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Vector Sum of a Standing Wave Confusion

  1. Jun 7, 2014 #1
    Hi,
    I was taught that a standing wave is formed when a progressive wave meets a boundary and is reflected. I was also taught that waves that meet a fixed end, reflect on the opposite side of the axis to the side that they met it at. (I hope that makes sense)
    If this is true, when the wave is reflected, I don't understand why any wave is formed at all. Surely, if the two waves are opposites in the way they are, their amplitudes would interfere destructively and not form a wave at all.
    I have been looking around for answer to this question elsewhere online but cannot seem to come up with one and wasn't sure where else to post this.
    I have a physics exam on Monday and I don't understand this particular bit! :surprised
    Thanks
     
  2. jcsd
  3. Jun 7, 2014 #2
    Standing wave diagrams are a still frame of where the wave will have traveled over time as it travels to one end of a medium and back.

    This is just like the graph of a wave on a coordinate plane. Some parts of the wave are crests, some are troughs, but they don't interfere because the crests and troughs are never in the same place at the same time.

    A standing wave doesn't interfere with itself because, although a crest and a trough do exist in the same place, they do not exist in that place at the same time.

    EDIT:

    I wrote my response thinking of a single wave pulse, in which case no interference would occur at all.

    In the case of a standing wave, we create multiple pulses at regular time intervals, and constructive and destructive interference occur at different points along the medium.

    The reason the wave doesn't completely cancel itself out is the dimension of time. The relative location of each crest and trough along the wire in time explains the resulting interference pattern.
     
    Last edited: Jun 7, 2014
  4. Jun 7, 2014 #3
    as the incident wave and reflected wave travel in opposite direction there will be times when one wave's crests coincide with the other wave's valleys. When that happens the two waves cancel out as you described. But at some later time the waves move in opposite directions and eventually one wave's crest will coincide with the other wave's crest. At that instant the two waves add up to a bigger wave (constructive interference). You will see the wave go back and forth between those two opposite possibilities which gives a wave that expands and contracts without moving known as a standing wave.
     
  5. Jun 7, 2014 #4

    ehild

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    A very simplified derivation will be shown:
    A wave travelling along the x axis to the right can be represented by the function y1=Asin(wt-kx). A wave travelling in the opposite direction, having the same amplitude and opposite phase is y2=-Asin(wt+kx).

    The resultant function is Y=y1+y2=Asin(wt-kx)-Asin(wt+kx). Apply the addition formula to expand the sines:

    Y=A[ sin(wt)cos(kx)-cos(wt)sin(kx)-sin(wt)cos(kx)-cos(wt)sin(kx)]= -2Acos(wt)sin(kx)

    The result is a standing wave. It describes the vibration at each x with angular frequency w, and amplitude 2Asin(kx). At certain points the amplitude is zero (where kx=integer multiple of pi) these are the nodes. At other points, where kx is odd multiple of pi/2, the amplitude is maximum, 2A. These are the antinodes of the standing wave.

    ehild
     
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