It is reported as an experimental study (Goya et al. 2002) that the optimal water depth for streamlined gliding motion in swimming is from 0.4 m to 0.8 m. It is considered that wave drag under streamlined gliding motion increases as a swimmer approaches the water surface, but that interference drag between a swimmer and the bottom of a swimming pool increases as he/she approaches the bottom of a swimming pool. Therefore, it is expected the optimal water depth must be somewhere from the water surface to the bottom of a swimming pool. The purpose of this study is to verify the optimal depth of water for streamlined gliding motion applying hydrodynamics to Rankine Body. There is a theoretical analysis (Wigley 1953) concerning wave drag decreasing as the depth of water increases. Therefore I use the result of this study. On the interference drag, the interference drag can be calculated applying Lagally theory to the method of images. Lagally theory (Landweber and Yih 1956) can calculate the forces which occur when fluid velocity, generated by hydrodynamic singularities around the body, flows into the hydrodynamic singularities in the body. The results are as follows : Wave drag is nearly zero under the condition 3.5 d = h as the beam of Rankine Body is "d" and the depth of water is "h" . Interference drag is nearly zero under the condition a = d as the length from the bottom of the swimming pool to Rankine Body is "a" .

The objective of this study was to investigate the effect of abduction angle of foot and impact point on ball velocity, ball rotation, launch angle, ball deformation and impact force in side-foot soccer kicking.

Five experienced male university soccer players performed side-foot kicks using a one-step approach in varying the attack angle and the impact point. The kicking motions were captured three-dimensionally by two high-speed cameras at 2,500 fps. The attack angle was calculated as the angle between the swing vector that was the velocity vector of the center of mass of the foot and the face vector normal to the medial aspect of the foot. The impact point was calculated as the distance from the center of mass of the foot (projected onto the medial aspect) to the contact point. The effect of the attack angle and the impact point on the ball-foot velocity ratio, ball rotation and launch angle was examined. The impact force was calculated from ball deformation based on the Hertz contact theory.

Impact on the area from the center of mass of the foot to the ankle produced the greatest ball velocity. The ball rotation didn't occur at zero attack angle and increased with an increase in the attack angle. Also, the launch angle increased medially as the impact point approached the heel with large attack angle.