JEE Main & Advanced Physics Elasticity Question Bank Youngs Modulus and Breaking Stress

  • question_answer
    A wire of length L and radius r is rigidly fixed at one end. On stretching the other end of the wire with a force F, the increase in its length is l. If another wire of same material but of length 2L and radius 2r is stretched with a force of 2F, the increase in its length will be [NCERT 1980; AIIMS 1980; MP PET 1989, 92; MP PET/PMT 1988; MP PMT 1996, 2002; UPSEAT 2002]

    A) l

    B)                 2l

    C)                 \[\frac{l}{2}\]        

    D)                 \[\frac{l}{4}\]

    Correct Answer: A

    Solution :

        \[l=\frac{FL}{AY}=\frac{FL}{\pi {{r}^{2}}Y}\therefore l\propto \frac{FL}{{{r}^{2}}}\] (Y = constant) \\[\frac{{{l}_{2}}}{{{l}_{1}}}=\frac{{{F}_{2}}}{{{F}_{1}}}\times \frac{{{L}_{2}}}{{{L}_{1}}}{{\left( \frac{{{r}_{1}}}{{{r}_{2}}} \right)}^{2}}=2\times 2\times {{\left( \frac{1}{2} \right)}^{2}}=1\] \ \[{{l}_{2}}={{l}_{1}}\] i.e. increment in its length will be l.        


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