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