repeated blows to the head, the kind that you get on a football field can have lasting reprecussions. especially for young people. some states are writing new laws. in fact, one just took effect this month here in california. any player on a high school or middle school team who gets a concussion must be cleared by a trainer or doctor before going back on the field. even then, though, the risk is hard to gauge. you can t really see damage to the brain unless you actually look inside. of course, by then it s too late. reporter: football is a violent game, full of big hits. what are all those collisions doing to the brain inside those helmets? i met with kevin, a research esh from the university of north carolina. he can actually measure the intensity of those hits. so i m going to give a moderate hit and see what happens. so it s recorded up here at 23.6 g s of acceleration. he recently won a mcarthur genius grant for his work on concussions in football. he is going