RAJASTHAN PMT Rajasthan - PMT Solved Paper-2005

  • question_answer
    When the kinetic energy of a body executing S.H.M. is 1/3 of the potential energy. The displacement of the body is x percent of the amplitude, where x is:

    A)  33                                         

    B)  87

    C)  67                         

    D)         50

    Correct Answer: B

    Solution :

    The relation for kinetic energy of S.H.M. is given by \[=\frac{1}{2}m{{\omega }^{2}}({{a}^{2}}-{{y}^{2}})\]                                             ...(1) Potential energy is given by \[=\frac{1}{2}m{{\omega }^{2}}{{y}^{2}}\]      ...(2) Now for the condition of question and from eqs. (1) and (2)                                 \[\frac{1}{2}m{{\omega }^{2}}({{a}^{2}}-{{y}^{2}})=\frac{1}{3}\times \frac{1}{2}m{{\omega }^{2{{y}^{2}}}}\] or            \[\frac{1}{2}m{{\omega }^{2}}{{y}^{2}}=\frac{1}{2}m{{\omega }^{2}}{{a}^{2}}\] or            \[{{y}^{2}}=\frac{3}{4}{{a}^{2}}\] So,          \[y=\frac{a}{2}\sqrt{3}=0.866\,a\]                                   \[\approx \,\text{87 }\!\!%\!\!\text{ }\,\text{of}\,\text{amplitude}\]


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