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NEET Sample Paper NEET Sample Test Paper-46

  • question_answer
    One end of an elastic string is attached to a block of mass m and the other end is held keeping the string relaxed and vertical. Relaxed length of the string is \[{{\ell }_{0}}\] and there is a mark P at height 0.8 \[{{\ell }_{0}}\] from the lower end. Now the upper end is slowly raised. When the block leaves the ground, the mark P reaches the point where upper end of the relaxed string was. Expression for the work done by the force applied by the hand is -

    A) \[\frac{mg{{\ell }_{0}}}{4}\]

    B)                    \[\frac{mg{{\ell }_{0}}}{8}\]

    C)                    \[\frac{mg{{\ell }_{0}}}{16}\]

    D)                    None of these

    Correct Answer: B

    Solution :

    In such case elongation in string \[=\text{ }\frac{{{\ell }_{0}}}{4}\,\,=\,\,x\] and \[kx\text{ }=\text{ }mg\] energy store \[u\,\,=\text{ }\frac{1}{2}k{{x}^{2}}\] \[=\,\,\frac{1}{2}\frac{mg}{x}{{x}^{2}}\] \[=\,\,\,\frac{mgx}{2}\] \[=\,\,\,\frac{mg{{\ell }_{0}}}{8}\]


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