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Homework Help: Find the Time period using First Integral

  1. Jul 30, 2016 #1
    Recently I lend the Classical mechanics book written by Goldstein from the library, In the last page, someone scribbled this problem without any solution, I am just curious and want to give a try the problem mentioned below. I just want to know whether my approach and my solution is correct or not.
    1. The problem statement, all variables and given/known data

    Find the Time period of the particle confined to the Potential energy of the form $V(x)=-\frac{V_{0}}{\cosh^2(\alpha{x})}$.

    2. Relevant equations
    Where $'E'$ is the Total Energy
    and $'m'$ is the mass of the particle

    3. The attempt at a solution
    Since the potential energy is negative, the total Energy required to escape from the potential is zero, i.e, $E=0$,
    The Potential diagram is attached with this thread, as we can see, the potential goes to 0 as $x$ goes to $\infty$
    or $-\infty$, If I take one half of the Potential say from $0$ to $\infty$ and substituting in the above integral becomes,


    Apparently the right hand side goes to infinity. And the time period of the particle confined also goes to infinity. Am I right? I solved the problem but I don't know whether my solution is correct or not. Is there any mistake in my solution?

    Attached Files:

    Last edited: Jul 30, 2016
  2. jcsd
  3. Jul 30, 2016 #2


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    If you take E = 0 there is no classical turning point and the motion is not periodic. In order to have a periodic solution you need to assume a bound state with E < 0.
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