at for more than a year now. growing 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 b