Key concepts
Coefficient of restitution
One way to figure out how something works is to look inside—always with permission, of course! This approach works for all sorts of things, even items that are not mechanical or overly complicated. When you grab a baseball, the first thing you might think is how hard it is. You don't necessarily think of a baseball as bouncy. But that's exactly what a ball does when it collides with a bat. Like a bouncing ball, when hit with a bat a baseball deforms for an instant then springs back to its original shape. The bouncier the ball, the farther it will travel when hit.
If you have ever looked inside a baseball—or looked up pictures online—you will already know that it is made up of various layers of different materials. Why does a baseball require so many different layers? In the early years of the game, prior to 1848, baseballs were very bouncy, built with a solid rubber core and only about three inches in diameter. Hitting these "lively" balls, some pro teams tallied as many as 100 runs per game.
As the game evolved bigger, heavier, less bouncy balls were developed. Referred to as "dead balls," they didn't travel as far or as fast as the lively ones. Before the baseball’s characteristics were regulated, home teams could choose which kind of ball they would use: If you had better batters than the other team, you'd choose a lively ball; if your team's strengths were pitching and fielding, you'd choose a dead ball. Through decades of testing ball makers and ballplayers developed a finely tuned combination of materials and density, so today's balls are neither too lively nor too dead when walloped by a bat. Scientifically speaking, a ball's liveliness is called the coefficient of restitution—the ratio of its relative speed after collision to its relative speed before collision.

  • Official Major League Baseball ball (that you are allowed to cut apart)
  • Sharp knife or small saw (Always have adult assistance when using sharp tool, and use special caution when cutting into objects that are hard—and can roll.)
  • When you take something apart, don't rush, take your time so you can examine it closely and think about what each part does. Be sure to save the pieces so you can see how they fit together.
  • Carefully cut the red stitching and pull it free from the ball. What purposes do you think the stiches serve? What about the white leather? Why do you think these colors were chosen for a ball that would be played against green grass?
  • Peel back the leather covers. What do you see in this layer? What do you think the twine does for the leather that covers it?
  • Unwrap the thin twine. What is the next layer? Many cheaper baseballs have only a solid mass of cork/rubber amalgam under the first layer of yarn but with a pro ball the yarn has only just begun. Do you see small bits of red thread? Can you tell what their purpose is?
  • Unwrap the dark-colored yarn. What do you discover next? How might the yarn affect a ball's bounce? Imagine, if it helps, dropping a really tightly wound ball of yarn versus a really loosely wound one.
  • Unwrap the light-colored yarn. What comes next? How much of the baseball does this last layer of dark yarn contribute to its overall size and mass?
  • Unwrap more dark yarn. What do you find at the core of the baseball? Drop this ball on a safe, hard surface. What does it do? How do you think baseballs would behave if they only made of rubber—without all of those yarn layers?
  • Locate the red rubber-covered ball in the center and carefully cut this ball in half to reveal its interior. Do you see a tiny core? Can you tell what it is made of? Are you surprised to find so many materials in a simple baseball?
  • Extra: Now that you know what is inside a baseball, try making your own ball by winding materials around a central core. For a covering, try some type of wide adhesive tape. Try different designs. What materials work best for you? How would you design balls differently for different purposes?

Observations and results
No ball is perfectly elastic; the elasticity of a particular ball depends on its construction. If you change a ball’s bounciness, you radically change the game you play with it. Imagine, for example, a basketball that hardly bounced. The modern baseball is strictly regulated: as you have discovered, it has a rubber-covered cork core, which is then wound tightly with yarn and covered with alum leather. Official baseballs must be between nine and nine and one quarter inches in circumference, and five to five and one quarter ounces in weight. They should have a coefficient of restitution of no more than 0.578, no less than 0.514. Now you know some of the serious science behind those rare grand slams!
More to explore
Putting Something On the Ball, from Exploratorium
Thrown for a Curve, from Exploratorium
Bouncing Balls, from Exploratorium
How Far Can You Hit One?, from Exploratorium
Biological Baseball, from Exploratorium
“What's Inside a Baseball?” was developed by Exploratorium and is featured on page 62 of Exploralab: 150 Ways to Investigate the Amazing Science All around You. Created by Exploratorium, Exploralab is a book that takes curious kid scientists, ages 8–12, through 24 hours' worth of household investigations, experiments and discoveries.

This activity brought to you in partnership with Exploratorium