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Planetary gravity question

  1. Mar 17, 2013 #1
    Since gravity gets stronger the closer you get to the center, would a slightly egg shaped planet just continue to squeeze until it flattens out? The closer in areas would feel an ever stronger force of gravity and the poles of the "egg" will feel a progressively weaker force of gravity.
  2. jcsd
  3. Mar 17, 2013 #2


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  4. Mar 17, 2013 #3
    What he describes doesn't happen in nature because under the surface of the planet the gravitational force is proportional to the r, not r squared, and since the mass of the planet diminishes as it comes closer to the centre, gravitational force effectively diminishes. It is 0 at the centre
  5. Mar 17, 2013 #4


    Staff: Mentor

    Have you ever taken a piece of clay and pushed down on the top? What happens to the sides? Do they stay right where they are or do they squish out?

    Now, think about a mountain. The weight of a mountain presses on the base of a mountain just like your hand presses on the clay. And in the same manner, the base wants to spread out. If the mountain is too high then there is too much pressure on the base and it will squish, spreading the base out and reducing the height until the remaining base is strong enough.

    An egg shaped planet is a planet with a big mountain.
  6. Mar 18, 2013 #5


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    The gravitational field does not get stronger as you get nearer to the centre, once you are below the surface. It is Zero at the centre. Gravitational potential, however, gets more and more negative as you get nearer the centre (i.e. things will always tend to fall towards the centre of a spherically symmetrical body)
    A non-spinning, fluid / 'soft' planet will take the shape of a sphere, because that is the shape with lowest gravitational potential (just as small water drops and bubbles are spheres - but in that case, it's because of the electrical inter-molecular forces.)
  7. Mar 18, 2013 #6
    The closer areas actually feel less gravity. They might be closer to the center, but because of this fact they also have less mass below them exerting a gravitational force. The further areas might be further, but this also means that they have MORE mass pulling on them gravitationally, so the net effect is that they actually feel a stronger force.

    As an example, the surface gravity of the sun is 29x larger than the earth....despite the surface being MUCH MUCH MUCH further from the center (and the sun being less dense).

    As another example, YOUR surface (your skin) is MUCH MUCH MUCH closer to your center then the surface of the earth is to its center...and yet the gravity you exert on your skin is practically nothing.
  8. Mar 18, 2013 #7
    The Earth is slightly squashed shaped because it spins. You weigh slightly less at the Equator than at the North or South Poles. (Centrifugal force) There is a complete BALANCE of Gravitational forces at the Center of the Earth because it is surrounded by the Earth's mass that is pulling in all directions. Therefore you would be weightless at the Earth's core. (But you would be crushed by the atmospheric density, assuming you could drill a hole that deep. )
  9. Mar 18, 2013 #8
    But atmospheric density is again mainly due to gravity, so no, one wouldn't be crushed by it.
  10. Mar 18, 2013 #9
    There would still be a 4000 mile column of air or rock or whatever directly above you, which DOES feel the gravity of half the earth on the other side. If you're in between, you WILL get crushed. You just might be weightless while doing so.
  11. Mar 18, 2013 #10


    Staff: Mentor

    Atmospheric density is due to pressure, not gravity. At the center of the earth you would have zero gravity but enormous pressure.

    [EDIT: and it is Lsos with the win!]
    Last edited: Mar 18, 2013
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