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Homework Help: Conservation of Angular Momentum and Energy

  1. Mar 23, 2008 #1
    1. The problem statement, all variables and given/known data
    We measure a comet at perihelion to have a radial distance r1 and a velocity v1. Find the radial distance and velocity when it reaches aphelion.

    2. Relevant equations
    L=mvr=I*[tex]\omega[/tex]
    E=.5mv^2+U

    3. The attempt at a solution
    My professor skipped the chapter on Newton's Gravitation, F=GMm/r^2, so I don't think it applies here.

    The book is entirely symbolic--the answer expected should be in terms of v1 and r1.

    I labeled the radial distance and velocity r2 and v2, respectively. Since L is constant, m*v1*r1 = m*v2*r2, or v1*r1=v2*r2. Of course, this equation only gives the unknown radial distance in terms of the unknown velocity, or vice versa.

    I don't think U = -GMm/r would apply here, since, again, my professor skipped the chapter on gravitation. I tried solving using a second equality, .5m*v1^2-GMm/r1=.5m*v2^2-GMm/r2, but when I solved the system of equations with Mathematica, I got a gigantic mess for an answer.

    I appreciate your time.
     
  2. jcsd
  3. Mar 24, 2008 #2
    I think both your equations correct, you just did something wrong with the calculations. :smile:

    BTW, what's mathematica ? Are you using a http://en.wikipedia.org/wiki/Mathematica" [Broken] to solve these equations ?
     
    Last edited by a moderator: May 3, 2017
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