There is one more thing that bothers me. I've read that some people got concussions from hitting there head (very) lightly against a hard, non moving surface(wall, table). It happens unfortunately from time to time to probably everyone(you can't be cautios enough)

Back to our g calculations, how can such a light bump ever cause a concussion? I mean there is not enough acceleration of the brain that could reach the required g force, even though the deceleration is of course higher.