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Gravitational problems

  1. Mar 26, 2009 #1
    1. The problem statement, all variables and given/known data

    1. Suppose you leave the solar system and arrive at a planet that has the same mass per unit volume as the earth but has 10 times the earth's radius. What would you weigh on this planet compared with what you weigh on earth ??

    2. Suppose that Kepler had found that the period of a planet's circular orbit is proportional to the square of the orbit radius. What conclusion would Newton have drawn concerning the dependence of the gravitational attraction on distance between two masses.

    2. Relevant equations

    F= -Gm1m2/r^2

    3. The attempt at a solution

    1. So I use Newton's law of gravitational and get you weigh 1/100 (radius is 10, square is 100) as you do on Earth. I am wrong. So how should I approach this problem ?

    2. Can you guys hint me how should I do this one ?
     
  2. jcsd
  3. Mar 26, 2009 #2

    LowlyPion

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    2 things changed. Radius and what else?
     
  4. Mar 26, 2009 #3
    Radius change so mass will change too right ??
     
  5. Mar 26, 2009 #4

    LowlyPion

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    Correct.

    How does the total mass change with the radius if the density is a constant?

    And of course the distance from the center changes directly with the radius.
     
  6. Mar 26, 2009 #5
    Mass changes at the same rate as radius does. Right ?
     
  7. Mar 26, 2009 #6

    LowlyPion

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    What is the formula for that?
     
  8. Mar 26, 2009 #7
    So F= -Gm(10M)/ 100R^2 ??
     
  9. Mar 26, 2009 #8

    LowlyPion

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    No what is the formula for the mass given the density?

    Isn't that Mass = density * volume?
     
  10. Mar 26, 2009 #9
    Oh yeah, I misunderstood you.

    but how can this help with finding your weight ?
     
  11. Mar 26, 2009 #10

    LowlyPion

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    What is the volume of a sphere again?
     
  12. Mar 26, 2009 #11
    V= 4/3*pi*r^3
     
  13. Mar 26, 2009 #12

    LowlyPion

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    So that means then that M ∝ r3 ?
     
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