# It's not playing ball - you're playing physics

The first ballplayers were probably the ancient Mayans. Thousands of years ago, two teams of four to six players wearing wood-and-leather pads would bat a 12-inch ball of solid rubber around a stone arena. Spectators watched them from the stands. Sports technology has come a long way since then, but the science of how a ball bounces, flies through the air, and spins remains the same.

Throwing a curve

Big-league pitchers teach a lesson in physics with every curveball they throw. The Baseball Hall of Fame in Cooperstown, N.Y., points to William "Candy" Cummings who played for the Brooklyn, N.Y., Excelsiors. He threw the first curveball in 1867, by his own account.

It's the spin that makes a curveball curve. The spin, and Earth's atmosphere.

The simplest explanation is that the spin of the ball sets up a little whirlpool of air around it. As the ball moves forward, it runs into the whirlpool, or vortex, of air it created. The air current pushes the ball sideways, and it curves. Some pitches have been known to curve 17 inches on their way to the plate!

Here's a more detailed explanation: A curveball "curves" because the air on one side of the ball is moving faster than the air on the other. Think about it: As a spinning ball is thrown forward, one side of the ball is spinning in the same direction as the forward motion of the ball. But the other side spins against the air rushing past. The air is moving faster (in relation to the ball's surface) on one side of the ball than it is on the other.

Faster-moving air exerts less pressure, and natural law states that a moving body follows the path of least resistance. The ball moves toward the low-pressure side - and curves. This is called the "Magnus effect," as it was discovered by Gustav Magnus in 1852.

Dimples and drive

A golf ball with dimples goes about four times farther than one without dimples. (That's probably why there's no such thing as an undimpled golf ball.)

Nineteenth-century golfers were the first to notice that "uglier is better" when it came to golf balls. Roughed-up golf balls went farther than new, unblemished ones.

Why? Again, it's physics. It has to do with turbulence and drag.

Normally, turbulence and drag slow things down. Turbulence is stirred-up air, which is harder to move through than calm air is. Drag is the air's resistance to something moving through it. But a little turbulence can actually reduce drag.

That's where the dimples come in.

Air moves around a golf ball in flight. In an undimpled ball, the air follows the curve of the ball until it gets about halfway around. At the "separation point" (see diagram, this page), the air stops following the surface of the ball. Instead, it becomes a drag-producing wake. The wake slows down the ball.

Small pits, or dimples, on the surface of a golf ball create tiny areas of turbulence. The small-scale turbulence creates a "boundary layer" of turbulent air around the ball. The boundary layer allows the air passing around the ball to cling to it longer. The separation point moves farther around the ball. That means a smaller wake. A smaller wake means less drag, and less drag means the ball goes farther.

Two California scientists patented a golf ball in 1975 with only half as many dimples. The dimples were in a band around the middle of the ball. The rest of the ball was smooth. It didn't fly as far as a fully dimpled golf ball. But it significantly reduced the ball's tendency to "hook" or "slice" (curve left or right) when it was hit incorrectly.

(Remember the Magnus effect? The grooves in the head of a golf club make the ball spin backward. If the ball is hit squarely, the backspin gives the ball lift. But if the club hits the ball at a slight angle, the spin can make the ball curve right or left.)

Many golfers have trouble hitting the ball so that it goes straight every time. Why doesn't everyone use the banded balls today? They were banned.

The United States Golf Association ruled that the partially dimpled balls made golf too easy. It said they would "reduce the skill required to play golf and threaten the integrity of the game."

In the 1980s, golf-ball manufacturers suddenly thought: "If some dimples are good, maybe more dimples are better." They started making golf balls with many more, smaller dimples on them. Did the new balls work better? No. Lots of tiny dimples made the ball start to act as though it was smooth.

What's the best number of dimples to have? Golf-ball manufacturers are cagey about the number and size of the dimples on a ball. But experts say the best number is between 300 and 500 dimples.

(For more on the history of golf balls and the odd material with which they were stuffed, see the end of this article.)

How fast will it go?

What would happen if you dropped a baseball off the top of the Empire State Building? Would it smash itself deep into the pavement below?

