JEE Main & Advanced Physics Wave Mechanics Question Bank Self Evaluation Test - Oscillations

  • question_answer
    A pendulum made of a uniform wire of cross sectional area A has time period T. When an additional mass M is added to its bob, the time period changes to\[{{T}_{M}}\]. If the Young's modulus of the material of the wire is Y then\[\frac{1}{Y}\] is equal to: (g = gravitational acceleration)           

    A) \[\left[ 1-{{\left( \frac{{{T}_{M}}}{T} \right)}^{2}} \right]\frac{A}{Mg}\]

    B)        \[\left[ 1-{{\left( \frac{T}{{{T}_{M}}} \right)}^{2}} \right]\frac{A}{Mg}\]

    C) \[\left[ {{\left( \frac{{{T}_{M}}}{T} \right)}^{2}}-1 \right]\frac{A}{Mg}\]

    D)        \[\left[ {{\left( \frac{{{T}_{M}}}{T} \right)}^{2}}-1 \right]\frac{Mg}{A}\]

    Correct Answer: C

    Solution :

    [c] As we know, time period, \[T=2\pi \sqrt{\frac{\ell }{g}}\] When additional mass M is added then \[{{T}_{M}}=2\pi \sqrt{\frac{\ell +\Delta \ell }{g}}\] \[{{T}_{\frac{M}{T}}}=\sqrt{\frac{\ell +\Delta \ell }{\ell }}\,\,\,or\,\,\,{{\left( \frac{{{T}_{M}}}{T} \right)}^{2}}=\frac{\ell +\Delta \ell }{\ell }\] or, \[{{\left( \frac{{{T}_{M}}}{T} \right)}^{2}}=1+\frac{Mg}{Ay}\]           \[\left[ \therefore \,\Delta \ell =\frac{Mg\ell }{Ay} \right]\]   \[\therefore \,\,\frac{1}{y}=\left[ {{\left( \frac{{{T}_{M}}}{T} \right)}^{2}}-1 \right]\frac{A}{Mg}\]


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