Adapted from the book Viscoelastic Materials, R. S. Lakes '09
    In many sports, the rebound of a ball following an impact is highly relevant to the character of the sport. For example in baseball [10.10.2, 10.10.3], the batter likes to hit the ball as far as possible. The baseball itself consists of a winding of wool and cotton yarn around a core of cork, and covered by cow-hide [10.10.2]. The ball is rather inelastic: it exhibits a height ratio of 0.3 in a drop test. This corresponds to a coefficient of restitution of only about 0.55 [10.10.3]. The coefficient of restitution is related to the viscoelasticity of the materials of which the ball is made. The coefficient of restitution of a major league baseball is required to be e = 0.546 plus or minus 0.032 [10.10.4] based on a standard impact with a block of ash wood. Even such a small allowable variation can result in as much as a 15 foot (4.6 m) difference in the range of a well-hit baseball. It is not really meaningful to speak of a loss tangent for a baseball, since it is actually a heterogeneous structure. Since the ball's constituents are natural polymers, the viscoelasticity depends on temperature and humidity. Recall that the coefficient of restitution is the velocity of separation divided by the velocity of approach. The height ratio for a ball dropped on a rigid surface is the square of the coefficient of restitution. For a homogeneous material,
tan delta is approximately [1/pi] [height ratio]. The baseball is actually heterogeneous in structure.
    Adair [10.10.2] adduces several apocryphal stories of efforts by home teams to gain advantage by cooling some or all of the baseballs prior to the game. The rules have been changed so that all balls must be provided to the umpires two hours before game time. The rebound velocity of the ball depends on the properties of both the bat and ball. Since the bat is stiffer than the ball, its viscoelastic properties are less important. Even so, since aluminum bats dissipate less mechanical energy than wood ones, it is possible to hit the ball further with an aluminum bat. The mechanical damping of the bat also manifests itself in the sound of the impact: a 'ping' sound in the case of an aluminum bat which has low damping.
    Baseball games are played in regions which differ widely in both temperature and humidity. Some people have considered a low humidity environment to be advantageous. Measurements of the coefficient of restitution [10.10.4a] showed a change from 0.55 at zero humidity to about 0.5 at 100 % humidity. This full humidity range corresponds to a change in the distance of a batted ball by as much as 30 feet. Some writers have suggested that the baseballs themselves have become more lively in recent years, a possible cause for the increase in home runs [10.10.4b]. Recent balls have a higher proportion of synthetic fibers than older baseballs. Their cores seem to have more rebound, but such a test is not definitive since natural and synthetic polymers undergo physical aging, that is, their properties change with time after manufacture.
    Rebound is pertinent to other sports including football, basketball, squash, and hockey.

References
10.10.2 Adair, R. K., The Physics of Baseball, Harper Collins, NY, 1994.
10.10.3 Adair, R. K., "The Physics of Baseball", Physics Today, 48, 26-30, May 1995.
10.10.4 Kagan, D. T., "The effects of coefficient of restitution variations on long fly balls", Am. J. Physics, 58, 151-154, 1990.
10.10.4a Kagan, D. and Atkinson, D., The coefficient of restitution of baseballs as a function of relative humidity, The Physics Teacher, 42, 330-333, 2004.
10.10.4b Rist, C., The physics of ... baseballs, Discover, May, 2001. See also http://discovermagazine.com/2001/may/featphysics.