You better believe it. And the reason is simple: players are bigger and faster than they used to be. How much bigger? Between 1970 and 2006, the average weight of N.F.L. players at every position increased (defensive tackles, for example, grew by 20 percent). Speed is a bit more difficult to assess, since there are no systematic records of 40-yard-dash times for N.F.L. players. It's reasonable, though, to assume that the percentage increase in player speed over this period was similar to the percentage change in 100-meter-dash times: taking world-record times in this event, we can infer that player speed increased 4 percent. Now let's focus on a collision between a running back and a linebacker, and analyze it according to Newton's Second Law of Motion. This basic law of physics tells us that the force of a hit is equal to a player's mass times the acceleration he feels as a result of the hit. Consider a running back who breaks into the defensive secondary and reaches top speed as he motors for the end zone. Unfortunately for him, he's blasted head-on by a middle linebacker who has also built up a full head of steam. They collide violently and fall to the turf. Newton's Second Law tells us that the force of this impact can be approximately three-quarters of a ton. (The scientific term for this is ''getting your bell rung.'') That force is proportional to the product of the player's speed and mass, if we assume a given time (about 0.15 seconds) for the duration of the collision. By factoring in gains in mass and speed, we can estimate that the force of a full-speed hit in the N.F.L. has increased by as much as 25 percent since 1970. Hence the clich?'football is a contact sport.''

PHOTO (PHOTOGRAPH, LEFT, BY MARC SEROTA / GETTY IMAGES.