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Homework Help: Escape speed

  1. Mar 26, 2007 #1
    1. The problem statement, all variables and given/known data

    By considering the gravitational potential energy of an object at the surface , find an expression for the escape speed from the surface and determine its value.

    2. Relevant equations

    GPE= mgh

    3. The attempt at a solution

    KE= 1/2* m * v^2

    GPE= mgh

    GPE+ KE = 0

    mgh = - 1/2 * m * v^2
    gh= - 1/2 * v^2


    v^2= -2gh

    v= sqare root of (-2gh)

    this is the escape velocity...
     
  2. jcsd
  3. Mar 26, 2007 #2

    Dick

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    You've computed velocity needed to reach a height h in a uniform gravitational field. Is that the same concept as 'escape velocity'?
     
  4. Mar 26, 2007 #3

    HallsofIvy

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    You also appear to be assuming gravitational force is constant. If that were true, there would be no "escape velocity". It is impossible to "escape" a constant force!
     
  5. Mar 26, 2007 #4
    Well if ur looking for escape velocity....it should be[tex]\sqrt{\frac{2GM}{R}}[/tex]
     
  6. Mar 26, 2007 #5
    thanks for the info...
    so how shall i start off instead
     
  7. Mar 26, 2007 #6
    you shud use the work energy theorem
     
    Last edited: Mar 26, 2007
  8. Mar 26, 2007 #7

    Dick

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    You should change your expression for gravitational potential energy to one appropriate for a nonconstant field.
     
  9. Mar 26, 2007 #8

    HallsofIvy

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    Remember that gravity is an "inverse square formula"!
     
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