1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Orbital dynamics: "The familiar arc-cosine form"

  1. Feb 22, 2016 #1
    This arises in Joos's discussion of planetary motion, at the following URL:


    I've modified the notation in obvious ways.

    He asserts that the following expression is "the familiar arc-cosine form":

    $$-\int \frac{1}{\sqrt{a+2 bx-hx^2}} \, dx=\frac{1}{\sqrt{h}}\arccos \left[\frac{b-hx}{\sqrt{a+b^2h}}\right]$$

    It's not familiar to me, and I am having trouble confirming it. The following is the approach I have taken.

    $$\theta =\arccos (u)$$

    $$\cos (\theta )=u$$

    $$-\sin (\theta ) \frac{d\theta }{dx}=\frac{du}{dx}$$

    $$\frac{d\theta }{dx}=-\frac{\frac{du}{dx}}{\sin (\theta )}=-\frac{\frac{du}{dx}}{\sqrt{1-u^2}}$$



    $$\frac{d\theta }{h}=\frac{dx}{\sqrt{1-u^2}}=\frac{d\arccos (u)}{h}$$

    I seem to lose a minus sign that appears in the original form, but that may not be significant.

    $$\int \frac{1}{\sqrt{1-u^2}} \, dx=\frac{\arccos (u)}{h}$$

    $$\int \frac{1}{\sqrt{1-u^2}} \, dx=\frac{\arccos (u)}{h}$$

    $$\int \frac{1}{\frac{\sqrt{1-u^2}}{\sqrt{h}}} \, dx=\frac{\arccos (u)}{\sqrt{h}}$$

    $$\frac{1-u^2}{h}=\frac{1}{h}\left(1-\frac{b^2-2 bhh+x^2x^2}{a+b^2h}\right)=\frac{a+b^2h-\left(b^2-2 bhh+x^2x^2\right)}{h\left(a+b^2h\right)}=\frac{a2 b+hhx-h^2x^2}{h\left(a+b^2h\right)}=\frac{a+2 bx-hx^2}{a+b^2h}$$

    That's where I get stuck. Notice that the numerator in the final expression is identical to the radicand of the denominator of the lhs in the first equation. In order for that to work, ##a+b^2h=1## must hold. No matter how I try, I just can't seem to pound it into submission.

    $$C+\theta =-\int \frac{2 c}{\sqrt{\left(v_0^2-\frac{2 GM}{r_0}\right)+2 GM\mu -4 c^2 \mu ^2}} \, d\mu$$

    $$-\int \frac{1}{\sqrt{a+2 bx-hx^2}} \, dx=\frac{1}{\sqrt{h}}\arccos \left[\frac{b-hx}{\sqrt{a+b^2h}}\right]$$

    $$x=\mu ;\text{Null}h=4 c^2;\text{Null}a=v_0^2-\frac{2 GM}{r_0};b=GM$$

    $$a+b^2h=G^2 M^2+\left(V_0^2-\frac{2 GM}{r_0}\right)4c^2$$

    How might I approach this problem effectively?
  2. jcsd
  3. Feb 22, 2016 #2
    Whoops! ##\frac{du}{dx}=\frac{-h}{\sqrt{a+b^2h}}##. I guess I have to embarrass myself before I can find my mistakes. I think that correction will get me most of the way there.

    I would still like to understand where that "familiar form" came from. That is, how is it arrived at "working forward"?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook