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JEE Main & Advanced JEE Main Paper (Held On 10-Jan-2019 Morning)

  • 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 speed of the train is reduced to 17 m/s, the frequency registered is\[{{f}_{2}}\]. If speed of sound is 340 m/s, then the ratio \[{{f}_{1}}/{{f}_{2}}\] is - [JEE Main Online Paper (Held On 10-Jan-2019 Morning]

    A) 19/18                           

    B) 20/19

    C) 21/20               

    D)   18/17

    Correct Answer: A

    Solution :

    \[{{f}_{2}}={{f}_{0}}\,\left( \frac{v}{v-17} \right)\,\] \[{{f}_{1}}={{f}_{0}}\,\left( \frac{v}{v-34} \right)\,\] \[\frac{{{f}_{2}}}{{{f}_{1}}}\,\,=\,\,\frac{v-34}{v-17}\,=\,\frac{340-34}{340-17}\,\,=\,\,\frac{306}{323}\,\,=\,\,\frac{18}{19}\] \[\frac{{{f}_{1}}}{{{f}_{2}}}\,\,=\,\,\,\frac{19}{18}\]


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