Each of three blocks $P$, $Q$ and $R$ shown in figure has a mass of $3 \mathrm{~kg}$. Each of the wire $A$ and $B$ has cross-sectional area $0.005 \mathrm{~cm}^2$ and Young's modulus $2 \times 10^{11} \mathrm{~N} \mathrm{~m}^{-2}$. Neglecting friction, the longitudinal strain on wire $B$ is____________ $\times 10^{-4}$. $\left(\right.$ Take $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2$ )
JEE MAIN 2024, Diffcult
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$ a=\frac{10}{3} \mathrm{~m} / \mathrm{s}^2 $
$ 30-T_1=3 \times a $
$ T_1=20 \mathrm{~N} $
$ \text { strain }=\frac{\text { stress }}{Y} $
$ =2 \times 10^{-4}$
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