Two masses m and `(m)/(2)` are connected at the two ends of a massless rigid rod of length l. The rod is suspended by a thin wire of torsional constant k at the centre of mass of the rod-mass system(see figure). Because of torsional constant k, the restoring torque is `tau=ktheta` for angular displacement `theta`. If the rod is rotated by `theta_(0)` and released, the tension in it when it passes through its mean position will be :