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KVPY Sample Paper KVPY Stream-SX Model Paper-4

  • question_answer
    A musician using an open flute of length 50 cm produces second harmonic sound waves. A person runs towards the musician from another end of a hall at a speed of 10 km/h. If the wave speed is \[330 m/s,\] the frequency heard by the running person shall be close to:

    A) \[666 Hz\]         

    B) \[753 Hz\]

    C) \[500 Hz\]                     

    D) \[333 Hz\]

    Correct Answer: A

    Solution :

    [a]
    Frequency of the sound produced by open flute \[f=2\left( \frac{v}{2\ell } \right)=\frac{2\times 330}{2\times 0.5}=660Hz\]
    Velocity of observer, \[{{\operatorname{V}}_{0}}=10\times \frac{5}{18}=\frac{25}{9}m/s\]
    As the source is moving towards the observer therefore, according to Doppler?s effect.
    \[\therefore \]frequency delected by observer, \[f'=\left[ \frac{v+{{v}_{0}}}{v} \right]f=\left[ \frac{\frac{25}{9}+330}{330} \right]660\]
    \[=\frac{2995}{9\times 330}\times 660\operatorname{or},f'=665.55=666Hz\]        


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