In a sense, Walker is updating Newton's second law of motion: Force equals mass times acceleration. In Newton's theoretical universe, colliding objects retain their original shapes (think of two billiard balls), but in the real world, things don't always happen so neatly. Superfast bullets crumple on impact, and ceramic-based body armor dents or shatters in a variety of ways. Computing the forces involved is mind-bendingly complex--so complex, in fact, that until recently, no one had bothered to do the math. Engineers have taken the "shoot-and-look approach," says physicist Charles Anderson, Walker's boss. "What James has done is to bring some real rigor to this field."