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Graph of stationary wave after moving for half period

  1. May 2, 2016 #1
    1. The problem statement, all variables and given/known data
    A stationary wave travels along a stretched string. Part of this wave at a particular
    instant is shown.

    https://www.google.co.id/imgres?imgurl=http://1.bp.blogspot.com/-0ltZhcjZgMk/VFjPCek0qDI/AAAAAAAAFpQ/v5tEMYIVsrQ/s1600/stationary-wave-june2011p23q6c.png&imgrefurl=http://physics-ref.blogspot.com/2014/11/physics-9702-doubts-help-page-7.html&docid=o0LHAI0i1F88PM&tbnid=cX0R4qmI0GkYfM:&w=917&h=272&bih=667&biw=1366&ved=0ahUKEwjrpYGWv7rMAhWFFqYKHXJEAB8QMwhOKCkwKQ&iact=mrc&uact=8

    The period of vibration of this wave is 20 ms. Sketch the stationary wave 10 ms after the instant shown.


    2. Relevant equations
    stationary wave

    3. The attempt at a solution
    I think the wave will be reversed after moving half period. Let say the equation of the wave is sin, then after moving for 10 ms the equation will be - sin. But the answer says that the graph will be horizontal line. I don't understand how it can be horizontal line only (I assume the answer means that all the amplitude is zero)

    Thanks
     
    Last edited: May 2, 2016
  2. jcsd
  3. May 2, 2016 #2

    Simon Bridge

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    ... but the problem statement says:
    ... which is it: progressive or stationary?
     
  4. May 2, 2016 #3
    Sorry my bad. Should be stationary wave. I have edited the question
     
  5. May 2, 2016 #4

    Simon Bridge

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    Yeah I don't see how you can start at non-zero displacement and end up with zero displacement after half a period.
    You can go from max to zero in a quarter of a period.

    Note: you are still writing: "A stationary wave travels along a stretched string"
    If it travels, it is not stationary.
     
  6. May 3, 2016 #5
    Ah my bad again. But if the question is progressive wave, will it still be reversed and not zero after moving half period?
     
  7. May 4, 2016 #6
    This question comes from CIE specimen paper 2 from 2015 Q7b(iv) - the mark scheme is wrong according to the question. Either the question should say:

    The period of vibration of this wave is 20ms. On Fig. 7.2, sketch the stationary wave 5ms after the instant shown in Fig. 7.2. ​

    - in which Fig 7.2. looks like the image above OR the mark scheme should describe the wave as being 180o out of phase.

    The problem with specimen papers is that they frequently cut and paste from previous years and then when they alter them a little - they don't update the mark scheme to reflect that. In this case the original question came from June 2011 paper 23 question 6 and the original text read like this:

    The period of vibration of this wave is τ. On Fig. 6.2, sketch the stationary wave 0.25 τ after the instant shown in Fig. 6.2.​

    Clearly 0.25 τ is a quarter of a period or 5ms later not 10ms.

    Check it out here:

    http://www.sciwebhop.net/sci_web/science/asp/struc/default_ks5.asp?q=1472
     
  8. May 7, 2016 #7
    Thanks a lot for your help and thanks to you too Simon
     
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