Physics of ‘the Hit’
Julie Jacobson/Associated Press
Baltimore running back Willis McGahee was knocked unconscious after a collision with the Steelers' Ryan Clark in the A.F.C. championship game.
Published: January 30, 2009
TAMPA, Fla. — Isaac Newton’s apple hurt considerably less than Ryan Clark’s coconut. But they did have a few things in common.Skip to next paragraph Clark’s shockingly violent hit on the Baltimore Ravens’ Willis McGahee two Sundays ago — a full-speed, helmet-to-helmet crash that left McGahee unconscious and Clark all but — didn’t just follow the N.F.L.’s rules, but Newton’s as well. Force equaled mass times acceleration. Momentum was conserved. And the bodies finally came to rest, McGahee’s on a stretcher.
“How I look at it, you can be the hammer or the nail,” the inner scientist in Clark explained this week. “I try to be the hammer.”
The tackle, the art of making the ball carrier not stay in motion, is football’s most primeval action. Amusing physicists the way batting averages do actuaries, collisions lead the highlight reels, impart the force of a deadly car crash, and rely upon kinematics that date to a considerably different big bang.
Sunday’s Super Bowl could very well ride on how well the Steelers’ defense — known as perhaps the most fearsome and bone-clattering in the N.F.L. — can tackle the Arizona Cardinals’ fast and evasive wide receivers. From angles and acceleration to speed and centers of gravity, players might not understand the physics of tackling, but they know how to wield it.
“It’s all about timing and leverage,” Cardinals safety Adrian Wilson said. “Being able to time the hit the right way, and the leverage you’ve got to have once you make impact so the other player goes back, and not you.”
Trying to trip up or throw down a ball carrier with only one’s arms can be a risky maneuver. Barreling straight into him with 200-plus pounds of muscle at 20 miles an hour is a more reliable impediment.
From there, Newton’s second law of motion (force equals mass times acceleration) and conservation of momentum take over. Mass is the players’ weight, which in the N.F.L. grows higher every decade. Acceleration is not that of the incoming tackler, as is often assumed, but how quickly both the defender and runner slow down through impact.
It is this duration of impact, between one- and two-tenths of a second by many estimates, that has tremendous effect on the force of a football collision. Hard objects repel each other quickly; equally heavy but softer objects have “give” that allows their contact to last longer and accept the force less jarringly. It’s the difference between being hit by a baseball and being hit by an overripe peach.
“The tackler doesn’t want his body to be a big spring — these players lower their shoulder and tense up and launch to make their force go up,” said Stefan Duma, a professor of mechanical engineering at Virginia Tech who has studied the similarities between football collisions and car crashes. “It’s like trying to break down a door — you try to get all your mass behind you and drive it through one point. You want to get all your mass to act as one mass, one missile.”
Reaching the ball carrier at full speed is crucial, as any deceleration before impact saps force from the hit. This is where angles come in, said Timothy Gay, a professor of physics at the University of Nebraska-Lincoln and the author of “Football Physics: The Science of the Game.” Football instincts allow the best safeties to anticipate where the runner or receiver will be and then take the shortest route to him, maintaining speed and even allowing for one final push.
“Jack Tatum was vicious — that helps — but he had a way of popping with the perfect angle and timing,” Gay said of the former Oakland Raiders safety called the Assassin in both reverence and fear. “The best hitters accelerate at the last instant. That final jolt of speed allows them to apply a bigger force to their victim.”
Ask a physicist and a coach where that force should be applied, and they can answer differently. Gay said that the hit should be applied at the runner’s center of mass — just below the rib cage in the center of the chest — to direct all the force into stopping his forward motion. Missing that spot by too much, Gay said, “Causes the ball carrier to rotate around his center of mass, and he might not go down. The announcer would say, ‘He bounced off him.’ ”
But Ray Horton, the Steelers’ defensive backs coach, preaches a different approach. He is all for the ball carrier rotating, as long as he does so violently enough to wind up on the turf.
“We teach you tackle at the knees — if you tackle at the thigh to the shoulders, that’s his power box,” Horton said. “If I want something to tip over, I don’t want to hit it in its center of gravity, because it might go straight back and stay upright. If I want it to go down, I want to hit below the center of gravity, and that’s why we hit by the knees.”
Horton added: “Low man wins. If you hit him too high, he’s going to run you over because of the physics of how big these guys are.”
Which is why trying to run over the most massive running backs, from Earl Campbell to Brandon Jacobs, is asking for your action to get an equal and opposite (not to mention embarrassing) reaction, with you on the ground and the runner continuing onward. Because momentum — defined as mass times velocity — is conserved in all collisions, Jacobs moving at any decent speed is almost impossible to stop by an outweighed defender’s merely running into him. Tripping him or wrapping him up and waiting for help is a far better option, as long as you are not under him when he finally falls. Skip to next paragraphRunning backs do not sustain the hardest shots in football, though. Few plays get more oohs and aahs than when a lithe receiver crosses the middle and, with or without the ball, gets hit squarely by an oncoming safety.
Duma suggested imagining the body as a primary mass (the torso) in the middle with several other masses connected by springs (the limbs and neck) attached to it. When the tense and intent defender hits the center of this object, the torso accelerates back while the head and feet stay behind temporarily, before flopping back. These are the hits that make the highlights.
Duma said that in his experience — he watches dozens of N.F.L. games each season — these hits are more frequent at the end of games already in hand.
“When you’re up, your defense hits harder,” Duma said. “They take more risks. If you come across the middle on the slant, they’re going to go after you and not worry about missing the tackle and giving up the touchdown.”
He added: “I think that’s what you saw on the Clark-McGahee hit at the end of the last game. I saw it a lot in these playoffs. Baltimore against the Dolphins, they were just leveling people. Ed Reed could run all over the backfield, and if he was out of position, they wouldn’t lose the game.”
Clark appeared to concur. On Wednesday, he said: “The McGahee hit, that was a point where I probably could have stopped and waited and tried to tackle him, but it’s sad to say I think I closed my eyes and I was praying that I’d wake up when I hit the ground.”
In the end, players leave physics out of their own definitions of hard hits. Anquan Boldin, the Cardinals’ receiver who was hit so high and hard by a Jets defender earlier this season that he now has 7 plates and 40 screws in his face, said he defined the perfect hit as “when you get hit hard enough to make you rethink about having anything to do with the ball,” which apparently the Jets hit still was not. Clark said, “What makes a good hit is not getting fined.”
Clark said he was not particularly familiar with Newton’s laws — but then offered his own theory of momentum, one he plans to use in Sunday’s Super Bowl.
“A good hit can change the momentum of the game,” he said. “If we come out there and hit them, be physical with them, and get a good hit early, I think they might go back to the quarterback and say, ‘How ’bout you not throw the ball in there?’ ”
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