A steel ring of radius r and cross section a is fitted onto a wooden disc of radius R  ( R>r ). If the Young’s modulus be E, then the force with which the steel ring is expanded is

Initial length ( circumference) of the ring is  2πr Final length ( circumference ) of the ring is 2πRChange in length =  2πR−2πrStrain=2πR−2πr2πr=R−rrNow Young's modulus E=FAlL=F/A(R−r)/r⇒F=AE(R−rr)