Mechanical Properties of Solids

JEE Main level, numerical answer, medium

51 of 118

Question 51 of 118

A wire of length LL and radius rr is clamped rigidly at one end. When the other end of the wire is pulled by a force FF, its length increases by 5cm5cm. Another wire of the same material of length 4L4L and radius 4r4r is pulled by a force 4F4F under same conditions. The increase in length of this wire is _____ cmcm.

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Answer: 5

Young's modulus is given by Y=FLA∆LY=\dfrac{FL}{A∆L}, here, AA is cross-section area and ∆L∆L is change in length.
Now, when force
FF is applied, change in length ΔL1=FLAY=FLπr2Y=5cm\Delta {L}_{1}=\dfrac{FL}{AY}=\dfrac{FL}{\pi {r}^{2}Y}=5cm
Now, when force
4F4F is applied, change in length
ΔL2=4F4Lπ16r2Y=FLπr2Y=5cm\Delta {L}_{2}=\dfrac{4F4L}{\pi 16{r}^{2}Y}=\dfrac{FL}{\pi {r}^{2}Y}=5cm

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