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Thursday, 18 August 2011

Physics GRE - #18

A string consists of two parts attached together.


The right part of the string has mass \mu_r per unit length and the left part of the string has mass \mu_l per unit length. If a wave of unit amplitude travels along the left part of the string, what is the amplitude of the wave that is transmitted to the right part of the string?

  1. 1
  2. \dfrac{2}{1+\sqrt{\mu_l / \mu_r}}
  3. \dfrac{2\sqrt{\mu_l / \mu_r}}{1+\sqrt{\mu_l / \mu_r}}
  4. \dfrac{\sqrt{\mu_l / \mu_r}-1}{\sqrt{\mu_l / \mu_r}+1}
  5. 0

Solution :

The correct answer is choice 3.

To solve the problem we can look at some nice limiting cases.
First, suppose \mu_l =\mu_r, then there should be no difference in amplitude as the wave travels (i.e. amplitude is 1 under this condition). Choices 1, 2, and 3 satisfy this.


Second, suppose \mu_r\rightarrow\infty. Then the wave should be completely reflected at the part where the two strings join (imagine a string attached to a wall). Thus, we should have amplitude equal to 0 under this condition. Only choice 3 satisfy this.

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[hide] Jacob said...

Good call on evaluating limiting cases. I just wanted to add that the transmission/reflection coefficient formulas are applicable to every 1-D wave equation (or normal incidence in 3-D) in all the different fields of study. I just had the result memorized for EM propagation from materials of different permittivity, which is analogous to mass per unit length.

on 20 August 2011 at 02:47
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