The speed of the ball as it approaches the hitter also affects its
flight after being hit. The greater the speed of the ball before impact,
the greater its rebound speed will be. This is due to the elasticity,
or bounciness, of the ball. Though it seems quite hard, the modern
baseball is very elastic, which means that when it is deformed by
an impact, it tends to bounce back, returning the kinetic energy of
the impact into motion in the other direction. The measure of this
bounce is called the "coefficient of restitution," which is a measure
of how much of the energy of the collision is returned into the motion
of the ball, rather than being dissipated as heat.
No ball is perfectly elastic:
The elasticity of a particular ball depends on its construction.
After 1920, baseballs were made much livelier to encourage power
hitting. The next season, the number of major league home runs more
than doubled, and the nature of the game was drastically altered.
Over time, changes in materials have changed the nature of the game.
But the forces of nature remain the same.
The above chart shows the percentage of energy
returned to the ball after one bounce.