MCQ
A train whistling at constant frequency is moving towards a station at a constant speed V. The train goes past a stationary observer on the station. The frequency n′ of the sound as heard by the observer is plotted as a function of time t Identify the expected curve:
  • A

    Image



  • B

    Image


  • C

    Image



  • D

    Image


Answer

  1.  

Explanation:

When qbserver is at rest and source of sound id moving towards observer then observed frequency n'.

$\text{n}'=\Big(\frac{\text{v}}{\text{V}-\text{v}_\text{s}}\Big)\text{n}_0$

Where n0 original frequency of source of sound

v = speed of sound in medium

$\therefore​​​​​​\text{n}'>\text{n}_0\ \ \text{v}_\text{s}=$ speed of source

When source is moving away from observer

$\text{n}'=\frac{\text{v}}{(\text{v}+\text{v}_\text{s})}\text{n}_0\ \text{n}''<\text{n}_0$

Hence, the frequencies in both cases are same and $\text{n}'>\text{n}''.$ so graph (c) verifies the answer.

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