Three blocks, each of same mass m, are connected with wires w1and w2 of same cross-sectional area a Young's modulus Y. Neglecting friction the strain developed in wire w2 is
2mg3aY
The acceleration of the system is
a=Fnetmtotal=mg3m=g3.
Let T1 and T2 be the tensions in w1 and w2. Then
T1=ma=mg3.
Also,
T2−T1=ma
⇒T2=2mg3
The strain in the wire w2 is
strain=T2aY=2mg3aY