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Question on gravitation law derivation

  1. Feb 2, 2013 #1
    Hi, guys.
    I was wondering on Newton's Gravity Law derivation, and I found this page: http://www.relativitycalculator.com/Newton_Universal_Gravity_Law.shtml
    Everything seems clear, but the first step is just killing me, because I can't get it.
    Assuming small incremental changes in s; [tex]
    \lim_{t\rightarrow 0} {s} \rightarrow 0
    we have the following ratios
    [itex]\frac{\omega}{\nu}[/itex]=[itex]\frac{s}{r}[/itex], and [itex]\frac{t}{T}[/itex]=[itex]\frac{s}{2πr}[/itex]
    Could someone help me out? Explain, or just say, which part of math do I have to cover in order to understand that?
    (btw, I did pre-calculus, and calculus, so concept of limits is familiar to me)
    Thanks in advance.
    Last edited: Feb 2, 2013
  2. jcsd
  3. Feb 3, 2013 #2


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    Staff: Mentor

    It's just telling you the ratios of everything. The vector changes at the same rate that the distance traveled does since v and r are equal. If you double w you double s as well.

    In the 2nd ratio, T is the total time of one orbital period and 2*Pi*R is the total distance of the orbit. As t changes, which is the time it takes to transverse the incremental time period s, s changes as well. If you double t you double s. Does that make sense?
  4. Feb 3, 2013 #3
    Yes, thank you a lot! I had kind of intuitive feeling about it, but I wasn't sure if it is correct.
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