Slum dunk science: Smash! When a backboard shatters, who should take the penalty-the slam dunker or science?

Nigel Dixon is a big guy with a monster dunk. During a game against Arkansas State last January, the 6'11", 320-pound center for the Western Kentucky Hilltoppers slammed the ball through the hoop. CRASH! The glass backboard shattered into pieces, which scattered as far as center court. "Everyone was in shock," says Dixon, who had been showered with glass.

The incident forced a 35-minute delay while a crew swept up the mess and replaced the goal. Doctors removed slivers of glass from Dixon's arms, neck, and hand. Then, with a few bandages in place, the tough player returned to the game. "I pretended like nothing happened," Dixon says. "I just wanted to get the win."

POWER PLAY

What caused the backboard blast? Turns out that behind the driving, dribbling, and passing you see on the court, lots of forces are at play--forces of physics, that is. A force is any action--a push or pull--that causes an object to move, says Peter Brancazio, retired professor of physics at Brooklyn College and author of the book Sport Science.

When Dixon slam dunked, he grabbed onto the rim, which was bolted to a metal plate sandwiching the glass backboard. His powerful pulling force resulted in a twisting motion, known as torque. Torque forced the metal plate to flex forward, making the glass buckle. Then--pop!--it was raining glass.

Like all basketball moves, position is everything. "If Dixon had come up for the shot from the side and grabbed the rim closer to the backboard, the glass wouldn't have shattered," explains Brancazio. Why? By definition, torque increases as the force (Dixon's pull) moves farther away from the axis or pivot point--in this case, the point where the bolts meet the glass. Because Dixon grabbed the rim head-on (as far from the bolts as possible), the excessive torque shattered the glass, Brancazio says.

Another example of torque: Is it easier to push a door open if you stand closer to or farther from the door's hinges?

MASTER BLASTER

Dixon's towering height and hulking mass (amount of matter in a per son or object) also played a role in the startling stunt. "Slam dunks are my favorite shot," Dixon says. In fact, the glass-breaker was his third dunk in the game. At nearly 7-feet tall, Dixon, and other players his size, look like they could slam dunk in their sleep.

The reason: With his arms extended straight up toward the basket, Dixon has a standing reach of about 9 feet. To get his wrist and the ball to rim level (10 feet 6 inches), he has to have a vertical leap, or jump, of only 18 inches. Says Brancazio: "That's a leap the average playground player can do." By contrast, a 5'6" teenager needs a vertical leap of 45 inches to make the dunk. "Any vertical leap over 36 inches is exceptional," adds Brancazio. (Calculate your vertical leap, below right.)

Dixon's extra height means he can "put less energy into jumping and more into dunking," sometimes with shattering results, Brancazio says. Even by NBA standards, Dixon is large. The NBA's "Average Joe" stands at 6 feet, 7 inches and tips the scales at 231 pounds. A full four inches taller and 90 pounds heavier, Dixon can exert more force on the rim, resulting in greater torque and more broken glass.

BACKBOARD ATTACK

Still, no one would call NBA players petite. With big players like 340-pound Shaquille O'Neal pounding the courts, why don't you see backboards exploding in the NBA? "They did twenty years ago," says Kevin R. Murphy, vice president of American Athletic Inc., a sports equipment manufacturer in Jefferson, Iowa. In the early 1980s, Darryl Dawkins, a slam dunker for the 76ers and Nets, won a reputation as the "Master Blaster," smashing six backboards in one season. Soon after, the NBA switched to "break-away rims," which are mounted on springs (devices that return to their original shape after being compressed or stretched), explains Murphy. When a player slams down with 230 pounds of weight or more, the rim collapses downward. "The springs absorb the shock and the rim returns to normal," Murphy says.

Too bad the rim in Arkansas wasn't a break-away: Dixon pocketed a penalty for his smashing strait and the Hilltoppers lost the game. But Dixon claims the slam was a highlight of his career. His dream is to dunk his way into the NBA, where he hopes to shatter records, not backboards.

What's Your Leap?

COMPARE YOUR VERTICAL LEAP WITH THE JUMPS OF ALL-STAR DUNKERS.

1. Grab a piece of chalk and stand facing a wall.

2. Hold your arm up straight. Using the chalk, mark your reach on the wall. This is your standing reach (S).

3. Keep your arm up and jump as high as you can. Mark this reach on the wall. This is your jumping reach. (J).

4. Subtract your standing reach from your jumping reach. This is your vertical leap (J-S).

HOW DOES YOUR LEAP RATE? BELOW 18 INCHES: With practice, you'll rule in gym class.

18-24 INCHES: If you were as tall as Dixon, you'd be dunking.

24-36 INCHES: Even in the NBA, your leap is tops.

ABOVE 36 INCHES: You leap like Michael Jordan!

Did You Know?

* The first basketball game was played on December 21, 1891. The sport was invented by Dr. James Naismith, a teacher at the School for Christian Workers in Springfield, Massachusetts. Naismith was given 14 days to create an indoor game that would provide "athletic distraction" for his students during the harsh New England winter. Naismith nailed a peach basket to two opposite ends of a YMCA basement gymnasium. He found a soccer ball and set out 13 rules--some of those rules still apply today. A stepladder was used to retrieve the ball from the peach baskets.

* The record score for a player in one NBA game is 100. On March 2, 1962, Philadelphia center Wilt Chamberlain racked up these points by netting 36 field goals and 28 free throws in a game against New York.

Resources

"Mechanics of a Slam Dunk," by Peter Brancazio, Popular Mechanics, November, 1991.

Sports Science: Physical Laws and Optimum Performances, by Peter Brancazio, Simon & Schuster, 1984.

You can learn about the history of basketball at: www.hoophall.com/history/history.html