Two particles of mass m each are attached to  the end of a mass less spring. This dumb-bell  is moving towards right on a smooth horizontal  surface at speed V with the spring relaxed. Another  identical dumb-bell is moving along the same line  is opposite direction with the same speed. The  two dumb-bells collide head on and collision is  elastic. Assuming collisions to be instantaneous,  how many collisions will take place?