What this display shows is a string of nine atoms initially at rest in our reference frame. The forces on each atom are initially balanced out so that the net force is zero. By clicking the Action button what you do is to grab the atom on the left end of the string and shift it about 1.5 atomic radii to the right. This happens so quickly that the movement is over before the second atom in the string has a chance to respond. The net effect is work being done on the leftmost atom as it is moved to a higher potential energy position. Because the atom is left at rest after its displacement, the momentum of the system remains at zero.