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Standing wave forms

  1. Jan 22, 2015 #1
    1. The problem statement, all variables and given/known data
    Why does Asin(kx)sin(wt) also represent a standing wave? Which two interfering waves may superpose to make it?
    Acos(kx+wt) and Acos(kx-wt) could if we were subtracting them, but we're adding so that doesn't make much sense? Also, is there something like a phase shift in the time term?


    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Jan 22, 2015 #2

    Quantum Defect

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    If you think about situations where you see a standing wave, what kinds of waves are you dealing with?

    Ex: Waves in a jump rope --> you wiggling one end of the rope, the other end tied to the wall. Think about what happens when you launch a single impulse down the rope.
     
  4. Jan 22, 2015 #3

    BvU

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    Remember the rules for ##\cos(\alpha+\beta)## ? Those are the ones to write the standing waves (a product) as propagating waves (sums).

    [edit] You used those already, I realize.

    If a sign is in the way, you can always add a phase to change it ... something to do with ##\cos(\alpha+\pi)##
     
  5. Jan 22, 2015 #4
    ^^That's precisely the problem. If I change the phase, how will it still remain a standing wave?
     
  6. Jan 22, 2015 #5

    haruspex

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    Not sure exactly what you're asking. You're looking at two general ways of writing standing waves, one as a trig product and one as a trig sum, but seem to be complaining that a specific example of one form does not turn into a specific example of the other form.
    If you start with ##\cos(kx+\omega t)+\cos(kx-\omega t)## then in general it can be turned into ##A\sin(kx+\alpha)\sin(\omega t+\beta)##. That will be a standing wave, regardless of the values of the three constants introduced. Specifically, you will get A = 2, ##\alpha = \beta = \pi/2## (or something like that).
     
  7. Jan 22, 2015 #6

    BvU

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    If you change the phase it will just stand in a shifted location ! One of the two waves that travel in opposite directions is shifted.
     
  8. Jan 22, 2015 #7
    Ok fine. That makes sense.
     
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