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AFMC AFMC Solved Paper-2000

  • question_answer
    A source and an observer move away from each other, with a velocity of 10 m/s with respect to ground. If the observer finds the frequency of sound coming from the source as 1950 Hz, then actual frequency of the source is : (velocity of sound in air = 340 m/s)

    A) 2486 Hz                                

    B)  2132 Hz

    C)  2068 Hz                               

    D)  1950 Hz

    Correct Answer: C

    Solution :

                     Key Idea: Movement of the source and observer alters the wavelength and the received frequency of sound. As both source and observer are moving  away, so observed frequency decreases. \[\therefore \]    \[{{f}_{observed}}=\left( \frac{v-{{v}_{o}}}{v+{{v}_{s}}} \right){{f}_{s}}\] where, v is velocity of sound = 340 m/s Here, \[{{v}_{o}}=10\,m/s,\,{{v}_{s}}=10\,m/s\] \[\therefore \]      \[1950=\left( \frac{340-10}{340+10} \right)\times {{f}_{s}}\] \[f\approx 2068\,Hz.\]


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