A particle of mass `m` carrying charge `q` is projected with velocity `v` from point `P` toward aninfinite line of charge from a distance `a` . Its speed reduces to zero momentarily at point `Q`, which is at a distance `a//2` from the line of charge. If another particle with mass `m` and charge `-q` is projected with the same velosity `v` from `P` toward the line of charge, what will be its speed at `Q` ?