Problem not clear

Please attach picture not video. Basically, you can take assume that at highest point velocity of mass was zero as question doesnt give that. Now, at lowest point, total tension will be mv*v / r ( centripetal acceleration ) + mg (weight) 

from conservation of energy, 0.5 m v*v (KE) = Delta PE = diameter * m * g = 2lmg 

Now, solve both equations and apply young's modulus formula to get strain.