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New to Physics. Can you explain?

  1. Jun 30, 2011 #1
    I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

    They say the earth keeps spinning because it started spinning and now there is no external force applied to it to stop it.

    The Sun is an external force.

    I actually believe that only Stars can make bodies of mass spin. Hence the Earth and other planets like Jupiter have their satellites in a locked position(not spinning).

    Why doesn't the Sun keep the Earth in a locked position like the Earth keeps the Moon in a locked position? or like Jupiter keeps its satellites in a locked position?
  2. jcsd
  3. Jun 30, 2011 #2
    I am pretty sure that eventually the earth will stop rotating, but it will take quite a long time. The earth is quite massive.
  4. Jun 30, 2011 #3


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    The moon isn't quite in a locked position, it rate of rotation coincides with it's rate of orbit, so that the same hemisphere faces the earth, but due to the axis of rotation versus plane of it's orbit, and the elliptical orbit of the moon, (and the position of an earth observer) over time an earth bound observer can see 59% of the moons surface due to libration:


    That wiki article contains a link to tidal locking:

    Last edited: Jun 30, 2011
  5. Jun 30, 2011 #4


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    This is actually a guess on my part, but I think it's because the torque that causes the effect falls off like 1/r which means something with a smaller separation distance (such as the Earth/Moon distance) will fall into tidal locking faster than something with a large separation distance. That's a guess though, I haven't put pen to paper on that one.
  6. Jun 30, 2011 #5
    Re-read my question.

    The Moon doesn't spin 100% like the Earth does around the Sun. It oscillates because its fighting to get out of Earth's orbit. 59% isn't much. 59% - 50% = 9% of oscillation.

    The Earth doesn't seem to have the "Power" to spin a body of mass like the Sun does.
  7. Jun 30, 2011 #6


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    This is incorrect. Any spinning object keeps spinning unless a force is applied, like the law states. The Earth and all the planets and moons and everything else are spinning because they were spinning when the gas cloud that formed the solar system first collapsed and formed them. Had there not been enough mass within this cloud to give the Sun enough mass to ignite fusion, and hence it would not have been a star, it would still have been spinning and so would anything else that had formed with it.

    The moon is tidally locked to the earth because of Tidal Locking: http://en.wikipedia.org/wiki/Tidal_locking

    The Earth isn't tidally locked to the sun because the gravitational gradiant isn't sufficient enough to cause that effect. If you look at Mercury however, it IS tidally locked in a 3:2 orbital resonance. The eccentricity of its orbit causes this to be stable. The moon is in a 1:1 orbital resonance with the earth because of its nearly circular orbit with low eccentricity. (And even then it isn't perfectly synced, there is a small shifting back and forth that allows us to view a little bit more than 50% of the moon over the course of its orbit.)

    The Earth caused the moon to STOP rotating as fast as it was and become locked. It did not cause the moon to spin.
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