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

Initial length (circumference) of the ring = 2πrFinal length (circumference) of the ring = 2πRStrain = change in lengthoriginal length = 2π(R-r)2πr = R-rrNow Young's modulus E = FAIL = FA(R-r)r∴  F = AE(R-rr)