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Gravitation and velocity problem

  1. Jan 3, 2012 #1
    1. The problem statement, all variables and given/known data
    object.jpg

    2. Relevant equations
    2ΔEk=-ΔEp


    3. The attempt at a solution

    -Gm1m2(1/r1-1/r2)-Q=mv2

    plug in given values solve for v.

    my answer is 5.35x106m/s
    given answer is 2100m/s
     
    Last edited: Jan 3, 2012
  2. jcsd
  3. Jan 3, 2012 #2

    CompuChip

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    Any ideas?
     
  4. Jan 3, 2012 #3

    D H

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    Watch your units, be careful to distinguish between altitude and radial distance, and most important of all, show your work. There's no way to tell you where you went wrong if you don't show your work.

    BTW, the given answer is correct.
     
  5. Jan 3, 2012 #4
    its 400000m not 400m
     
  6. Jan 3, 2012 #5
    1600v2=-G(1600)(5.98E24)/(6.38E6+400000) - (-G)(1600)(5.98E24)/ (6.38E6+100000) - 8.3E8

    v=1407 m/s

    still not getting the correct solution.
     
    Last edited: Jan 3, 2012
  7. Jan 3, 2012 #6

    D H

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    Showing your work does not mean spewing out an equation without units and without justification. What are the relevant equations?
     
  8. Jan 3, 2012 #7
    i am stumped. i tried using

    ΔEk+Q=-(0.5)ΔEp

    where Ek= 0.5mv2

    and Ep= -GMm/r

    Q is heat energy. Given in problem.
     
    Last edited: Jan 3, 2012
  9. Jan 4, 2012 #8
    why do you have a 0.5 in the energy conservation equation?
     
  10. Jan 4, 2012 #9
    upon further reading i found that

    ΔEk+Q=-0.5ΔEP for CIRCULAR ORBITS ONLY

    am i correct in assuming that

    ΔEk+Q=ΔEp would apply for a object falling to Earth
     
  11. Jan 4, 2012 #10
    so i tried it and did get correct answer

    0.5mv2+Q=-GMm(1/r1-1/r2)

    r1=radius earth + 400000km
    r2=radius earth + 100000km

    when i solve for v i get 2133 m/s
     
    Last edited: Jan 4, 2012
  12. Jan 5, 2012 #11
    I got 2096 m/s ≈ 2100 m/s
     
  13. Jan 5, 2012 #12

    BruceW

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    I think you've done it right, but there might be some rounding error, which means your answer isn't exactly the same as the given answer. good job!
     
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