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WB JEE Medical WB JEE Medical Solved Paper-2012

  • question_answer
    A train approaching a railway platform with a speed of \[20\,m{{s}^{-1}}\] starts blowing the whistle. Speed of sound in air is \[340\,m{{s}^{-1}}.\] If the frequency of the emitted sound from the  whistle is 640 Hz, the frequency of sound to a person standing on the platform will appear to be

    A)  600 Hz          

    B)  640 Hz

    C)  680 Hz          

    D)  720 Hz

    Correct Answer: C

    Solution :

     Given \[v=340\,m{{s}^{-1}},\]\[{{u}_{s}}=20\,m{{s}^{-1}}\]and \[{{v}_{0}}=640\,Hz\] From Dopplers law, \[v=\left( \frac{340}{340-20} \right)640\] \[v=680\,Hz\]


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