A string \( A \) of length \( 0.314 \, \text{m} \) and Young's modulus \( 2\times 10^{10}\, \text{N}/\text{m}^2 \) is connected to another string \( B \) of length and Young's modulus both twice of those of \( A \). This series combination of strings is then suspended from a rigid support and its free end is fixed to a load of mass \( 0.8 \, \text{kg} \). The net change in length of the combination is ______ \( \text{mm} \).
(radius of both the strings is \( 0.2 \, \text{mm} \) and acceleration due to gravity \( =10\, \text{m}/\text{s}^2 \) (Mass of both strings is to be neglected as compared to the mass of load
