JB, I've got that concept down pretty well, having treated bullet wounds and fired them before. But not all the momentum is absorbed sometimes. We had one fellow trying to qualify for the USAF using a 44 mag revolver, and shooting John Wayne style. He put the hammer into his forehead with that relaxed stance.

My main point here is that people are saying that bicycle helmets are not effective in collisions with cars, and I wanted to point out that what they are comparing is the equivalent of a bullet or more in momentum (10,000x more, actually). The bullet, because of the very small surface area, simply goes right through a person. But the car, with the 10,000x momentum, accelerates the entire body of the person and throws the person, sometimes 100 feet or more. The injuries can be massive. But if the "hit' is only to the handlebar on the left side of the rider, and the rider simply falls to the ground from the loss of balance, then the helmet can do wonders. Why? For the same reason you point out here, that it increases the surface area impacted, thus decreasing the force on the contact point of the skull. It also crushes, allowing for a decrease in the acceleration forces that injure the brain from the motion. So your point about surface area is well taken. This is also why drivers don't realize the amount of damage that they can do with their vehicles--they have no concept of the amonut of momentum that they really have in that vehicle that they are driving.

John