This topic was presented by, Anton Tuomi, a reader from Sweden. Many thanks to Anton for the very rich situation! I have the theory that: if you drive a vehicle into a static, unbreakable wall, you will feel the same G-force and get the same injuries as if you would drive into your exact copy but mirrored (same car, weight, velocity, angle) head to head. Everybody [says] there would be more energy transferred to the drivers and more injuries . but I don't agree. I think the effect should be exactly the same when the energy is divided between the bodies no matter if it's a wall or your mirror image/clone. Answer: There's a lot going on in this example, so I'll be focusing on only a few of the points you bring up -- mainly the distinction (or lack thereof) between force and energy. Force - Colliding With a Wall Consider case A, in which car A collides with static unbreakable wall. The situation begins with car A traveling at a velocity v and it ends with a velocity of 0. The force of this situation is defined by Newton's second law of motion. Force equals mass times acceleration. In this case, the acceleration is (v - 0)/t, where t is whatever time it takes car A to come to a stop. The car exerts this force in the direction of the wall, but the wall (which is static and unbreakable) exerts an equal force back on the car, per Newton's third law of motion. It is this equal force which causes cars to accordion up during collisions. It is important to note that this is an idealized model. In case A, the car slams into the wall and comes to an immediate stop, which is a perfectly inelastic collision. Since the wall doesn't break or move at all, the full force of the car into the wall has to go somewhere. Either the wall is so massive that it accelerates/moves an imperceptible amount or it doesn't move at all, in which case the force of the collision actually acts on the entire planet - which is, obviously, so massive that the effects are negligible. Force - Colliding With a Car
