During the entire firing sequence for the ballistic pendulum experiment there are four distinct states that the system progresses through: (1) spring compressed and all components stationary: (2) spring fired, ball traveling at v1, pendulum stationary: (3) ball and pendulum traveling together at v2 just after collision, (4) ball and pendulum stationary at some height Ah. So there are 3 transitions between these 4 states when energy and momentum might be conserved.
