ΔAmplitude of wave on rope with a change in linear density

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Discussion Overview

The discussion centers around the effects of varying linear density on wave amplitude in a rope, exploring theoretical implications and behaviors of waves as they encounter changes in medium density. Participants consider the relationship between amplitude, linear density, tension, and frequency, as well as the effects of reflections at density boundaries.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes that if the linear density of a rope changes by a factor of k, the amplitude of a wave would change by a factor of \(\frac{1}{\sqrt{k}}\), assuming fixed tension and frequency.
  • Another participant highlights the importance of wave reflection at points where the medium changes density, suggesting that a denser medium could reflect most of the wave's power.
  • A question is raised about whether the frequency of the reflected wave matches that of the wave traveling along the denser part, with a suggestion that this may relate to a 'common point' between the two densities.
  • Concerns are expressed about the behavior of waves transitioning from higher to lower density, questioning if reflections still occur and how power distribution might change with varying densities.
  • A participant speculates on the creation of a 'partially standing' wave due to reflections at density changes, suggesting the existence of 'semi-nodes' where amplitude is limited by reflected power.
  • Another participant inquires about the implications of a linearly varying density function on wave behavior, pondering if this would lead to continuous reflections and the complexity it might introduce.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the effects of linear density changes on wave behavior, particularly concerning reflections and power distribution. The discussion remains unresolved with no consensus on the implications of varying density on wave amplitude and reflection dynamics.

Contextual Notes

Participants note the dependence of their arguments on assumptions about tension and frequency being constant, as well as the need for further exploration of how power is distributed in relation to relative densities.

Nathanael
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Would it be correct to assume that if the linear density of a rope changes by a factor of k, then the amplitude of a wave traveling on the rope would change by a factor of \frac{1}{\sqrt{k}}? (Assuming tension and frequency are fixed.)
I get this by assuming the average power of the wave is unchanged by the change in linear density (and also because the average power depends on the square the amplitude and the linear density).

This is essentially what I'm wondering:
If the linear density (\mu) varies with the position (x) as described by some function, \mu(x), then would the amplitude (A) as a function of x be A(x)=A_0\sqrt{\frac{\mu(0)}{\mu(x)}} ?
(Again, assuming tension and frequency are fixed.)
 
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You also need to consider that part of the wave will be reflected where the medium changes density. If you think about the limiting case where the new medium has a very large mass, it will essentially act as if the lighter part was fixed in a wall. Most of the power would then go into the reflected wave.
 
Orodruin said:
You also need to consider that part of the wave will be reflected where the medium changes density.
Would the frequency of the reflected wave and that of the wave that moves along the denser part be the same? (If this is true, is the reason for it's truth related to the idea that the two rope densities have a 'common point'?)

Orodruin said:
If you think about the limiting case where the new medium has a very large mass, it will essentially act as if the lighter part was fixed in a wall.
What if the wave is traveling from a higher density to a lower density? A wave would still be reflected? Would the limiting case (as the density goes to zero) then be as if it were fixed to a frictionless ring? (A "free end" is what I think my book called it.) Then, the smaller the density of the second part, the more power would go into reflected wave?

Orodruin said:
If you think about the limiting case where the new medium has a very large mass, it will essentially act as if the lighter part was fixed in a wall. Most of the power would then go into the reflected wave.
I haven't been able to see any way of determining how the power is distributed based on the relative density. Any insight?

If I were to continuously generate a wave, and then it were to cross some point where the density changes, part of it being reflected and part of it continuing on; would I then be creating a 'partially standing' wave? That is, would there be 'semi-nodes' where the amplitude never gets bigger than some limit? (The limit of the amplitude of the "semi-node" would depend on how much power is reflected.)
 
What if the density of a rope varied linearly, [\mu (x)=\mu_0+kx] then what would you be able to say about a wave traveling along that rope?

Would it be as if a wave of increasing power are continuously being reflected? (But if that's so, and if I was right about waves reflecting when the density suddenly decreases, then wou have something like waves reflecting back and forth everywhere. That could either drastically confuse things or drastically simplify things; either way, I'm drastically curious.)
 

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