Wave question: TBR Ch. 5 #39

[attixx]

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The question asked what is/are true about the appearance of a wave.
I. The waves must propagate.

I said this was true, but TBR tells me this is false. The passage even starts by saying "a wave is the propagation of a disturbance through a medium."

The explanation is that "waves don't always have to move through space; consider a standing wave"..

Standing waves are still moving, are they not? They are just moving back and forth and interfering with each other. TBR even says "waves are still traveling to the right, as your hand shakes, and to the left, as those waves reflect off the tree. However, the way in which the left and right traveling waves interfere with each other gives rise to this apparently stationary wave. Standing refers to the wave's apparent lack of propagation."

But waves are still propagating!

 

[lovelyweapon]

Hello! I'm not a wave person and I have difficulty with this topic myself so bear with me, but as I understood the question it is asking which of the following are true about the appearance of waves, not the characteristics of the wave.

"However, the way in which the left and right traveling waves interfere with each other gives rise to this apparently stationary wave. Standing refers to the wave's apparent lack of propagation." These two statements are directly referring to the appearance, or "apparent lack of propagation", and are not necessarily implying that they are not propagating.

The appearance is due to the fact that a boundary blocks further propagation of the wave, reflecting it. The two waves are now in antiphase and cancel each other out, so that there is no net propagation over time. So in space (what you see) they are NOT moving, but they are still propagating. The appearance you end up seeing is something like a wave in simple harmonic motion, like a "resonance structure" of the two waves (for lack of a better phrase). So, just by looking at it you will not be able to tell whether it is propagating or not, because all you see is a wave that is not moving with a much higher amplitude than the two waves really have.

Kind of like you holding the end of a phone cord and gently move your hand a bit, the wave the phone cord makes produces a much higher amplitude than the amplitude your hand made (if that makes any sense). At the right wavelength, the phone cord would look like it's not moving and you'd get a standing wave. You know that the wave is propagating back and forth, it just looks like it isn't because it is in a defined space. Note that it would have to be at a very specific wavelength to look like it's not moving, I get the impression that in real life this is really difficult to do.

Therefore, it makes sense that the answer should be false. I think it's important to see that when you answer 'true' you are assuming this statement applies to ALL waves, and while all waves propagate, not all waves appear to propagate.

 

[attixx]

Ahh thanks for the response and clarification. Now that I think about it, your right; the key is "appearance." So while they might be actual propagating, they appear not to. Blah I hate this test.