JEE Main & Advanced Sample Paper JEE Main - Mock Test - 17

  • question_answer
    Figure shows an arrangement of a rod of length \[\ell \]and mass M and a bead of mass m attached to a weightless string passing over a frictionless pulley. At \[t=0,\]the bead is in level with the upper end of the rod. The bead slides down the string with considerable friction, and is opposite to the other end of the rod after t second. Assuming friction between the bead and the string to be constant all through, the frictional force is

    A) \[\frac{2Mm\ell }{(M-m)\,{{t}^{2}}}\]               

    B) \[\frac{2Mm\ell }{(M+m)\,{{t}^{2}}}\]

    C) \[\frac{Mm\ell }{(M-m)\,{{t}^{2}}}\]                

    D) \[\frac{4Mm\ell }{(M-m)\,{{t}^{2}}}\]

    Correct Answer: A

    Solution :

    Let \[{{a}_{1}}\] and \[{{a}_{2}}\] be the acceleration of M and m respectively. Then   \[Mg-F=M{{a}_{1}}\]       .........(1) and     \[mg-F=m{{a}_{2}}\]    .........(2) Now, \[\ell +\frac{1}{2}{{a}_{2}}{{t}^{2}}=\frac{1}{2}{{a}_{1}}{{t}^{2}}\]or \[{{a}_{1}}=\frac{2\ell }{{{t}^{2}}}+{{a}_{2}}\]  .........(3) Solving eq. (1), (2) and (3) we get \[F=\frac{2Mm\ell }{(M-m){{t}^{2}}}\]


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