There are perennial arguments about the circumstances of the assassination of President Kennedy in 1963, and in particular whether more than one shooter is required by the evidence (such as the Zapruder film). Those who know little about physics frequently claim that the sharp backwards motion of JFK’s head as the fatal shot hit him is proof that there must have been a gunman in front of the car carrying Kennedy. This is simply untrue. As I show here, the movement of the head is consistent with a bullet arriving from behind, from Oswald on the sixth floor of the Texas Book Depository in Dealey Plaza, Dallas. There is no need, in terms of the physics, for a second gunman.
It’s that time of year again. On 22nd November 1963 (it was already the 23rd in New Zealand) President Kennedy was shot from behind by Lee Harvey Oswald from the sixth floor of the Texas Book Depository in Dealey Plaza, Dallas. Every year people gather in that place to recall the tragic events of that day, and often there is a repeated media ruckus as someone, somewhere, comes up with yet another conspiracy theory that they think will explain what happened.
Above I very deliberately wrote “shot from behind” so as to provoke a negative response from any reader who believes that the fatal shot must have come from the front, and the famous ‘grassy knoll’. Their reasoning (such as it is) is that JFK’s head jerked sharply backwards as the final shot hit his head, and they imagine (repeat, imagine) that this must imply that the shot came from in front of the cavalcade and the limousine in which the President was riding. In terms of basic physics, that’s not true. There is no requirement, on the basis of physics (specifically, mechanics) for the shot to be from the front. This is something I have written about repeatedly over the years, such as here:
There are many ways in which a physicist might contribute to an analysis of what actually happened in the assassination. See, for example, this paper by Luis Alvarez. Just in terms of analysing the famous film shot by Abraham Zapruder (a stabilised version of that movie is available here), there is much that could be said and written, and indeed has been. Here, though, I will restrict my commentary to just one piece of the puzzle that physics can be used to address: the implication of JFK’s head quickly moving backwards as the fatal shot hit him.
Here is the layout of the crime, a sketch from the paper published in 1976 in the American Journal of Physics by Luis Alvarez.
Oswald’s position is shown, on the sixth floor of the Book Depository. Nowadays there is a Sixth Floor Museum located there. The grizzly murder of the President attracts tourists continually.
The way that many people interpret incorrectly the sharp rearward movement of JFK’s head as shown in the Zapruder film is that they implicitly assume that there were only two interacting objects: the bullet, and the head. Although that might have been true in the split-second before the bullet hit its target, in fact soon after that there were three objects involved: the bullet, the remnants of the head, and a jet of brain material, blood, skin and skull that can be clearly seen in Zapruder frame 313 projecting largely forwards, but actually splattering all over the place. It’s gruesome, but that’s the way it is.
In terms of the physics, it’s a relatively simple problem. A first-year undergraduate should be able to solve it, in terms of the theory at least: it’s just conservation of momentum, and conservation of energy. The only ‘difficult’ part might be involved in solving the relevant equations numerically.
Here following are grabs from one of my ‘standard talks’ that is all about the JFK assassination. In that talk I discuss many other aspects of the evidence, and what can be deduced from the Zapruder film (and other sources), but here I am considering only the backward snap of the head after the final bullet struck.
So, I solve the equations numerically (whereas in his paper, Alvarez arrived at an analytical solution indicating the head would move backwards). Here is my code: youngsters will be horrified to see that I programmed it in Turbo Pascal (but this was back in 1992; and I did learn to write programs in Fortran on IBM 80-byte cards…).
From that code I have produced an executable (.EXE) file, which I have put into a zip file and uploaded here on Dropbox so that anyone should be able to run it; just put the EXE file onto your desktop. (I have scanned the zip file for viruses, but you should also do so on grabbing it for your own use.)
So, here is a sample result from running that program:
Hours of your precious time can be spent trying to find combinations of input parameters that result in anything other than negative values for vR, the velocity of the remnant of the head, indicating a recoil towards the direction from which the bullet arrived. There are inputs that render positive/forward values for vR, and good luck in identifying some. Similarly there are some results obtained for shots from the front that result in the head moving backwards, but these are all generally inconsistent with other observations (such as the jet of brain spraying forwards; and the sizes of the entry and exit wounds).
The bottom line here is this: whilst (as is true in all science) one cannot prove that the shot came from behind, the observations are consistent with that being the arrival direction of the bullet, and it’s difficult to find sensible combinations of parameters that would result in a backward head motion if the shot were from the front (the grassy knoll). People who claim that the backwards snap of the head prove that the shot came from the front are simply wrong.