NEET Physics Wave Mechanics NEET PYQ-Wave Mechanics

  • question_answer
    When a string is divided into three segments of lengths \[{{l}_{1}},{{l}_{2}}\]  and \[{{l}_{3}},\] the fundamental frequencies of these three segments are \[{{v}_{1}},{{v}_{2}}\] and \[{{v}_{3}}\] respectively. The   original fundamental frequency (v) of the string is                                                                             [AIPMT (S) 2012]

    A)  \[\sqrt{v}=\sqrt{{{v}_{1}}}+\sqrt{{{v}_{2}}}+\sqrt{{{v}_{3}}}\]

    B)       \[v={{v}_{1}}+{{v}_{2}}+{{v}_{3}}\]

    C)  \[\frac{1}{v}=\frac{1}{{{v}_{1}}}+\frac{1}{{{v}_{2}}}+\frac{1}{{{v}_{3}}}\]

    D)  \[\frac{1}{\sqrt{v}}=\frac{1}{\sqrt{{{v}_{1}}}}+\frac{1}{\sqrt{{{v}_{2}}}}+\frac{1}{\sqrt{{{v}_{3}}}}\]  

    Correct Answer: C

    Solution :

    The fundamental frequency of string
                \[v=\frac{1}{2l}\sqrt{\frac{T}{m}}\]
    \[\therefore \]      \[{{v}_{1}}{{l}_{1}}={{v}_{2}}{{l}_{2}}={{v}_{2}}{{l}_{3}}=k\]               …(i)
    From Eq. (i)
                \[{{l}_{1}}=\frac{k}{{{v}_{1}}},{{l}_{2}}=\frac{k}{{{v}_{2}}},{{l}_{3}}=\frac{k}{{{v}_{3}}}\]
    Original length \[l=\frac{k}{v}\]
    Here,     \[l={{l}_{1}}+{{l}_{2}}+{{l}_{3}}\]
                \[\frac{k}{v}=\frac{k}{{{v}_{1}}}+\frac{k}{{{v}_{2}}}+\frac{k}{{{v}_{3}}}\]
                \[\frac{1}{v}=\frac{1}{{{v}_{1}}}+\frac{1}{{{v}_{2}}}+\frac{1}{{{v}_{3}}}\]    


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