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Homework Help: Equation for velocity

  1. Apr 21, 2012 #1
    i was working on a differential equation to find the initial velocity of a rocket to get to height h.

    so i have, which i got and its correct cause its the solution in the book, v^2 = 2g(R/(R+h))-2gR+V(0)^2

    v = velocity
    g = acceleration of gravity at earths surface
    R= radius of earth
    h = height
    and i assume v(0) is the velocity v at height=0 so v(0)=0?

    Problem is when i keep getting a negative number. so i assume I'm making an amateur mistake.

    What am i doing wrong? ive tried converting etc. im using km
     
  2. jcsd
  3. Apr 22, 2012 #2

    rude man

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    Really? I'm concerned because the terms in your equation don't match dimensions. Specifically, 2gR/(R+h) does not match 2gR.

    Make that very concerned. :uhh:
     
  4. Apr 22, 2012 #3
    I'm trying to use this
    diff-eqn-rocket.jpg
    y = R (the radius of the earth) +h ( the height i want to reach).
     
  5. Apr 22, 2012 #4

    rude man

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    The last equation looks OK except there's a minor typo in it. The equation before that is OK, so use it to solve for v0.
     
  6. Apr 22, 2012 #5
    whats the typo?
     
  7. Apr 22, 2012 #6

    rude man

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    Bottom equation, " v2 = v2 ". Equating velocity with velocity-squared is a no-no.
     
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