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JEE Main & Advanced JEE Main Paper (Held on 10-4-2019 Afternoon)

  • question_answer
    A metal coin of mass 5 g and radius 1 cm is fixed to a thin stick AB of negligible mass as shown in the figure. The system is initially at rest. The constant torque, that will make the system rotate about AB at 25 rotations per second in 5 s, is close to: [JEE Main 10-4-2019 Afternoon]

    A) \[4.0\times {{10}^{6}}Nm\]            

    B) \[2.0\times {{10}^{5}}Nm\]

    C) \[1.6\times {{10}^{5}}Nm\]            

    D) \[7.9\times {{10}^{6}}\text{ }Nm\]

    Correct Answer: B

    Solution :

    \[\alpha =\frac{\Delta \omega }{\Delta t}=\frac{25\times 2\pi }{5}=10\pi \,rad/{{\sec }^{2}}\]           \[\tau =\left( \frac{5}{4}M{{R}^{2}} \right)\alpha \]           \[=\frac{5}{4}\times 5\times {{10}^{-3}}\times {{({{10}^{-2}})}^{2}}\times 10\pi \]           \[=1.9625\times {{10}^{-5}}Nm\]           \[\simeq 2.0\times {{10}^{-5}}Nm\]


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