The calculation is not difficult. It all depends on how much mass is expelled from the head and what portion of the bullet energy it carries.
The momentum is: p = square root of {2m(KE)} where m is the mass of the ejected matter and KE is the kinetic energy of that matter.
Let's say the mass of ejected blood and brain matter was, conservatively, 100 g. It was likely more.
The energy of that expelled mass can only make up a small fraction of the energy of the incoming bullet. This is because much of the bullet energy is used in deforming the bullet when penetrating the skull. The energy of the deformed bullet plowing through the brain is converted into compression energy (pressure x volume of matter) of the brain material that is then converted to kinetic energy of the pressurized brain matter when the front of the skull ruptures.
Conservatively, let's say only 10% of the bullet energy is converted into kinetic energy of the expelled brain matter. A 10 g bullet moving at 1900 fps (580m/sec) carries kinetic energy (mv^2/2) of 1680 Joules. This would mean that the 100 g. of ejected matter carried 168 J. of kinetic energy. Using the formula for momentum, that means that the momentum imparted to the ejected matter was p = sqrt{2 x .1 x 168) or about 6 kg m/sec of momentum. This would propel the head (having a mass of, say, 13 lb or 6 kg) back at a speed of about 1 m/sec.
[Note: Another factor is gravity. Once JFK's body was pushed far enough left, gravity would take effect.]
So even using these conservative estimates for the amount of matter ejected and its energy, there would be a significant momentum imparted to the head - enough to cause a visible rearward and leftward motion of the head.
Note: Detached skull fragments were drawn at a larger scale than the head. |
I'm wondering if the bullet entered at the back of the skull and created a few linear fractures radiating from the impact point, that ran into the lower rear of the skull.
Some of the surface of the skull at the point of impact was pushed forward such that it jutted forward of the intact skull above (see lateral X-ray):
The bullet fragmented into multiple pieces at the entry point and may not have had enough energy to fracture the intact skull opposite the entry. Possibly it was the bullet pressure wave that caused the fragmentation of skull above the right ear, or the gaping wound.
Nalli writes:
"It is noted that the massive “defect” was not the “exit wound”
of the bullet (as is commonly misunderstood), but rather
corresponded roughly to the area where the maximum
explosive energy was deposited by the bullet during its passage "
"Here the large wound inflicted on the President's head was not a
bullet exit wound, but rather the region of maximum temporary
cavitation associated with KE transfer. This KE deposit
propagated radially outward in the form of an expanding pressure
wave resulting in a rupture and explosion of the skull."
"However, all this said, note well that because such explosions are
not necessarily the bullet outshoots, the momentum directly carried
forward by a given bullet during passage may not be the primary
player in a recoil effect."
The HSCA drawing seems to imply the right-side gaping wound was caused by the path of a bullet, or more precisely, one of its fragments, that impacted the inside of the skull to fracture it.
