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What causes a 'simple pendulum' to stop?

  1. Sep 5, 2009 #1
    1. The problem statement, all variables and given/known data
    I need all the variables that causes the pendulum to stop.


    2. Relevant equations



    3. The attempt at a solution

    I know friction causes the pendulum to stop but what type of friction is it and what from?

    thanks
     
  2. jcsd
  3. Sep 5, 2009 #2

    Hootenanny

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    Welcome to Physics Forums.

    A simple pendulum will never stop once set into motion.
     
  4. Sep 5, 2009 #3
    doesn't it stop due to air resistance etc.
     
  5. Sep 5, 2009 #4

    Hootenanny

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    By definition a simple pendulum ignores air resistance, friction, etc.
     
  6. Sep 5, 2009 #5
    is that even possible ignoring air resistance?
     
  7. Sep 5, 2009 #6

    Hootenanny

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    I don't quite understand what you mean.
     
  8. Sep 5, 2009 #7
    But real pendulums are subject to friction and air drag, so the amplitude of their swings declines.
     
  9. Sep 5, 2009 #8

    negitron

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  10. Sep 5, 2009 #9

    Hootenanny

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    Is a simple pendulum the same a real pendulum?
     
  11. Sep 5, 2009 #10
    OK, what causes a real pendulum to stop?
     
  12. Sep 5, 2009 #11

    Hootenanny

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    Well, you've already given one cause:
    Another cause could be friction from the pivot.
     
  13. Sep 5, 2009 #12
    how does the pivot cause friction?

    Thanks Hootenanny.
     
  14. Sep 5, 2009 #13
    Energy is also transferred from the pendulum via the support system.If the support is firmly fixed in position the pendulum will swing for a longer time.
     
  15. Sep 5, 2009 #14

    Hootenanny

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    The usual way is friction between axle and the bearing. Take the simple case of a piece of string tied round a pin. If the knot is loose then as the pendulum oscillates the string will 'rub' against the pin.
     
  16. Sep 5, 2009 #15
    SHM is just a mathematical picture right? It would not exist in the real world unless it were place in a vacuous region of the universe
     
  17. Sep 5, 2009 #16

    Hootenanny

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    You are indeed correct. We would also presumably need to invent frictionless bearings or ideal springs to practically realise SHM. We would also need to find massless strings and point particles.
     
  18. Sep 5, 2009 #17
    is 'SHM' what a pendulum creates?
     
  19. Sep 6, 2009 #18
    A simple pendulum is just one example of a system that vibrates with SHM.If we were to be fussy we would say that the motion is only exactly simple harmonic if the string were massless the bob had zero size and if the amplitude of swing was negligible,in other words,a real pendulum does not move with SHM.I suspect,however,that this fussiness is confusing you and that at your current level you do not need to be fussy.Provided that the size of the bob is small,the string is light and the amplitude of swing is small the motion is approximately SHM.
     
    Last edited: Sep 6, 2009
  20. Sep 6, 2009 #19

    ideasrule

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    A pendulum can actually approximate SHM extremely well. The string doesn't need to be massless, nor does the bob need to be a point mass; extended pendulums still swing with SHM provided the amplitude of its oscillations is small.
     
  21. Sep 6, 2009 #20
    It's true that the pendulum can approximate SHM very well but it is still only an approximation.By definition a system moves with SHM if it accelerates towards a fixed point it's acceleration being direcly proportional to the displacement from that point and these criteria are not met exactly with a real pendulum.As I stated above this is being too fussy and may confuse the OP.
     
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