A string of area of cross-section $4 \mathrm{~mm}^2$ and length 0.5 is connected with a rigid body of mass 2 kg . The body is rotated in a vertical circular path of radius 0.5 m . The body acquires a speed of $5 \mathrm{~m} / \mathrm{s}$ at the bottom of the circular path. Strain produced in the string when the body is at the bottom of the circle is $\_\_\_\_$ $10^{-5}$. (Use Young's modulus $10^{11} \mathrm{~N} / \mathrm{m}^2$ and $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2$ )