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JEE Main & Advanced Physics Wave Mechanics Question Bank Doppler's Effect

  • question_answer
    A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer is \[{{f}_{1}}\]. If the train?s speed is reduced to 17 m/s, the frequency registered is \[{{f}_{2}}\]. If the speed of sound is 340 m/s then the ratio \[{{f}_{1}}/{{f}_{2}}\] is  [IIT-JEE (Screening) 2000]

    A)            18/19                                       

    B)            1/2

    C)            2    

    D)            19/18

    Correct Answer: D

    Solution :

               By using \[n'=n\,\,\left( \frac{v}{v-{{v}_{S}}} \right)\]                    Þ\[{{f}_{1}}=n\,\left( \frac{v}{v-{{v}_{S}}} \right)\]\[=n\,\left( \frac{340}{340-34} \right)=\frac{340}{306}n\]                    and \[{{f}_{2}}=n\,\left( \frac{340}{340-17} \right)=n\,\left( \frac{340}{323} \right)\]\[\Rightarrow \]\[\frac{{{f}_{1}}}{{{f}_{2}}}=\frac{323}{306}=\frac{19}{18}\]


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