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Chhattisgarh PMT Chhattisgarh PMT Solved Paper-2005

  • question_answer
    A motor car blowing a horn of frequency 124  vib/s moves with a velocity 72 km/h towards a tall wall. The frequency of the reflected sound heard by the driver will be                 [velocity of sound in air is 330 m/s]

    A)  109 vib/s                            

    B)  132 vib/s

    C)  140 vib/s                            

    D)  248 vib/s

    Correct Answer: C

    Solution :

    Here observer is also acting as a sources, therefore    apparent frequency heard by observer is \[n'=n\left( \frac{v+{{v}_{o}}}{v-{{v}_{s}}} \right)\] \[=124\left( \frac{330+20}{330-20} \right)\] \[\because \]  \[[{{v}_{o}}={{v}_{s}}=72\times \frac{5}{18}=20m/s]\] \[=124\times \frac{350}{310}\] \[=140vib/s\]


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