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Hole through earth

  1. Jan 25, 2010 #1
    Hi,
    i am preparing for a test and i really have no idea how to solve the following question.
    i would appreciate if some one could give me the direction of solution explaining why things should be done the way the should. i am really clueless here.

    The residents of a small planet have bored a hole straight through its center as part of a communications system. The hole has been filled with a tube and the air has been pumped out of the tube to virtually eliminate friction. Messages are passed back and forth by dropping packets through the tube. The planet has a density of 3820 kg/m3, and it has a radius of R=5.31E+6 m.

    What is the speed of the message packet as it passes a point a distance of 0.480R from the center of the planet?

    How long does it take for a message to pass from one side of the planet to the other?

    thanks
     
  2. jcsd
  3. Jan 25, 2010 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Hi zhenyazh! :smile:

    What is the gravitational acceleration at radius r inside the planet?

    If it's g(r), then d2r/dt2 = -g(r). :wink:
     
  4. Jan 25, 2010 #3
    thanks for your response but i still don't get it.
    can u give a more detailed explanantion?
     
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