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Trying to understand this concept

  1. Nov 29, 2009 #1
    If a force is exerted on the earth by the moon, how come the earth doesn't move? I don't get how this works with F=ma because shouldn't there be some acceleration caused the moon exerting an equal and opposite force on the earth?
  2. jcsd
  3. Nov 29, 2009 #2
    There is a force exerted to the Earth by the moon, but the net force results in the direction of the bigger force exerted to the Earth by the Sun.
  4. Nov 29, 2009 #3


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    But the Earth DOES move as the result of the moon's attractive force.

    Two objects attracting each other through the force of gravitation will typically orbit around their common center of mass.

    Because the Earth is so much more massive than the Moon, it will go in an orbit of tiny extent around the center of mass, whereas the Moon will go in a fairly large orbit.

    (The Sun complicates this simplified picture!!!)

    The same holds true for a star with respective to a planet around it.
    The star will WOBBLE a bit, as the result of the planet's effect on it, and this WOBBLING is detectable in how the wavelengths of the light emitted by the star looks like.

    This is, in fact, the major means at our disposal to discover planets orbiting other stars in the universe.
  5. Nov 29, 2009 #4


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    Furthermore, the most spectacular effect of the Moon's attractive effect on the Earth are the tides.

    The "loose" water facing the Moon is attracted more strongly than the Earth center, whereas the "loose" water on the backside of the Earth is attracted more weakly than the Earth's centre.

    Therefore, the water's differing acceleration fromthe Earth centre, coupled with that the earth's crust moves at the same speed as the Earth centre, results in bulges of water, one facing the Moon on one side of the Earth, another tidal bulge on the other side.
  6. Nov 29, 2009 #5
    Orbital mechanics are complicated but the basic principle is centripital force. The moon has tangential velocity so it is constantly falling toward the earth without getting much closer or farther away (it is changing its orbit over very long periods of time, I think, getting farther out).


    You can swing a heavy object (carefully) around in a circle to get feel for the pull of the moon on the earth. The pull is necessary to keep the object moving in a circle (or general orbital path in more advanced studies).

    Search online for Newton's Cannon to see how Newton compared the moon's orbit to a projectile fired from a mountain on Earth.
  7. Nov 29, 2009 #6


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    Read up on Earth Moon Barycenter.

    The Earth-Moon system rotates around a point 4600 kiliometers from the Earth's center of mass - that's a point 1700km below the Earth's surface.
  8. Nov 29, 2009 #7
    Awesome guys! I'll have to read up on some of this stuff! Thanks for the replies!
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