evidence repeated blows to the head, the kind you get on the football field can have lasting rep recushions. a law take effect in california. a player who gets a concussion must be cleared by a trainer or doctor before going back on the field. the risk is hard to gauge. you can't see damage to the brain unless you actually look inside. of course by then, it's too late. football is a violent game, full of big hits. what are all those collisions doing to the brain inside the helmets? i met with kevin, he's a researcher from the university of north carolina. he can measure the intensity of the hits. a moderate hit and see what happens. >> it's recorded up here at 23.6 gs of acceleration. >> he won a grant for his work on concussions in football. >> he's going to with stand an impact 157 gs. >> wow. it's similar to a car accident. >> right. the question is how many of those big impacts can a player with stand over the course of a season orca rear before there's damage? >> on average, high school players sustain more than 650 blows to the head every season. that worries him because their brains