For the 'let it roll' experiment, Tracker automatically records the decreasing angular speed of the mark er . The recording speed indicates the time that the marker uses to move from frame to frame during the course of rotation. The data may be plotted in terms of ln ω versus the re volution time as shown in figure 4 , allowing the use of linear regression to find the parameter b . The solid line is the fitting result. From the analysis, we have the value of the coefficient of rotational friction of 6 . 5 × 10 − 5 kg m 2 s − 1 , when I = 4 . 54 × 10 − 4 kg m 2 is used in the model. Also, the result from the 'let it roll' experiment provides a nearly identical value of b to that