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Need Help on Gravitation Potential Energy Problem

  1. Apr 2, 2008 #1
    [SOLVED] Need Help on Gravitation Potential Energy Problem

    1. The problem statement, all variables and given/known data
    (a) A 5.3 kg particle and a 3.0 kg particle have a gravitational attraction with a magnitude of 2.6 x10^-12 N. What is the gravitational potential energy of the two-particle system?


    (b) If you triple the separation between the particles, how much work is done by the gravitational force between the particles?

    (c) How much work is done by you?


    2. Relevant equations
    F=(GmM)/r^2
    U=-(GmM)/r


    3. The attempt at a solution

    First I used the given information to solve for r using newtons law of gravitation. Then I pluged r=1.96E1 into the potential energy equation to get -5.41E-11J but that was wrong.
     
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  3. Apr 2, 2008 #2

    Hootenanny

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    Recheck your arithmetic, you separation value is incorrect.
     
  4. Apr 2, 2008 #3
    is R=2.012E1 and U=-5.25E-11 correct
     
  5. Apr 2, 2008 #4

    Hootenanny

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    Correct indeed :approve:
     
  6. Apr 2, 2008 #5
    Also would the solution to part b be U=-(GMm)/3r
     
  7. Apr 2, 2008 #6

    Hootenanny

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    Not quite, the work done is change in gravitational potential energy.
     
  8. Apr 2, 2008 #7
    Work=((-GMm)/3r)-((-GMm)/r)
     
  9. Apr 2, 2008 #8

    Hootenanny

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    Much better :approve:
     
  10. Apr 2, 2008 #9
    I got 3.5E-11J but that was also incorrect
     
  11. Apr 2, 2008 #10

    Hootenanny

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    I get something different. Furthermore, be careful with the sign.
     
  12. Apr 2, 2008 #11
    I got ((-GMm)/3r)=-1.75E-11J and ((-GMm)/r)=-5.25E-11J
    is((-GMm)/3r)=-1.75E-11J correct?
     
  13. Apr 2, 2008 #12

    Hootenanny

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    Yes, your calculations are correct, just be aware of the sign of your final answer.
     
  14. Apr 2, 2008 #13
    work=-3.50E-11J
     
  15. Apr 2, 2008 #14
    because work is equal to the negative change in potential energy
     
  16. Apr 2, 2008 #15
    and part c is 3.50E-11J
     
  17. Apr 2, 2008 #16

    Hootenanny

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    Correct.
     
  18. Apr 2, 2008 #17
    thank you for your help
     
  19. Apr 2, 2008 #18

    Hootenanny

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    A pleasure
     
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