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JEE Main & Advanced Sample Paper JEE Main - Mock Test - 19

  • question_answer
    A siren is fitted on a car going towards a vertical wall at a speed of\[36\text{ }km/hr\]. A person standing on the ground behind the car, listens to the siren sound coming directly from the source as well as that coming after reflection from the wall. The apparent frequency of the wave coming directly from the siren to the person and coming after reflection respectively are (Take the speed of sound to be\[340\text{ }m/s.\])

    A) \[515Hz,486Hz\]  

    B)     \[486Hz,515Hz\]

    C) \[510Hz,490Hz\]       

    D) \[490Hz,510Hz\]

    Correct Answer: B

    Solution :

    Here the observer is at rest with respect to the medium and the source is going away from the observer. The apparent frequency heard by the observer is, therefore, \[v'=\frac{v}{v+{{v}_{s}}}v=\frac{340}{340+10}\times 500Hz\] \[v'=486Hz\] The frequency received by the wall is \[v''=\frac{v}{v-{{v}_{s}}}v=\frac{340}{340-10}\times 500=515Hz\] The wall reflects this sound without changing the frequency. Thus, the frequency of the reflected wave as heard by the ground observer is \[515\text{ }Hz\]


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