Different Varities of Baseball

On the market, there are several different types of baseballs made from different "centre" material such as cork, rubber and polyurethane. The most important feature of these types of baseballs is that it offers cheaper alternatives to the traditional yarn wound baseballs. Though they are generally considered of lower quality and are not used in major leagues.

In a baseball, there is a pill, which is a small sphere of cork and rubber enclosed in a rubber shell. The pill is strongly wounded with three different layers of yarn and finished with a winding of cotton or polyester yarn. It forms the core. A final yarn is then applied to keep the ball tight and "together".

The ball is then dipped in a latex cement to give tacky surface to aid in adhering the cover to the core, as having this surface allows the cover to stay in contact with the core during the sewing process. Generally, the highest quality leather (alum-tanned leather) is used as cover material. It provides a soft supple feel, while providing good sewing uniqueness and good impact durability. Split leather from cowhide is also used to make cheap cost leather ball. Various synthetic covers are used to reduce the cost of ball production.

The ball is then hand-stitched with a red cotton thread in a 104-stitch pattern.

According to Major League Baseball's authorized rules for baseball

"The ball shall be a sphere formed by yarn wound around a small core of cork, rubber or related material, covered with two stripes of white horsehide or cowhide, tightly stitched together. It shall weigh not less than five or more than 5 1/4 ounces avoirdupois and measure not less than nine nor more than 9 1/4 inches in circumference." The manufacture of a baseball varies. Generally a baseball has a circumference of approximately 9 inches (23cm) and a mass of approximately 5 ounces (132g).

Curveball

As the ball spins, its top surface moves in the similar direction in which the air moves. However at the base of the ball, the ball's surface and the air move in opposite directions. As a effect, the velocity of the air relative to that of the ball's surface is larger on the bottom of the ball. The high velocity puts stress on the air flowing around the bottom of the ball and this makes air flowing around the ball "break away" from the ball's surface. The air at the top of the spinning ball is subjected to not as much of stress due to the lower velocity difference and hence can "hang onto" the ball's surface longer before it breaks away. The air flowing over the top of the ball will leave it in a direction pointing a little downwards relatively the usual straight back. According to Newton's 3rd law, the spinning ball throws the air down and the air pushes the ball up in reaction. The faster the rate of spin, the further the ball's path curves.

Knuckleball

Some people consider that when a baseball is thrown without a spin, it would dance through the air in an unpredictable manner due to any passing breeze. However this might not be the actual reason why the ball appears to "dance". Researchers have found out small changes to the orientation of the ball with respect to the flow of air result in dramatic changes in the forces acting on the baseball. Both the magnitude and the direction of the forces acting on the baseball change, thus the baseball appears to "dance" due to it having a slow spin. But the mechanism by which the forces change magnitude and direction has not been determined. It was clear that the stitch on the baseball is the reason behind it. The stitches on the baseball will most likely cause the boundary layer to trip to a confused state. Once the boundary layer becomes confused, a separated flow tends to reattach. This reattachment will significantly alter the forces acting on the ball. Similarly as the baseball rotates, due to the position of the stitches, a region that was confused might become laminar. The laminar flow will separate from the turbulent flow and cause the forces on the baseball to swing.