The graph in the figure (1.4) presents the speed’s dependence on the position in this first part of the particle’s motion (towards the wall).

After the collision with the wall, the speed of the particle, , has the same magnitude as the speed just before the collision but it is directed in the opposite way. In the graphical representation of the speed as a function of position, the collision with the wall is represented as a jump of the speed from a point lying on negative side of the speed axis to a point lying on positive side of the speed axis. The absolute value of the speed just before and immediately after the collision is the same as represented in the figure 1.5.