Manipal Engineering Manipal Engineering Solved Paper-2013

  • question_answer
    Two spheres of equal masses, one of which is a thin spherical shell and the other a solid, have the same moment of inertia about their respective diameters. The ratio of their radii will be

    A) \[5:7\]                                  

    B) \[3:5\]

    C) \[\sqrt{3}:\sqrt{5}\]          

    D)        \[\sqrt{3}:\sqrt{7}\]

    Correct Answer: C

    Solution :

    Let the radii of the thin spherical and the solid sphere are\[{{R}_{1}}\]and\[{{R}_{2}}\]respectively. Then the moment of inertia of the spherical shell about their diameter                 \[I=\frac{2}{3}MR_{1}^{2}\]                                        ... (i) and the moment of inertia of the solid sphere is given by                 \[I=\frac{2}{5}MR_{2}^{2}\]                                        ... (ii) Given that the masses and moment of inertia for both the bodies are equal, then from. Eqs. (i) and (ii)                 \[\frac{2}{3}MR_{1}^{2}=\frac{2}{5}MR_{2}^{2}\Rightarrow \frac{R_{1}^{2}}{R_{2}^{2}}=\frac{3}{5}\] \[\Rightarrow \]               \[\frac{R}{{{R}_{2}}}=\sqrt{\frac{3}{5}}\Rightarrow {{R}_{1}}:{{R}_{2}}=\sqrt{3}:\sqrt{5}\]


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