JEE Main & Advanced JEE Main Paper (Held On 11 April 2015)

  • question_answer
    A pendulum with time period of 1s is losing energy due to damping. At certain time its energy is 45 J. If after completing 15 oscillations, its energy has become 15 J, its damping constant (in\[{{s}^{-1}}\]) is : [JEE Main Online Paper (Held On 11 April 2015)]  

    A) \[\frac{1}{30}\ln 3\]

    B) \[\frac{1}{15}\ln 3\]

    C)  2

    D) \[\frac{1}{2}\]

    Correct Answer: A

    Solution :

      Amplitude in a damped oscillation is given by \[A={{A}_{0}}{{e}^{-\beta t}}\] energy \[\propto {{A}^{2}}\] \[\therefore \] \[\sqrt{E}=\sqrt{{{E}_{0}}}{{e}^{-\beta t}}\] where \[{{E}_{0}}\] is initial energy \[\sqrt{15}=\sqrt{45}{{e}^{-\beta 15\sec }}\] \[{{3}^{\frac{1}{2}}}={{e}^{-15\beta }}\] on taking log both sides \[-\frac{1}{2}\ln \left( 3 \right)=-15\beta \] \[\beta =\frac{\ln 3}{30}=4\]


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