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In the derivation of the wave equation in classical mechanics in one dimension in a string. It's assumed that the angle between the tension and the horizontal line is small. This is assumed to allow us to let $$\sin (\theta)\approx \theta\approx \tan(\theta)= \frac{\partial y}{\partial x}$$ and complete the derivation.

My Question is, Isn't it true that such an assumption does neglect some wave functions? I mean that those waves in which the angle is not relatively small will not be solutions of the wave equation and so the equation does not consider them. Is this true? If yes, What to do then? If no, could you explain please?

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Yes the derivation breaks down for very large waves. This happens in the real world as well - the behavior is no longer linear. It can lead to a few different effects:

  • dispersion (different frequencies and different amplitudes travel at different speeds)
  • generation of harmonics (multiples of the frequency appear)

These are real effects that can be important. You usually need to use numerical modeling to solve for them as there are no simple closed form solutions for the general case.

So this approximation is just that - an approximation. It works "quite well, most of the time." A lot of physics is like that.

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