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BITSAT 2024
Exam
BITSAT
Year
2024
Subject
Physics
Chapter
Mechanical Properties of Solids

BITSAT 2024 · Physics · Mechanical Properties of Solids

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

Two wires of the same material (Young's modulus =Y=Y ) and same length LL but radii RR and 2R2 R 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 2 R respectively, are joined end

Options

  1. A3w2L4πR2Y\frac{3 w^2 L}{4 \pi R^2 Y}
  2. B3w2L8πR2Y\frac{3 w^2 L}{8 \pi R^2 Y}
  3. C5w2L8πR2Y\frac{5 w^2 L}{8 \pi R^2 Y}
  4. Dw2LπR2Y\frac{w^2 L}{\pi R^2 Y}

Correct answer

C. 5w2L8πR2Y\frac{5 w^2 L}{8 \pi R^2 Y}

Step-by-step solution

Δl1=wL(4πR2)Y,Δl2=wLπR2Y∴U=12K1(Δl1)2+12K2(Δl2)2=12×Y(4πR2)L×[wL4πR2Y]2+12×Y(πR2)L×[wLπR2Y]2(∵K=YAL)=5w2L8πR2Y\begin{aligned} & \Delta l_1=\frac{w L}{\left(4 \pi R^2\right) Y}, \Delta l_2=\frac{w L}{\pi R^2 Y} \\ & \therefore U=\frac{1}{2} K_1\left(\Delta l_1\right)^2+\frac{1}{2} K_2\left(\Delta l_2\right)^2 \\ &=\frac{1}{2} \times \frac{Y\left(4 \pi R^2\right)}{L} \times\left[\frac{w L}{4 \pi R^2 Y}\right]^2+\frac{1}{2} \times \frac{Y\left(\pi R^2\right)}{L} \\ & \times\left[\frac{w L}{\pi R^2 Y}\right]^2\left(\because K=\frac{Y A}{L}\right) \\ &=\frac{5 w^2 L}{8 \pi R^2 Y}\end{aligned}
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