JEE Main & Advanced Physics Simple Harmonic Motion JEE PYQ-Simple Harmonic Motion

  • question_answer
    A simple pendulum oscillating in air has period T. The bob of the pendulum is completely immersed in a non-viscous liquid. The density of the liquid is \[\frac{1}{16}\]th of the material of the bob. If the bob is inside liquid all the time, its period of oscillation in this liquid is :                                                              [JEE Main 9-4-2019 Morning]

    A) \[4T\sqrt{\frac{1}{15}}\]

    B)      \[2T\sqrt{\frac{1}{10}}\]

    C) \[4T\sqrt{\frac{1}{14}}\]

    D)      \[2T\sqrt{\frac{1}{14}}\]

    Correct Answer: A

    Solution :

    [a] For a simple pendulum \[T=2\pi \sqrt{\frac{L}{{{g}_{eff}}}}\]
    situation 1 : when pendulum is in air \[\to {{g}_{eff}}=g\]
    situation 2 : when pendulum is in liquid
    \[\to {{g}_{eff}}=g\left( 1-\frac{{{\rho }_{liquid}}}{{{\rho }_{body}}} \right)=g\left( 1-\frac{1}{16} \right)=\frac{15g}{16}\]
    So,\[\frac{T'}{T}=\frac{2\pi \sqrt{\frac{L}{15g/16}}}{2\pi \sqrt{\frac{L}{g}}}\Rightarrow T'=\frac{4T}{\sqrt{15}}\]
    Option [a]


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