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JEE Main
Exam
JEE Main
Subject
Physics
Chapter
Mechanical Properties of Solids

JEE Main · Physics · Mechanical Properties of Solids

Two wires of the same material (Young's modulus Y ) and same length...

Two wires of the same material (Young's modulus YY) and same length LL but radii RR and 2R2R respectively are joined end to end and a weight ww is suspended from the combination as shown in the figure. The elastic potential energy in the system is

Two wires of the same material (Young's modulus Y ) and same length L but radii R and 2R respectively are joined end to

Options

  1. A3w2L4πR2Y\dfrac{3w^{2}L}{4{πR}^{2}Y}
  2. B3w2L8πR2Y\dfrac{3w^{2}L}{8{πR}^{2}Y}
  3. C5w2L8πR2Y\dfrac{5w^{2}L}{8{πR}^{2}Y}
  4. Dw2LπR2Y\dfrac{w^{2}L}{{πR}^{2}Y}

Correct answer

C. 5w2L8πR2Y\dfrac{5w^{2}L}{8{πR}^{2}Y}

Step-by-step solution

k1=Yπ(2R)2L{k}_{1} = \dfrac{Y \pi\left(2R\right)2}{L}, k2=Yπ(R)2L{k}_{2} = \dfrac{Y \pi\left(R\right)2}{L}

Equivalent 1k1+1k2=L4YπR2+LYπR2\dfrac{1}{k1} + \dfrac{1}{k2} = \dfrac{L}{4{YπR}^{2}} + \dfrac{L}{Y \pi R2}

Since, k1x1=k2x2=w{k}_{1}{x}_{1} = {k}_{2}{x}_{2} = w

Elastic potential energy of the system

U=12k1x12+12k2x22U = \dfrac{1}{2}{k}_{1}{x}_{1}^{2} + \dfrac{1}{2}{k}_{2}{x}_{2}^{2}

U=12k1(wk1)U = \dfrac{1}{2}{k}_{1} \left(\dfrac{w}{k1}\right)2 + 12k2(wk2)\dfrac{1}{2}{k}_{2}\left(\dfrac{w}{k2}\right)2<br/><br/>=12<br/><br/> =\dfrac{1}{2}w^{2} \left(\{\dfrac{1}{k1} + \dfrac{1}{k2}\right\} =\ddfrac{1}{2}w^{2} \left(\dfrac{5L}{4{YπR}^{2}}\right)<br/><br/><br/><br/>U = \dfrac{5w^{2}L}{8{πYR}^{2}}$
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