Please read what I post below, and tell me what you agree/disagree. Like/dislike about it. Please be nice what writing about this. Make sure you understand the physics involved. Discussion 1; If we think of ourselves sitting in a car that is travelling with an amount of velocity and acceleration. Our car slams into the back of another car, either moving with less or no velocity and/or acceleration than our`s, our bodies which are fastened to the car via a seat and seat belt, accelerate forward during that moment of impact. The transfer of force from the collision imparts an equally proportionate force on us as we are part of the car. Along with our bodies, our head also accelerates forward during impact. Due to the elasticity in the muscle and ligaments in our neck, we create an opposing force to quickly stop the acceleration and velocity forward, and reverse it. The reasoning for a headrest in this case would be so that we don`t do even more damage to our spine when our muscles and ligaments create that greater force after the moment of impact. Conversely, we are sitting in a car with little or no velocity and/or acceleration. We are rear-ended by another car with a greater force from velocity and acceleration, than ours. At the moment of impact, the car is projected forward, yet our bodies actually accelerate backward(negative acceleration with respect to the horizontal axis). The primary purpose of the headrest in this instance is to stop or minimize this negative acceleration, as well as limiting the recoil force of our neck and head moving forward with after the initial impact.