Sep 3, 2014 - the coefficient of restitution (COR). COR is important in sports engineering research especially in the development of new material of the sport's ...
Paper ID: MoHE091 p. 54
Movement, Health & Exercise (MoHE) Conference 2014 (1 – 3 September 2014) Kuantan, Pahang, Malaysia
A PROPOSED STANDARD METHOD FOR MEASURING THE COEFFICIENT OF RESTITUTION OF A TAKRAW BALL Norhafizan Ahmad, Iskandar Hassanudin and Wong Chee Yik Centre for Product Design & Manufacturing (CPDM), Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, 50603 Malaysia INTRODUCTION: The sport industry has become ever more adept at exploiting leading-edge technologies to create more efficient equipment. One parameter that requires the utilisation of such technological advancement is the coefficient of restitution (COR). COR is important in sports engineering research especially in the development of new material of the sport’s ball [1], sport injury analysis [2] as well as game performance [3]. This study proposed a standard test method to determine the COR using high-speed video in a two-dimensional setting for capturing the behaviour at different point of contact (i.e. along line of impact). Hitherto, there is no proper method in determining the takraw ball’s COR upon considering different point of impacts that may affect by the ball’s trajectory and spin. METHODS: The experimental setup is based on horizontal drop test procedure [4] for different drop heights (from 1 to 3 meter) and at each point of impacts as depicted in Fig. 1. Three different faces of the impact point were selected to have a good representation of the overall surface of the ball. Different ball brands from the official ISTAF Takraw ball were used. The velocity of the ball before and after the impacts were experimentally measured and the ratio of the ball velocity before and after the impact is used to determine the COR value [5]. The range of COR at different drop heights and point of impacts are measured to determine the percentage difference between the maximum and the minimum value of COR. The range of the CORs obtained were also compared with other test method based on the free fall first-bounce height data of various takraw balls [6]. Figure 1: Drop test experiment setup. RESULTS AND DISCUSSION: The COR obtained in this study ranges between 0.700 and 0.726. This proposed test method also was tested by dropping the ball at a height of 3.3 m of drop height and the value of COR obtained ranges between 0.696 and 0.720. The readings obtained is similar to the range obtained by dropping the ball drop between 1 and 3 meter. However, this range of COR is higher as compared to the range of COR calculated based on the free fall first-bounce height data [6] which only ranges from 0.505 to 0.674. This is mainly due to the internal ball sliding friction and the deviation of the ball’s rebound trajectory from the vertical line. CONCLUSION: The coefficient of restitution of a takraw ball for a vertical drop test can be measured in terms of the bounce velocity by using high speed camera. This standard method is easier to set up eliminate the error of result due to the effect the ball rebound trajectory from vertical line and the ball spin due internal sliding friction. REFERENCES: [1] R. Cross, "Dynamic properties of tennis balls," Sports Engineering, vol. 2, pp. 23-34, 1999. [2] J. Heald and D. Pass, "Ball standards relevant to risk of head injury," ASTM SPECIAL TECHNICAL PUBLICATION, vol. 1229, pp. 223-223, 1994. [3] S. Haake, S. Chadwick, R. Dignall, S. Goodwill, and P. Rose, "Engineering tennis–slowing the game down," Sports Engineering, vol. 3, pp. 131-143, 2000. [4] N. Ahmad, Z. Taha, S. Ujihashi, and K. Tanaka, "An Experimental Study of the Impact of a Sepak Takraw Ball on a Flat Surface," The Impact of Technology on Sports III, pp. 447-452, 2009. [5] R. Cross, "The bounce of a ball," American Journal of Physics, vol. 67, pp. 222-227, 1999. [6] B. Lorhpipat and B. Lorpipatana, "Takraw balls," ed: Google Patents, 1996
