As for the experiment based on conserv ation of energy , we use the centre of the t ennis ball as our marker . General information on physical parameters for the motion of the ball is presented in table 1 . The vertical motion of the ball is re gistered in T racker and interpreted in terms of change in potential ener gy ( mg y ) and the difference of the squared velocity ( v 2 ) of the marker at each instant in the recorded movie. The maximum torque due to rotational friction is approximately 1 . 1 × 10 − 3 N m or about 4 . 3% of the torque due to string tension as giv en in table 1 . It is again sensible to neglect the effect of rotational friction. Equation ( 6 ), with W f = 0, is used as a fitting model for linear regression to find I , where m is the mass of the tennis ball and r is the radius of the disc. As a result, we have the value of