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NEET Physics Wave Mechanics NEET PYQ-Wave Mechanics

  • question_answer
    The driver of a car travelling with speed \[30\,m{{s}^{-2}}\] towards a hill sounds a horn of frequency 600 Hz. If the velocity of sound in air is \[330\,m{{s}^{-1}},\] tile-frequency of reflected sound as heard by driver is [AIPMT (S) 2009]

    A)  550 Hz            

    B)       555.5 Hz         

    C)  720 Hz            

    D)       500 Hz

    Correct Answer: C

    Solution :

    Key Idea Apply Doppler's effect.
    Whenever there is a relative motion between a source of sound and the observer (listener), the frequency of sound heard by the observer is different from the actual frequency of sound emitted by source.
    Case I
    Case II
    [for case I]  \[n'=\frac{v}{v-30}n\]                       ...(i)
    [for case II]\[n''=\frac{v+30}{v}n'\]                      ... (ii)
    From Eqs. (i) and (ii), we get
    \[n''=\frac{v+30}{v-30}n=\frac{360}{300}\times 600=720\,Hz\]
    \[=720\,Hz\]


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