Seattle Seahawks quarterback Russell Wilson scrambled across the line of scrimmage but was about to get flattened by a trio of Philadelphia Eagles — short of a crucial first down in the fourth quarter of Sunday's game.
Then came the play that sent Eagles fans into a rage.
Wilson deftly shoveled the ball to running back Mike Davis, appearing to aim it sideways or even slightly backward, as the rules require.
But because the quarterback was running forward at the time, the net effect was that the ball continued forward after it left his hands. Though the Eagles opted not to challenge the call, it looked very much like an illegal forward pass.
Why did the ball drift forward?
Simple physics, said Richard Di Dio, an associate professor of physics and mathematics at La Salle University. Or more specifically, Newton's first law of motion.
If an object is moving in a certain direction, it will keep doing so unless acted upon by some external force — a property referred to as inertia.
"It's like dropping a bomb out of an airplane," Di Dio said.
In other words, when a bomb is dropped from a plane, it does not fall straight down. It continues to move forward at the same speed as the aircraft. Yes, it starts to slow down due to air resistance, but the impact is barely noticeable in the initial moments after launch, Di Dio said. Ditto for Wilson's shovel pass.
The Inquirer dug deeper, calculating velocities of the quarterback and the ball by slowing down video of the play to watch it one frame at a time.
Wilson was running in a diagonal direction, but what matters is his rate of progress directly downfield. Scrutiny of the video revealed that he was traveling downfield at a rate of 5 yards per second.
The ball, meanwhile, drifted forward at a rate of about 1.7 yards per second, before it was caught by Davis.
That shows that indeed, Wilson tried to nudge the ball backwards, as its forward progress was slower than that of the running quarterback (1.7 yards per second instead of 5 yards per second).
But Wilson did not nudge hard enough. The ball left his fingers just after he crossed the Seahawks' 46-yard line. It drifted forward so that Davis caught the ball just after he crossed the 47-yard line. A forward pass, even though it may not have looked like one with respect to where the quarterback was standing, said La Salle's Di Dio.
"You can't consider the ball's motion with respect to Wilson," Di Dio said. "It has to be with respect to the fixed markings on the field. It went forward."
Davis, the running back, rumbled on to the Eagles' 35-yard line, for a total gain of about 24 yards.
First down. And the Seahawks went on to score a decisive touchdown, making the score 24-10.