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Does plucking one point on the string really can cause standing wave?

  1. Aug 29, 2014 #1
    I was teaching myself standing wave's chapter. It is said in the book that when plucking in the center of the string, it will cause two traveling waves of different directions, one left, one right. Then both of them will reach ends of string and invert back. Before they encounter at the center, they will form standing waves at left part and right part. However, if we combine them into a complete image, it is different from normal standing wave pattern. What's more, it seems that the problem will continue because it is just like a cycle, the left part and right part always appear to be symmetrical instead like normal standing wave. (I have attached a detailed process from 0-2T.) Anyone could help me?
     

    Attached Files:

  2. jcsd
  3. Aug 30, 2014 #2

    AlephZero

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    In your diagrams, because the center point where you pluck the string doesn't seem to move. If you try the experiment with a real string, or an elastic band, that isn't correct.

    This is an animation of a string that you can "pluck" at any point along the length by clicking the mouse.
    http://www.falstad.com/loadedstring/
     
  4. Aug 30, 2014 #3
    The center point remaining constant is because I only chose some specific time point, such as T/2,T,3T/2 and 2T. At that time, the center point has returned to zero, however it has moved within these points(or there won't be any traveling waves). In addition to this, when plucking one point on the string, there are many harmonics, I selected fourth harmonics as an example. If I chose the fundamental frequency, it will wiggle up and down as the string in animation. But the problem is at phase, if we try to wiggle one point, two traveling waves must be out of phase. Therefore, two corresponding reflected waves will be out of phase too. I think this lead to strange effect compared with normal standing wave pattern
     
  5. Aug 30, 2014 #4
    This looks very interesting.. just take a look.. maybe what u have been thikning about




    Its a combination of a lot of harmonics together..
     
    Last edited by a moderator: Sep 25, 2014
  6. Aug 30, 2014 #5
    I have seen that animation, it is fantastic. Thanks. But I don't know its relation with my question. I found another animation here.
    http://www.acs.psu.edu/drussell/Demos/SWR/SWR.html.
    Pay you attention to the two standing waves. The first one is formed by two waves out of phase. The second one is formed by two in phase. And we can find the nodes and antinodes of these two are different. It is that what I want to say, after the two reflected waves encounter at the center point, it seems that antinodes and nodes of standing waves might change.
    http://www.youtube.com/watch?v=INqfM1kdfUc&list=PLE35BD0AB93E47FB6
    What's more, I noticed that it is only part of string have standing wave pattern, and the rest of them are traveling wave. And some of them could transfer between standing wave and traveling wave. I feel really shocked by these.
     
    Last edited by a moderator: Sep 25, 2014
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