JEE Main & Advanced JEE Main Paper (Held on 08-4-2019 Morning)

  • question_answer
    A thin circular plate of mass M and radius R has its density varying as \[\rho (r)={{\rho }_{0}}r\]with \[{{\rho }_{0}}\] as constant and r is the distance from its centre. The moment of Inertia of the circular plate about an axis perpendicular to the plate and passing through its edge is\[I=aM{{R}^{2}}.\]The value of the coefficient a is:                     [JEE Main 8-4-2019 Morning]

    A) \[\frac{3}{2}\]            

    B)   \[\frac{1}{2}\]

    C) \[\frac{3}{5}\]                        

    D) \[\frac{8}{5}\]

    Correct Answer: D

    Solution :

    \[M\int\limits_{0}^{R}{{{\rho }_{0}}r(2\pi rdr)}=\frac{{{\rho }_{0}}\times 2\pi \times {{R}^{3}}}{3}\]             \[\underset{(MOI\,about\,COM)}{\mathop{{{I}_{0}}}}\,=\int\limits_{0}^{R}{{{\rho }_{0}}r(2\pi rdr)}\times {{r}^{2}}=\frac{{{\rho }_{0}}\times 2\pi {{R}^{5}}}{5}\] by parallel axis theorem\[I={{I}_{0}}+M{{R}^{2}}\] \[=\frac{{{\rho }_{0}}\times 2\pi {{R}^{5}}}{5}+\frac{{{\rho }_{0}}\times 2\pi {{R}^{3}}}{3}\times {{R}^{2}}={{\rho }_{0}}2\pi {{R}^{5}}\times \frac{8}{15}\] \[=M{{R}^{2}}\times \frac{8}{5}\]


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