The experimental setup, as shown in figures 1 (a) and (b), consists of an acrylic cylinder whose thickness and diameter are 1 . 0 and 13 . 9 cm, respectively . A hole of 1 . 3 cm diameter is perforated at the centre of the cylinder for ball-bearing insertion. The disc is then connected to the supporting foundation with a metal tube of radius 0 . 25 cm. A black circle, as indicated by an arrow in figure 1 (b), is used as a monitoring point in Track er . This marker must be large enough to protrude from the background. Here, the radial size of the marker is 1 cm and the colour , black, is chosen so that it is distinct from the surroundings; as a consequence, the recorded video is clear and easy to work with in Tracker . A digital camera is placed 270 cm away 3 from the disc and the line of sight of the camera is perpendicular to and centred on the rotating plane. The camera is set to record in the audio-video-interleave (.avi) format at 30 frames per second (fps). The setup is shown in figure 2 (a). For each video, the spatial calibration must be done within Track er before any interpretation. The physical resolution of our video is about 1 . 67 mm per pixel.