  In this article, we investigate the behavior of the coefficient of
restitution (COR) which is an important parameter in many impact-related
fields. In many cases, the COR is considered as a constant value, but it varies
according to many variables. In this paper, we introduce an analytical variable
COR model considering aero dynamics along with its verification through
experiment. To introduce and analyze the variable characteristic of the COR
model, the collision phenomenon between a pendulum and two kinds of ball is
employed as an example and aerodynamics such as drag force is considered for
analyzing the after-effect of the collision. Collision velocity of the
pendulum, dynamic parameters of colliding bodies, contact time, drag
coefficient, the air density, and the cross-sectional area of the ball are
found as the typical variables of analytical COR model. This observation
generalizes the result in previous researches. To verify new COR model, the
travel distances for the curve-fitted constant COR model and the curve-fitted
variable COR model are compared through simulation and experiment. Moreover,
comparison between constant COR and variable COR is presented in several point
of views. Finally, using the variable COR model, the travel distance of the
ball for collision velocity, which is beyond the curve-fitted range, is
estimated.
