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JEE Main & Advanced Physics Wave Mechanics Question Bank Mock Test - Waves and Acoustics

  • question_answer
    A source of sound of frequency\[{{f}_{1}}\] is placed on the ground. A detector placed at a height is released from rest on this source. The observed frequency\[{{f}_{{}}}(Hz)\] is plotted against time \[t\](sec). The speed of sound in air is 300 m/s. Find\[{{f}_{1}}\]\[(g=10m/{{s}^{2}})\]

    A) \[0.5\times {{10}^{3}}Hz\]       

    B) \[1\times {{10}^{3}}Hz\]

    C) \[0.25\times {{10}^{3}}\]         

    D) \[0.25\times {{10}^{3}}Hz\]

    Correct Answer: B

    Solution :

    [b] \[f=\left( \frac{v+{{v}_{0}}}{v} \right){{f}_{1}}={{f}_{1}}+{{f}_{1}}\frac{{{v}_{0}}}{v}\] \[{{v}_{0}}=gt\] So \[f={{f}_{1}}+\left( \frac{{{f}_{1}}g}{v} \right)t\] Slope of graph \[=\frac{{{f}_{1}}g}{v}\frac{2\times {{10}^{3}}-{{f}_{1}}}{30}=\frac{({{f}_{1}})(10)}{300}\] Or \[{{f}_{1}}={{10}^{3}}Hz\]


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