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Homework Help: Separate the variables by using kinetic energy and potential energy

  1. Sep 14, 2006 #1
    Is it possible to solve [tex]x^2\ddot{x}=\frac{q_{1}q_{2}}{4\pi\epsilon_{0}m}[/tex] to get x(t)? I can't see how!! Maybe I'm just missing something...
  2. jcsd
  3. Sep 18, 2006 #2
    You can separate the variables by using kinetic energy and potential energy, and conservation of angular momentum. I did that. I put
    it all into polar coordinates - and I came with an integral of a function
    of r (radius) that there's probably a formula for somewhere. It's not
    an easy integral though.

    It's simpler if you assume the particles don't have any angular momentum relative to each other. That's how I got curious about it - I did an exercise about the enormous acceleration a proton would have, jetting out of the nucleus, if there weren't any strong nuclear forces holding it
    in. So I wondered, what's its equation of motion?

    You could figure out the proton's final velocity without doing any
    complicated integrals - that would be (sort of) interesting too.

  4. Sep 18, 2006 #3


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    Let the squared position stand in the denominator on the right-hand side, and multiply your diff. eq. with the velocity.
    You now will get a first integral (take note of the sign of the square root used!), this can be integrated one more time.
  5. Sep 18, 2006 #4

    Well, I got

    The sign threw me though, so I'm not sure of it.

    Thanks! I might try it relativistically, to stop my brain from rotting before I go back to uni.
  6. Sep 18, 2006 #5
    One thing, there's only one m taken into account, which must be the mass of the particle which moves a distance x, or is x the distance between the two particles? I'm assuming they're both free to move, so wouldn't you need to take both masses into account? Hmmm
  7. Sep 18, 2006 #6
    in two dimension this is known as the one body problem (using a equilvalent equation in 2D)... you might wanna try to solve that (it is impossible to get r(t) explicitly, but you can find out the shape of the orbit) its a lot of "fun". after that go to two body problem... more fun awaits...
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