For the 'let it roll' experiment, T racker automatically records the decreasing angular speed of the mark er . The recording speed indicates the time that the marker uses to mo ve 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 hav e 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