51. Two blocks of equal mass m placed on a frictionless flat surface are connected bya massless spring of a spring constant k, see Fig. 9. In the beginning, the spring isat its natural length, while the mass on the left is at rest and the mass on the rightis moving towards right with a speed of v. The two-mass system would go onmoving and oscillating relatively. During the oscillation, the maximum extensionof the spring is L. What is the minimum value of the kinetic energy of thetwo-mass system during the oscillation?