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Homework Help: Classical mechanics ~ Potential energy and periodic movement

  1. Sep 26, 2013 #1
    Hey all,

    suppose there's a particle with Potential Energy : U(x) = A*[ x^(-2) - x^(-1) ] , where A is a constant.

    I'm supposed to find the energy required to make the particle go from periodic movement to unlimited movement.

    First thing I did was U '(x) = 0 to find the balance points, now the problem is that there's only one root to the function, which means there's only one balance point at x=2 (stable).

    I thought I was going to find two or more balance points to determine the energy that divides the movements.

    It's my first time posting here so I hope I'm making myself clear, but can someone explain me where I'm wrong ?
    Last edited: Sep 26, 2013
  2. jcsd
  3. Sep 26, 2013 #2


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    Homework Helper

    Hi rmfw, welcome to PF.

    There are some rules here in the Forums, how to post your question. Read https://www.physicsforums.com/showthread.php?t=686781, please.

    That particle moves in a potential well, how does it look like? Make a sketch. Why do you think there should be more balance points? What is that equilibrium point, minimum or maximum? At what energy is the particle free to go to infinity and not confined into the well?

  4. Sep 27, 2013 #3
    Hi, on the first picture is the graph I got. On the second picture is the graph of what I expected. I know that if the particle was trapped in the potential well (picture 2) , with an energy of E>k the particle would go into unlimited movement.

    Now back to the first picture, if someone questions me what's the energy required to change the movement of the particle from periodic to unlimited, is the right answer E=0 ?

    EDIT: in the second picture both maximums are supposed to be at the same height.

    Attached Files:

    • 1.jpg
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    • 2.jpg
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  5. Sep 27, 2013 #4


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    Your first picture is almost right: Assuming A>0, the potential increases to infinity if x -->0.
    The potential has got a negative minimum value, and tends to 0 if x -->∞. When the energy of the particle is negative, Emin<E<0, the particle is confined in the potential well. Your answer is correct, if E=0 the movement of the particle becomes unlimited, it can go to positive infinity.

    The second picture is not relevant to the problem.

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