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Why the gravity is weaker in the poles than in the equator

  1. Nov 7, 2003 #1
    according to newtons law since the earth is flatten in the poles the distance to the center of the earth is bigger in the equator and therefore its gravity is stronger there.
    with this same argument you could say that if you got to the center of the earth the distance tends to 0 and therefore the gravity tends to infinite, hummm

    according to me if the earth didnt spin the gravity should be bigger in the equator because the vertical column of earth volume till the antipodes is higher there so there should be more gravity but when the earth spins the centrifugal force lowers the gravity in the equator but not in the poles.

    the reason for the eatrh to be flatenned in the poles is the same centrifugal force (wouldnt happen the same with a clay ball fast spinned)

    my teacher explained me that newton didnt say this but i know to be right
  2. jcsd
  3. Nov 7, 2003 #2
    i meant that the gravity is stronger in the poles than in the equator
  4. Nov 7, 2003 #3
    Also due of relativistic effects

    Gm(1-(v/c)^2)^½/r^2 = g
  5. Nov 7, 2003 #4


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    At the scale of planets, you can often consider everything to be liquid. The forces involved are so large that the shear strength of materials isn't sufficient to overcome it.

    Now, if you imagine the surface of the earth as covered in water, and without tides, for a moment, you can see that the net force of gravity at the surface must be equal everywhere. Otherwise, there would be flow towards the- area of greater attraction- like the flow caused by the tides. So, the shape of the earth effectively balances net surface force.

    BTW: Since the internals of the earth are not perfectly spherical, the surface of the oceans is not either. In fact, a survey of average surface of the ocean generates a map of the ocean floor.
  6. Nov 7, 2003 #5


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    This is actually backwards. At the center of the earth, the gravitational field is zero -- the mass above you exactly balances the mass below you, in every direction.

    The rule to remember is that the only mass that effects you is the "interior mass," which is that mass within the sphere defined by the distance between you and the center of mass. Mass outside this sphere does not affect you gravitationally, because it is balanced on both sides. When you're on the surface, the entire Earth is within that interior sphere, so you have the maximum gravitational field strength. When you get the center of the earth, the interior volume is zero, so the interior mass is zero, hence there is no gravitational field.

    - Warren
  7. Nov 7, 2003 #6


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    This is why it is best to actually make the effort to learn the physics rather than make a half-hearted attempt to understand it and then discard it due to misunderstanding it.

    I think this is a pathology of smart people, so don't be upset (I have it too ): Sometimes you'd rather figure something out on your own than learn it from someone else, even though if you try to figure it out on your own you may reach the wrong conclusion.
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