Question
What is the Young’s modulus for a perfect rigid body?

Answer

According to Hooke’s law,
$\text{(Y)}=\frac{\text{stress}}{\text{longitudinal strain}}=\frac{\text{F}}{\text{A}}\times\frac{\text{l}}{\Delta\text{l}}$
For a perfectly rigid body, change in length $\Delta\text{l}=0,$ therefore longitudinal strain in zero.
$\therefore\ \text{Y}=\frac{\text{F}}{\text{A}}\times\frac{\text{l}}{0}=\infty$
Hence, Young's modulus for a perfectly rigid body is infinite $(\infty)$

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