Or how about this: Which would you rather catch? A Roger Clemens fastball, or a baseball tossed from the Goodyear Blimp? Believe it or not, the "blimp ball" is probably the smarter choice. Why? Physics again. This time it has to do with something called "terminal velocity."

As the baseball falls from the blimp, it runs into the air. The air pushes up against the ball, slowing it down. Sooner or later, the gravity pulling down on the ball and the air pushing up on it reach a balance. The ball won't go any faster. It has reached its terminal velocity.

What about a ball that's been pitched by Roger Clemens from the Goodyear Blimp? If the blimp is high enough, air pressure will slow down the ball. Once again, the ball will hit at the same terminal velocity.

Terminal velocity is not the fastest speed something can be propelled. Terminal velocity is the final speed a falling body will reach after falling through the air long enough. You can launch a ball downward at high speed, but the air will eventually slow it to its terminal velocity.

People have tried catching baseballs dropped from ridiculous heights. In 1908, a catcher for the Washington Senators, Gabby Street, tried to catch balls dropped from the top of the 555-foot-tall Washington Monument.

He caught only one of the 13 baseballs dropped, but he was never in any danger of being driven into the ground by the impact. The balls never went faster than 87 miles per hour. That's slower than most big-league fastballs today.

Other ballplayers have tried to catch baseballs dropped from blimps hovering at 1,000 feet. But the higher altitude did not increase the velocity of the balls by much. They didn't go faster than 95 m.p.h., the terminal velocity of a baseball.

The bounce factor

The coefficient of restitution (COR) is the physicist's fancy way to measure bounciness. Bouncy balls are made of elastic materials that do not deform (squish) easily. They also return quickly to their original shapes after hitting something (the floor, a baseball bat) with little loss of energy. A ball's COR is actually a measure of the speed of the ball before and after a collision. The higher the COR, the bouncier the ball.

A Superball made of a synthetic rubber is remarkably elastic, is not that squishy, and returns quickly to its original shape when it hits something. Its COR is .90.

A shotput's COR, however, is zero.

A baseball has a rubber-coated cork center, but it is wrapped with cotton and wool string. The string tends to absorb energy and release it as heat. So when a baseball is dropped, it doesn't bounce very high.

Baseballs used to be even less bouncy. A regulation ball in 1938 had a COR of .46. Today, it's .57. Why? In 1980, the rules were changed and the baseball's COR was raised because pitchers were dominating the game. A bouncier baseball goes farther, giving hitters an advantage.

CORs are written into the rule books of other sports, too. "Official" basketballs must bounce between 49 and 54 inches when dropped from a height of six feet. This translates into a COR of between .76 and .80.

Glue and Ping-Pong

What does Ping-Pong have to do with glue? A lot, it seems.

"Speed glue" is what professional table-tennis players use to apply the spongy rubber facing on their paddles. But the glue does more than make sure the rubber facing doesn't fall off. A chemical in the glue alters the molecular structure of the rubber. It makes the rubber softer.

This is important, because the softer the rubber backing, the deeper the Ping-Pong ball will sink into it. The deeper the ball sinks in, the longer the ball will stay in contact with the paddle. And the longer it stays in contact with the paddle, the more a player can control the ball - directing it or spinning it.

The effect of the solvent is short-lived. The rubber coating must be pulled off and reglued before play. And after many regluings, the rubber facing becomes hardened and must be replaced.

Finally, 'featheries'

The first golf balls were wooden. In the 1700s, the Dutch perfected a new, improved ball: the "featherie." A hat full of feathers was soaked in water. The wet feathers were stuffed into a 1-1/2 inch leather pouch, which was then sewn up and set aside to dry. The drying leather would shrink as the drying feathers expanded, creating a hard but elastic ball. The ball's stitched seams helped it fly far.

About 1850, less-expensive rubber golf balls ("gutties") became popular. Golfers using gutties soon noticed that the old, scarred ones flew farther and more predictably. Players began hammering or pitting new gutties before using them. Pre-pitted balls with various patterns of grooves and pits were then sold.

In 1908, William Taylor patented an inverted bramble pattern that consisted of evenly distributed circular depressions: the first true golf-ball dimples.

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