Topical

The Times focuses on the physics, but considering our recent conversations, I think some of the underlying psychology is more interesting.

Clark’s shockingly violent hit on 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.

The hit was the functional equivalent of running into a padded wall at 40mph.

“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

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“Jack Tatum was vicious — that helps — but he had a way of popping with the perfect angle and timing,” Gay said of the ex-Oakland Raiders receiver 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.”

Discovery reports on some promising technology.