VMMC VMMC Medical Solved Paper-2006

  • question_answer
    The ratio of radii of gyration of a circular disc and a circular ring of the same radii and same mass about a tangential axis in the plane is:

    A) 1 : 2                                       

    B) \[\sqrt{5}\,:\sqrt{6}\]    

    C)        2 : 3                       

    D)        2 : 1

    Correct Answer: B

    Solution :

    \[{{I}_{ring}}=\frac{m{{r}^{2}}}{2}+m{{r}^{2}}=\frac{3}{2}m{{r}^{2}}\] \[{{I}_{disc}}=\frac{m{{r}^{2}}}{4}+m{{r}^{2}}=\frac{5}{4}m{{r}^{2}}\] \[\frac{{{I}_{disc}}}{{{I}_{ring}}}=\frac{\frac{5\,m{{r}^{2}}}{4}}{\frac{3}{2}m{{r}^{2}}}=\frac{5}{6}\] \[\frac{m{{k}^{2}}_{disc}}{m{{k}^{2}}_{ring}}=\frac{5}{6}\] \[\Rightarrow \]               \[\frac{{{k}_{disc}}}{{{k}_{ring}}}=\sqrt{\frac{5}{6}}\]                 \[=\sqrt{5}:\sqrt{6}\]


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