As it's a tough thing to replicate (nobody is exactly stepping forward to volunteer to be shot in the head), the explanations on the table will have to suffice. First off, consider that there are TWO reversals of motion in the Zapruder film, but the second is seen as unremarkable and not considered evidence of conspiracy - as that reversal happens when JFK rebounds off the back of the limo seat. The physics of that reversal is well known and readily accepted.
And secondly, although the refrain has always been "Back and to the Left", there's no evidence the body of JFK moves to the left at all. It appears that JFK, upon rebounding off the back seat, winds up no closer to Jackie than he was at the time of the head shot. So where's the evidence of movement to the left? It appears from here to be an illusion caused by JFK's leaning leftward toward JFK at the time of the headshot. JFK's leftward movement has always been *assumed*, not established.
So this backward motion has been explained in a number of ways:
1. As pointed out above by Sandy McCroskey, the head actually moves forward noticeably for one frame at the time of the impact of the bullet. This movement was discovered by Nobel Prize winning physicist Richard Feynman and discussed at length in David Lifton's book, BEST EVIDENCE.
2. The backward movement which starts only after the bullet has exited the head (and therefore cannot be due to the impact of the bullet, as the momentum would be transferred instantaneously) can be due to a neuromuscular reaction, as pointed out above by BKnight. This was tested by experiment using live goats and demonstrated in films shown to the Warren Commission.
3. The estimated mass carried forward by the cavitation of the head (times the speed pf the mass) is greater than the mass of the bullet (times the speed of the bullet) that struck the head. The physics of this exchange says, as I understand it, that the head should therefore move backward to maintain the stability of the system. This was determined by Nobel Prize winning physicist Luis Alvarez. This has been called "the Jet Effect".
4. The head can only go so far forward before it will reverse its motion. It appears JFK's chin is compressed pretty much as far as it can go into the chest by Zapruder frame 313. Like the physics of a basketball hitting a backboard and reversing its flight direction, JFK's head can be considered to be rebounding off the chest of JFK by Z-frame 313.
5. The rigid back brace held JFK upright, but prevented his body from moving forward too far. Once it reached the limits, it rebounded the body backward. This was mentioned by Tomtomkent above.
6. A bullet from somewhere to the front (not the knoll) caused the head to move backward. Let's examine this in detail:
(a) There's no evidence of a shooter in front of JFK, as there were a dozen men on the overpass directly in front of JFK at the time of the shooting, and none of them saw a shooter anywhere on the overpass.
(b) Further, there's no evidence of a shot striking JFK in the head from anywhere except from behind (as determined at the autopsy) and this finding was confirmed by neutral expert forensic pathologists who examined the extant autopsy materials in 1978 at the behest of the HSCA.
(c) There is also no damage evident to the skull from a shot at Z314, and no ejecta from a shot from the front.
(d) And all the fragments found anywhere that were large enough to be ballistically traceable where traceable to Oswald's weapon, so there's no evidence of a bullet fired from another weapon.
(e) The movement is too large to be caused by a bullet. It's not even close to the appropriate magnitude.
So it's my opinion this suggested explanation of a shooter striking JFK in the head from anywhere but the sixth floor southeast corner window of the Depository can be eliminated, as a shooter elsewhere in the plaza is the only one that has ZERO evidence in support, and no experts claiming this is what would happen. So I eliminate this explanation from consideration.
What explanations do you eliminate (if any) and why? Please be expansive in your response.
Thanks in advance.
Hank
