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The reason you feel your head thrown backwards is because of Newton's first law of motion, also referred to as the law of inertia. It goes something like this: "Objects that are in motion tend to remain in motion, and objects that are at rest tend to remain at rest." Your head, body, the seat you are sitting in, the car itself, are all at rest until Mr. Fun decides to abruptly change that peaceful resting position into one of forward motion. Your head feels it the most because there is no head-rest to catch it's backward thrust.
Inertia is an object's resistance to changes in motion. Things that have mass are subject to this law. Your head has mass; as such, the Earth exerts a pull on the amount of mass in your head and translates that mass into a force called weight. The more mass objects have, the more weight they have, and the more inertia they have.
Oh, and if you thought Mr. Fun was through teaching you all about inertia, just wait until he has the 1972 Trans Am going 60 miles per hour, then slams on his brakes. Guess what your head, which is now going 60 miles per hour, along with the car, is going to do when the car negatively accelerates quickly to 0 miles per hour? That's right, things in motion don't want to change, according to Sir Isaac Newton! You are about to experience the same jolting force, except this time in reverse, because your head doesn't want to change what it is doing, motion-wise.
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