The problem is to accelerate AB with electric field to get a speed respectively.

When b leaves the electric field, AB repel each other only by electrostatic force, if the distance between AB is s at this time.

The meaning of the problem is to find out the condition that B can catch up with A and continue to move in the original direction after the collision, or B can't catch up with A, and the repulsion between them is getting weaker and weaker, so that B will not change the direction of movement.