Once you are satisfied that the track is sufficiently level, set the rider at the 150 cm mark. Begin collecting data, and then give the rider a gentle push toward the left. Make sure that you are able to take data over a substantial portion of the return trip after it has bounced off the elastic bumper at the left end; also make sure that the data is smooth and without jumps. When you have collected this data, return to the Main Menu and proceed in the same manner as before to display a graph of displacement and a graph of velocity with respect to time.
Is the velocity graph what you expected? The time scales of the two graphs are the same, so you should be able to see how the rate of change in the displacement graph corresponds to the velocity. How does motion with constant velocity on the air-track compare with trying to walk with constant velocity?