JEE Main & Advanced Physics Rotational Motion JEE PYQ-Rotational Motion

  • question_answer
    One solid sphere A and another hollow sphere B are of same mass and same outer radii. Their moment of inertia about their diameters are respectively\[{{I}_{A}}\]and\[{{I}_{B}}\]such that where\[{{d}_{A}}\]and\[{{d}_{B}}\]are their densities.                                                                                                  [AIEEE 2004]

    A)   \[{{I}_{A}}={{I}_{B}}\]       

    B)        \[{{I}_{A}}>{{I}_{B}}\]

    C)   \[{{I}_{A}}<{{I}_{B}}\]       

    D)        \[\frac{{{I}_{A}}}{{{I}_{B}}}<\frac{{{d}_{A}}}{{{d}_{B}}}\]

    Correct Answer: C

    Solution :

    [c] Let same mass and same outer radii of solid sphere and hollow sphere are M and R, respectively.
                The moment of inertia of solid sphere A about its diameter
    \[{{l}_{A}}=\frac{2}{5}\,M{{R}^{2}}\]                          ...(i)
    Similarly, the moment of inertia of hollow sphere (spherical shell) B about its diameter
    \[{{l}_{B}}=\frac{2}{3}\,M{{R}^{2}}\]                           ...(ii)
    It is clear from Eqs. (i) and (ii), \[{{l}_{A}}<{{l}_{B}}\].
    Alternatively
    We can say that the object which has mass at greater distance will have higher moment of inertia as \[l=m{{r}^{2}}\]. So, hollow sphere   has large\[l\], because it has mass only on its circumference.


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