Compelling as JFK's dramatic rearward lurch in the Zapruder film may seem to be for a shot from the front and thereby conspiracy, it is not necessarily so. For one thing, it is preceded by a quick forward snap of his head. For another, its rearward acceleration extends over too many frames to be directly from a bullet. For a third, some degree of lurch ought to be expected in reaction to the mass of brain matter hurled forward as his head exploded. Nevertheless, the lurch remains the most popular piece of evidence for conspiracy in the assassination. 
    The question here is quantitative rather than qualitative. Is enough material hurled forward fast enough to throw the body backward with the observed speed? There is no reason this cannot be investigated with simple physics, yet it has not been done. One sets up the equations for conservation of momentum and total energy, provides reasonable estimates for the several quantities involved, and solves for the speed of the lurch, using as major factors the bullet, the head, the body, the two large fragments hurled forward, and the diffuse cloud also hurled forward. For those quantities that cannot be estimated reliably, ranges of values can be used.
    This monograph shows how to do all this, which gets complicated in practice even though not in principle. It begins with the measurements of JFK's motions as reported by Josiah Thompson in Six Seconds in Dallas. To these it applies sixteen simulations, two for the snap and 14 for the lurch. Both snap and lurch were calculated in rotational as well as translational coordinates, with the lurch getting seven levels of increasing complexity. More than 30 variables were eventually included. The important ones were identified in two ways, by examining how the answers changed when they were added to the simulations and by sensitivity tests for each simulation. Seven of the variables eventually stood out as being most important.
    The monograph asks and answers four basic questions:

  1. Can the forward snap be accounted for by a rearward shot from Lee Harvey Oswald's rifle? Answer: Yes, with ease.
  2. Can the initial rearward lurch of head and body be accounted for by a rearward shot from the same rifle? Answer: Yes, with ease.
  3. Must a rearward shot from this rifle have created a rearward lurch similar to that observed? Answer: Yes, provided only that a cloud of brain matter was thrown forward.
  4. Can a forward shot from the grassy knoll explain all or part of the rearward lurch? Answers: No and qualified yes.

    It does its work in five major sections. The first contains five introductory chapters that deal with the movements, the appropriate physics and wound ballistics, and the variables to be used. The second section contains two chapters that show that the forward snap is easily explained by a bullet from the rear. The third section contains 16 chapters that go through the 14 simulations for the lurch and show the default solutions and the most important variables. The default solutions converge on the actual initial lurch as soon as a three-dimensional cloud of fragments from the explosion is incorporated. In other words, the initial lurch can easily be explained physically. The fourth section is composed of seven chapters that examine the limits to the answers for the lurch, via a series of built-in constraints, such as that the mass of the head is limited to 5–7 lb. The observed rearward lurch is a natural consequence of a forward-moving cloud and nothing more. For example, one need not know the mass of the head, the mass of the cloud, or the speed of the cloud—it all takes care of itself in the math. These results strongly imply that there is little or no room in the solution for a hit from the front, such as from the grassy knoll. The fifth section follows up on this idea in more detail by examining whether a hit from the front could have produced all or part of the initial lurch. The answer is a solid no for the entire lurch, but a qualified yes for adding a small amount of speed to it (but there is no independent evidence for this). The last section explores the implications of these results for our understanding of the assassination. The explanation for JFK's motions resolves the last open piece of physical evidence. Not only does it destroy the myth of the frontal shooter once and for all, but it also unifies the physical evidence into a solid picture of all the damage being done by a single shooter from the rear with Oswald's rifle. The section continues by considering several objections to this work and answering them, and concludes by offering some refinements to be worked on in the future.

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