The Sonic Ranger is unable to detect objects closer than 41 cm, because it requires a certain amount of time between sending and receiving signals. Objects within this range will reflect signals too early for the Ranger to interpret the data correctly. Therefore, always work with distances greater than 50 cm from the Sonic Ranger to insure that the distance to the object is determined correctly.
Note also that the scale on the air-track increases as you read toward the right. Since the Sonic Ranger is aimed left, its scale increases toward the left, with the origin at the faceplate of the module, not the right end of the track (see Fig. 2.3). It is extremely important that you keep these two scales separate in your work. You should use the readings from the Sonic Ranger (which the computer indicates in meters) for all of your experimental calculations, and use the scale on the air-track (which is marked in centimeters) only for positioning the rider in the same place when you are repeating an experiment over several trials.
Figure 2.3:
The measurement scale of the Sonic Ranger starts at its faceplate
and increases toward the left, oppositely of the ruler on the air track.