Explaining Wave Equation Solution for Vibrating Strings

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Discussion Overview

The discussion revolves around the assumptions made in deriving the governing equation for a vibrating string, particularly focusing on the assumption of uniform tension and its implications. Participants explore the justification for this assumption and consider the effects of varying string density on the model.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks clarification on the assumption that the force of tension on each particle is proportional to the ratio of vertical to horizontal displacement, questioning its correctness and underlying reasons.
  • Another participant explains that tension is generally assumed to be constant throughout the string and discusses the transverse component of tension as a restoring force, suggesting that this can be derived rather than assumed.
  • A participant raises a concern about the implications of non-uniform density on the assumption of uniform tension, prompting further inquiry into how to proceed under such conditions.
  • In response, another participant argues that string density should not affect the assumption of uniform tension.
  • Further discussion questions the justification for assuming uniform tension, with one participant suggesting that uniformity leads to simpler solutions that align with observed reality, while also noting that assuming non-uniform tension would complicate the mathematics significantly.

Areas of Agreement / Disagreement

Participants express differing views on the implications of string density and the justification for assuming uniform tension. There is no consensus on whether the assumption of uniform tension is universally valid or if it should be reconsidered under different conditions.

Contextual Notes

Participants acknowledge that assuming non-uniform tension would require additional hypotheses about how tension varies along the string, which could complicate the analysis.

mahdert
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In deriving the governing equation for a vibrating string, there are several assumptions that are made. One of the assumptions that I had a hard time understanding was the following.

Once the string is split into n particles, the force of tension on each particle from the particles in the right and the left is assumed to be proportional to the ratio of the vertical displacement to the horizontal displacement.

Could you please explain to me how this assumption is correct. What are the reasons behind it. Thanks.
 
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mahdert said:
Once the string is split into n particles, the force of tension on each particle from the particles in the right and the left is assumed to be proportional to the ratio of the vertical displacement to the horizontal displacement.
Generally, one assumes that the tension is constant throughout the string. You need the transverse component of the tension--which is the restoring force tending to pull the string back to its equilibrium position. At any point, the string makes some angle θ. The transverse component of the tension = T sinθ, which for small angles ≈ T tanθ = T Δy/Δx.

(One should derive this, as above, not just assume it.)
 
I see. I can only suppose that this follows the assumption that the string is of uniform density. What if this is not the case? How would one proceed.
 
I don't think the string density would affect the assumption of uniform tension.
 
So what is the justification for assuming uniform tension across the string.
 
You could assume otherwise, but why? If uniformity gives simple solutions that match reality, isn't all you need? It's a hypothesis that works out to be correct, an example of successful science.

Assuming non-uniform tension would be the next step, in the case that the solutions didn't match reality. It would also complicate the math tremendously. First in that you would have to make another guess as how the tension behaves (which function T(x) ?).
 

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