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Homework Help: Just need some clarification

  1. Aug 12, 2006 #1
    I'm working through a problem that was posted here a while ago (https://www.physicsforums.com/showthread.php?t=115170). maltesers posted the equation (Gm)/(r-S)^2 = (GM)/r^2, so far this makes perfect sense, he then goes on to solve for S with S = r - (mr^2)/M, and it is here that he loses me. How did we eliminate the ^2 to go from (r-S)^2 to S = r - ...?

    I've been out of the math game a little too long!
     
  2. jcsd
  3. Aug 12, 2006 #2
    That equation is actually incorrect. It should have been

    [tex]\frac{Gm}{(r-S)^2} = \frac{GM}{S^2}[/tex]
     
  4. Aug 12, 2006 #3

    George Jones

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    This not the only thing wrong.

    Look at units. In maltesers' last equation, S is a length, while the last term on the right has units of length squared.
     
  5. Aug 12, 2006 #4
    Now that it's known that the original equation is wrong, why bother about what was derived from it. :tongue2:

    Thanks for pointing that out.
     
  6. Aug 12, 2006 #5
    Thanks alot, I wasted way too much time on this one.
     
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