If you believe Isaac Newton, says John Fetkovich , then you have to believe that the Immaculate Reception was a fair play.

Fetkovich, an emeritus professor of physics at Carnegie Mellon University, certainly believes in Newton, the English mathematician who formulated the laws of gravity and motion three centuries ago. But the die-hard Steelers fan had long suspected game officials might have blown the call on the most famous play in football.

"For a long time, I believed that the Steelers stole one," he said of that Dec. 23, 1972, playoff victory over the Oakland Raiders.

His mind began to change almost seven years ago, however, when a New York Daily News sportswriter, Hank Gola, sent him a tape of the play and asked him to analyze it for a story marking the play's 25th anniversary.

He didn't have much time before Gola's deadline. But when he closely watched the NFL Films tape, he thought he could make a strong case using the laws of physics that Terry Bradshaw's desperation pass had bounced off Raider free safety Jack Tatum before landing in the hands of Steelers running back Franco Harris. Harris would run the ball in for the winning touchdown.

Even after Gola filed his story, Fetkovich remained fascinated and kept studying the play. He even did some experiments by bouncing a football off the wall of his O'Hara garage. Again, the evidence convinced him that the ball had bounced off Tatum ---- the call on the field ---- and not off Steelers running back Frenchy Fuqua, which would have made Harris' catch illegal under NFL rules at the time.

"It's absolutely clear in my mind that the correct call was made," he said last week.

Fetkovich exhumed his notes and sketches after reading a story on this page two weeks ago about University of Nebraska physicist Timothy Gay and his new book, "Football Physics." Gay discusses the physics of the Immaculate Reception in his opening chapter and, in an interview, speculated that an analysis of the ball's trajectory might reveal who it bounced off.

A key to such an analysis is a fundamental law of physics called the conservation of momentum. This law says that when two bodies collide, one gains as much momentum as the other one loses. When Tatum hit Fuqua, for instance, he transferred all of his momentum to the running back, leaving Tatum standing and Fuqua flying to the ground.

What Fetkovich needed to figure out was whose momentum was transferred to the football.

The NFL Films tape didn't show the collision and didn't show the ball striking either player. But by slowing the tape, Fetkovich could see that the ball had already rebounded by the time the collision occurred.

"That's critical," he said, because if the two collided before the ball hit, they would have already exchanged momentum and made the analysis more difficult. But since the ball hit a player before the collision, then only that player's momentum would have been transferred to the ball.

Tatum was running upfield. If the ball hit him, "Tatum would have added a good deal of momentum [to the ball] in the upfield direction," Fetkovich explained, much as a baseball player adds momentum to a baseball by swinging his bat at a pitch.

By contrast, Fuqua was running across and down the field, with his left arm outstretched to catch the ball. If the ball hit him, he likely wouldn't have added any momentum to the ball, both because he was moving roughly in the same direction as the ball and because the ball likely would have hit him in the arm.

So a rebound off of Fuqua would have been, in baseball terms, a bunt.

From the tape, Fetkovich calculated that Bradshaw's pass was traveling at 55 feet per second. It was flying at about 30 feet per second when it rebounded and traveled about 24 feet upfield to reach Harris.

Fetkovich's gut feeling was that only Tatum could have caused such a rebound. But he still needed to understand the so-called coefficient of restitution, or COR, of the collision.

The coefficient of restitution is a way of expressing the springiness of a surface. A golf ball dropped on a concrete floor, which has a high COR, will bounce higher than a golf ball dropped on a carpeted floor, which has a low COR.

To calculate the COR of a person, he considered throwing footballs at his wife, "but she wouldn't go for that." So Fetkovich pitched an NFL regulation football at a brick wall, a surface with a presumably higher COR than a player.

He also threw the ball at 60 feet per second, faster than Bradshaw. To put that much velocity on the ball, he noted, "I was standing as close to the wall as I could without scraping my knuckles." After throwing the ball at a variety of angles, he recorded maximum rebounds of 10 feet for a spiralling ball hitting point first and 15 feet for a ball hitting the wall belly first.

So a slower ball striking a softer-than-a-wall player could not have rebounded more than 20 feet unless the player transferred some upfield momentum to the ball, he concluded.

"This really rules out a rebound from Fuqua," Fetkovich said. "The ball must have collided with Tatum before bouncing back to Franco Harris."

Addendum

Both Gay and Fetkovich studied the Immaculate Reception using the familiar, if incomplete, version by NFL Films. But readers of the Oct. 4 article about Gay's book, such as Vince Palamara of Mt. Lebanon, noted that NBC's superior video of the 1972 playoff game was replayed during the telecast of the 1998 AFC Championship game.

The replay generated little comment in most newspapers, but Bob Raissman of the New York Daily News called it "football's version of the Zapruder film," noting it clearly showed the ball hitting Tatum.

Also, Gay's book and the Oct. 4 article both said the artificial turf of Three Rivers Stadium was frozen during the 1972 game. In fact, that day was unseasonably warm.