A rubber cord L metre long and having A m2 area of cross-section is suspended vertically. If the wire extends 1 m under its own weight, then change in length (I) is (Take, density of rubber = D kgm-3 and Young's modulus of rubber = E Nm-2).

Volume of the rubber cord = ALWeight of the rubber cord = ALDg∴  Stress =ALDgA=LDgThe weight of the rubber cord acts at the centre of gravity which is  L2from the top.∴  Strain =lL/2=2lLSo,   Youngs modulus,   E= Stress  Strain =LDg2l/L=L2Dg2l or l=L2Dg2E