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Homework Help: Potential and field of a thing circular ring

  1. Nov 25, 2013 #1
    1. The problem statement, all variables and given/known data

    I'm trying to work through an example in my classical mechanics textbook (Fowles and Cassiday, 7th ed, example 6.7.2. The problem I am having is when he expands the integrand into a power series. I'll write out the first part of the solution. The question is "find the potential function and the gravitational field intensity in the plane of a thin circular ring

    2. Relevant equations


    3. The attempt at a solution

    [tex]\Phi=-G\int\frac{dM}{s}=-G\int^{2\pi}_0\frac{\mu R d\theta}{s}[/tex]where [itex]\mu[/itex] is the linear mass density of the ring,R is the radius of the ring and M is the mass of the ring. Then using the law of
    cosines we have[tex]\Phi=-2R\mu G\int^{\pi}_0\frac{d\theta}{\sqrt{r^2+R^2-2Rr\cos\theta}}[/tex][tex]\Phi=-\frac{2R\mu G}{r}\int^{\pi}_0\frac{d\theta}{\sqrt{1+(R/r)^2-2(R/r)\cos\theta}}[/tex]The next part is where I am getting stuck. It says to expand in a power series of x=R/r [tex]\Phi=-2x \mu G\int^{\pi}_0\left[\left( 1-\frac{1}{2}x^2+x\cos\theta\right)+\frac{3}{8}\left(x^2-2x\cos\theta\right)^2+\dots\right]d\theta[/tex]What is happening in this step? Sorry if this belongs in intro physics. It's a junior year course though.
  2. jcsd
  3. Nov 25, 2013 #2


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  4. Nov 25, 2013 #3
    oh my goodness. thank you.
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