Senior high school physics competition questions

when colliding, the three balls of A B C are taken as a whole, and the mass is 3m, so D must be bounced. Another point to note is that the abscissa of the center of mass of the A B C system is unchanged, because there is no horizontal impulse in the system. So the problem is simple. A ball with a mass of m hits a ball with a mass of 3m, and the D ball definitely bounces back, so the A B C system gains a downward speed. You don't have to consider how A B C moves, just consider their centroid movement. A momentum conservation equation, plus an energy conservation equation, can be found that the speed of D rebound is 1/2V, and the speed of the center of mass is -1/2V. Pay attention to the sign. Therefore, the speed of their relative movement is still v.

The ordinate of the mass center of ABC system is 1/3 of the ordinate of C, which is the root sign 3L/6. So the time is

the root sign 3L/6V.