closer to the Earth.
    Last edited: May 2, 2007
  5. May 2, 2007 #4


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    Like Phobos, Triton, and the theorized lost moons of Venus.
  6. May 2, 2007 #5
    Excellent. All good answers. Thank you.
  7. May 5, 2007 #6
    I appreciate the answers but I guess there is more to it. The earth spin axis follows a precession cone, to be completed every 26000 years. Newton has explained why, it's the torque exerted on the equatorial bulge by the gravitational force of the moon and a little of the sun.

    Torque is able to exchange angular momentum (vector) between bodies and if you realize that every 13000 years the vector direction is at opposing ends, then it should be clear that enormous amounts of momentum (vector direction) have been exchanged. I could imagine that friction forces in this process also make that exchange imperfect. I wonder if these aspects have been studied.
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