The ball M starts from point A and moves in the direction of AD with initial velocity V, and the motion of the ball is decomposed, one is uniform motion in the horizontal direction, and the other is simple pendulum motion in the vertical plane.

According to the simple pendulum period formula T=2πLg=2π s

The time for the ball M to move from point A to point C on the vertical plane is t = t2 (2n+ 1) (n = 0, 1, 2,3 …).

Because of the isochronism of fractional motion, the initial velocity v=lt= 10π(2n+ 1)? (n=0， 1，2，3…)

So the answer is: 10π(2n+ 1)? (n=0， 1，2，3…)