A light, elastic string, of natural length 2m and modulus of elasticity 50N, is attached to two points, A and B, by its opposite ends. The points A and B are at the same horizontal level and they are 3.5m apart. A particle of mass 2 kg is attached to the midpoint of the string. The particle is then projected downwards with speed u. Given that the particle comes to instantaneous rest 2m below the level of AB, find the value of u.