Final length (circumference) of the ring \(= 2\)\(\pi\)\(R \)
Change in length \(= 2\)\(\pi\)\(R -2\)\(\pi\)\(r.\)
\({\rm{strain}} = \frac{{{\rm{change in length}}}}{{{\rm{original length}}}}\)\( = \frac{{2\pi (R - r)}}{{2\pi r}}\)\( = \frac{{R - r}}{r}\)
Now Young's modulus \(E = \frac{{F/A}}{{l/L}} = \frac{{F/A}}{{(R - r)/r}}\)
\(\Rightarrow \) \(F = AE\left( {\frac{{R - r}}{r}} \right)\)