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Homework Help: Pendulum homework problem

  1. Sep 11, 2008 #1
    I need to solve this problem
    proof that sum of kinetic and potentional energy in every part of pendulums(every dot of pendulums swing) swing is equal...
    Sorry for bad eanglish
  2. jcsd
  3. Sep 11, 2008 #2


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    Is this for homework? What have you tried so far?
  4. Sep 11, 2008 #3
    no it is not for homework i had that question on exam and i didnt know the answer and i have thinked and thinked after that abaut that and i dont know the solution I have tried to solve that problem with trigonometry but nothing after that i try to solve energy for some dot A on the swing and for some dot B on the swing but my results for dot A and B are not the same....
  5. Sep 12, 2008 #4
    If you describe the motion of the pendulum by the angle, say theta, and the write down the kinetic energy T and the potential energy V you will see that T is quadratic in the time derivative of theta and V is only a function of position theta.
    In such a system the Hamiltonian H will be the total energy H = T + V = E and since H does not depend explicitly on time the partial derivative of H with respect to time is zero and hence H is conserved during the motion of the system.
  6. Sep 12, 2008 #5


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    Welcome to PF!

    Hi maracxos! Welcome to PF! :smile:

    What are you allowed to assume?

    Can you assume that gravitational force is conservative?

    If so, then just prove that there is no work done by the only other external force … the tension. :smile:
  7. Sep 12, 2008 #6
    Ok thank you!!!
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