In the great game the Denver Broncos happened to win, there was a critical personal foul called in which a Denver player blasted the New England receiver shortly after the receiver caught the ball, incidentally hitting the receiver helmet to helmet. Considering that the average NFL player can run 15-20 mph and weighs around 200 pounds (90 kg), that is a lot of linear momentum that the receiver is being hit with. When the player is hit, a lot of the energy is dissipated as the players come to rest almost immediately after the collision. Since this particular collision was helmet to helmet, this means that the momentum and energy from the defensive player was mostly transferred to the helmet of the receiver. Fortunately, helmets have padding designed specifically to increase contact time, and therefore lower the force felt from a collision. Despite the helmet lowering the force felt by the receiver's head, a large portion of the energy and momentum is transferred to the head of the receiver, whipping the head backwards. While this whipping motion can cause neck injuries due to the sudden acceleration of the head, it also causes concussions. The brain wants to stay still due to its inertia, but when the head whips, it causes the brain to whip back and forth. There is actually some room between the brain and the skull which allows this. The movement of the brain back and forth can damage nerves as well as the Brain itself, leading to a lot of NFL players developing mental health conditions after retirement. Unfortunately, players need to assess the risk of playing football because hard hits to the head are inevitable, even if everybody is doing as much as possible to prevent it. Denver deserved a penalty, but its hard to change your linear momentum when a player ducks into your tackle. This is why the NFL is investing so much money into concussion awareness and research.