So in our collision between bat and ball, what has happened? Both objects experience an equal change in momentum as a result of the collision. But the greater mass of the bat means that, for a given change in momentum, it will experience a much smaller change in velocity than the ball. The much lighter ball undergoes a large change in velocity, which results in the ball flying into the outfield.

So it would appear that a large, heavy bat would be the best for hitting a ball a long way. However, a large, heavy object also requires a great deal of effort to accelerate it from a standstill. It has LOTS of inertia. (Inertia is the tendency of a still object to remain still, and a moving object to keep moving) A batter has only split-seconds in which she must to decide to swing and accelerate the bat to meet the ball. A heavy bat requires great strength and excellent reflexes to use, otherwise it becomes a liability. If the bat is too heavy, the pitch will pass by before the batter can hit it. Some professional hitters prefer relatively light bats, which they can accelerate and swing quickly and accurately.

Hitters have illegally altered bats to make them lighter, by drilling out the center of the bat and filling it with cork or another light material. Most hitters who use lighter bats are "contact" hitters, specializing in carefully placed base hits, instead of "swinging for the fences". However, a few light-bat hitters have managed to hit a lot of home runs; Hank Aaron broke Babe Ruth's lifetime record of 714 home runs. A hitter with quick wrists using a light bat can generate tremendous bat speed, thereby generating great momentum.

Softball Batter
 Photo by Harold "Doc" Edgerton.

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 quite 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, 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, the baseball was made much livelier (see sidebar), to encourage power hitting. The next season, major league home run production more than doubled, and the nature of the game was drastically altered.

(END)
Want more baseball? The Exploratorium will be creating a full-feature on the Science of Baseball which will appear on the Exploratorium's Sport Science web site on September 1, 1998.
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