Quote:An impact from behind drove his head back?  Right.    You must be one of the "his chin hit his chest and his head rebounded backwards" believers.  

Thank you for showing a fundamental minsunderstanding of how momentum and force works and specifically what Newton's third law of motion actually means in the real world. 

 

Since you clearly have no idea what the hell you're talking about, let me break down the math for you.

 

A bullet travels at a very high velocity so people thnks it has a very high momentum. That's wrong because momentum is the product of mass and velocity (p = m * v). A bullet's mass is tiny so it's momentum is a lot smaller than people think. The 6.5 Carcano (or 6.5 Italian) round used by Oswald has a mass of 10.5 grams and a velocity of 700 m/s. Let's assume the positive effect of gravity cancels out the negative effect of wind drag (this is to the benefit of your hypothesis) and say the bullet struck Kennedy at 700 m/s. That gives a momentum of 10.5 x 700 = 7,350 g * m/s.

 

Now assuming Kennedy's head absorbs the full momentum of the bullet and no energy was wasted (both benefiting your hypothesis) we can calculate the speed at which Kennedy's head should have gone forward. An average human head weighs about 4.5 to 5 kg. Assuming the lower end (again to your benefit) that gives 7,350 / 4,500 = 1.64 m/s. That's just shy of 3.7 mph or a brisk walking pace.

 

So why did his head move backwards? Simple answer; because when the bullet struck the back of his skull it didn't move the entire head forward but instead it blew brain matter and skull fragments to the front and right. This is where Newton's third law of motion comes in. Briefly summarized; Every action has an equal and opposite reaction. This is what makes rockets work in space when there is nothing to "push of from." The rocket expels rapidly expanding gases from the combustion out the back and as a result the rocket goes forwards. In this case the expelled brain matter and skull fragments went to the front and right (the action); Kennedy's head went back and left (the reaction).

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