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B GMm/r^2 for r = 0?

  1. Jun 15, 2017 #1


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    This is probably a stupid question - but let's say you have a hollow Sun (M is hollow), with another mass in the center, so M and m share the same center of mass, the distance between the centers is zero.

    Would GMm/r^2 be infinite in this case?

    The force at the center of any object - if you define your system to be the center chunk of matter, and how that center chunk of matter sees the rest of the planet?


    sorry... I did not get enough sleep last night, that's my excuse!
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  3. Jun 15, 2017 #2


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    The 1/r^2 expression is only valid for the gravitational force on an object outside a spherically symmetric mass distribution.

    Your expression would go to infinity, but it would not be describing the force between the objects.
  4. Jun 15, 2017 #3


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    The force is, in fact, zero. Look up the Shell Theorem.

    Larry Niven wrote a novel called Ringworld which featured a ring around a star. He had to write a followup describing the giant ion engines the ring had to keep itself centred on its star because this fact was brought to his attention after publication...
  5. Jul 3, 2017 #4
    actually their will be no force acting because all the forces from different part of the sun will cancel out.
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