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Gravitational potential energy: derive expression for energy

  1. Jan 23, 2015 #1
    1. The problem statement, all variables and given/known data
    Part A:
    Derive an expression for the energy needed to launch an object from the surface of Earth to a height h above the surface.

    Part B:
    Ignoring Earth's rotation, how much energy is needed to get the same object into orbit at height h?

    Express your answer in terms of some or all of the variables h, mass of the object m, mass of Earth mE, its radius RE, and gravitational constant G.


    2. Relevant equations
    U=-GmmE/RE+h

    3. The attempt at a solution
    I figured that the energy you would need to bring the object to height h above the surface would equal the difference between the energy at the surface and the energy at h. I assume the object is at rest and the only energy it has is gravitational potential.

    E=-GmmE/RE-(-GmmE/RE+h)....E=GmmE/h

    This is not correct. Feedback I got was that the answer depends on RE. I've no idea how to go about part B.
     
  2. jcsd
  3. Jan 23, 2015 #2

    rock.freak667

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    From this line, how did you reach the right side?

    [tex] U = -\frac{Gmm_E}{R_E} - \left( - \frac{Gmm_E}{R_E +h} \right)[/tex]

    Your denominators don't cancel out.
     
  4. Jan 23, 2015 #3

    Dick

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    U=-G*m*mE/(RE+h). Review your algebra about subtracting fractions! You are doing it wrong.
     
  5. Jan 23, 2015 #4
    E=(-GmmE/Re)+(GmmE/Re)+(GmmE/h) First two terms cancel? I may have forgotten basic algebra.
     
  6. Jan 23, 2015 #5

    Dick

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    Yes, forgotten basic algebra. a/(b+c) is not the same as a/b+a/c. Find a common denominator etc.
     
  7. Jan 23, 2015 #6
    E=(-GmmE+h/Re+h)+(GmmE/Re+h)

    Can't believe I did that. Can I add +h to the numerator and denominator of the first term? Is that finding a common denominator? Then I get E=h/(Re+h).
     
  8. Jan 23, 2015 #7

    Dick

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    No, that's equally bad. a/b-c/d. Find common denominator, bd. Multiply first term by d/d, second by b/b getting ad/bd-bc/bd. Now you have a common denominator bd so you get (ad-bc)/bd. I hope this sounds familiar.
     
  9. Jan 23, 2015 #8
    Reworked and got E= [-GmmE(Re+h)+Re(GmmE)]/Re(Re+h)=GmmEh/Re(Re+h)

    Doing the algebra correctly, is the answer right? Is my reasoning sound? Sorry for forgetting elementary school, and thanks for your help.
     
    Last edited: Jan 23, 2015
  10. Jan 23, 2015 #9

    Dick

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    Yes, that looks much better. For part B you need enough energy to reach the radius of the orbit and then more energy to get to the correct velocity to maintain that orbit.
     
    Last edited: Jan 23, 2015
  11. Jan 28, 2015 #10
    Very helpful. I got to the right answer. Thank you!
     
  12. Mar 26, 2015 #11
    For part B, the energy that is extra and used to get the correct velocity, would that be kinetic or gravitational potential?
     
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