Would A Bullet Really Knock You Backwards? DEBUNKED

[Andrew Mason]

Why did the head immediately, or almost immediately (after just 10 ms) start moving backwards, and yet ended up 2 inches forward?

The impulses to the head are not in opposite directions. And they do not start at the same time.

The impulse to the head from the bullet is opposite to the direction of the bullet's change in momentum ie the impulse to the head from the bullet is forward. The impulse to the head from the jet effect is opposite to the direction of the ejection of matter. The matter from the head explodes out of the right front upper part of his head.  This pushes the head to the left side and down and to the rear. 

The different directions of these impulses also affect the body differently.  The sideways impulse takes the body with it whereas the forward impulse causes the head mainly to pivot forward.

[Joe Elliott]

The impulses to the head are not in opposite directions. And they do not start at the same time.

The impulse to the head from the bullet is opposite to the direction of the bullet's change in momentum ie the impulse to the head from the bullet is forward. The impulse to the head from the jet effect is opposite to the direction of the ejection of matter. The matter from the head explodes out of the right front upper part of his head.  This pushes the head to the left side and down and to the rear. 

The different directions of these impulses also affect the body differently.  The sideways impulse takes the body with it whereas the forward impulse causes the head mainly to pivot forward.

It is true, the ‘Jet Effect’ does not have to push the head straight back. With the side of the head exploding outwards, the ‘Jet Effect’ would push the head to the left. But what I am interested in the effect it would have on pushing the head to the rear. The ‘Jet Effect’ theory was developed to come up with a possible reason why the head started moving backwards.

Question: What was the change of speed, and the resulting speed of the head, rearward, as a result of the ‘Jet Effect’?

For instant, you could say:
     + The initial speed of the head forward was: 2.0 mph
     + When the head exploded, it added 2.5 mph motion to the rear
     + Resulting in a motion to the rear of 2.5 – 2.0 or 0.5 mph to the rear

Basically, I am interested in your estimate of the rearward component of the velocity vector.

For instant, the shot from Oswald’s position was coming down at an angle of ‘b’, 16 degrees, relative to the horizon.
It was moving at an angle of ‘a’, 3 degrees, to the left, relative to the limousine. On the horizontal plane, the shot came almost directly from behind.

So, the ‘x’ component of the velocity of the bullet, the ‘forward’ component, was cos (16) * cos (3) = 0.96 % of the scalar velocity. If the scalar velocity of the resulting head movement was 2.00 mph, the forward velocity was 1.92 mph. For all practicable purposes, this forward component of the velocity is the same as the scalar velocity.

The ‘y’ component of the velocity of the bullet, the ‘leftward’ component was, cos (16) * sin (3) = 0.05. If the scalar velocity of the resulting head movement was 2.00 mph, the leftward velocity was 0.05 mph, not significant.

The ‘z’ component of the velocity of the bullet, the ‘downward’ component was, sin (16) = 0.28. If the scalar velocity was the resulting head movement was 2.00 mph, the downward velocity would be 0.55 mph. If the head was able to move downward freely. Which it was not.

Question: Can you come up with a possible velocity (not a proven, but possible) of the head, rearward, immediately after the effect of the ‘Jet Effect’ takes place, that:
     + Takes place 10 ms after the bullet strikes.
     + That ends up with the head at frame 313 two inches ahead of the frame 312 position, as William Hoffman’s data indicates?

[Andrew Mason]

It is true, the ‘Jet Effect’ does not have to push the head straight back. With the side of the head exploding outwards, the ‘Jet Effect’ would push the head to the left. But what I am interested in the effect it would have on pushing the head to the rear. The ‘Jet Effect’ theory was developed to come up with a possible reason why the head started moving backwards.

Question: What was the change of speed, and the resulting speed of the head, rearward, as a result of the ‘Jet Effect’?

For instant, you could say:
     + The initial speed of the head forward was: 2.0 mph
     + When the head exploded, it added 2.5 mph motion to the rear
     + Resulting in a motion to the rear of 2.5 – 2.0 or 0.5 mph to the rear

Basically, I am interested in your estimate of the rearward component of the velocity vector.

Do you mean rearward momentum of the head and body relative to the body or to the car? There is a difference because JFK was turned to the left and leaning to his left and forward. Also, the head was leaning forward more than the body.  I don't see the head being driven back relative to the body. It was driven to the body left which was to the car rear-left and this caused the whole body to move rear-left.

This is going to be a rough estimate, but here goes:

The head and body pivot. The head pivots on the neck. The upper body, being seated, pivots from the seat.  So we really need to find the angular speed imparted to the head by the bullet and the angular speed of the head and body from the jet effect.  The angular speed (ω) of a body can be determined from the angular impulse or angular momentum (L) and moment of inertia (I) of the body by:
L=Iω.  We are going to use the moment of inertia of a sphere for the head pivoting about the neck and the moment of inertia of a cylinder pivoting on the seat for the head/torso.

My estimates of the motion of the head due to the bullet imparting momentum due to its impact to the head:

1. JFK was turned 15 degrees to the left. He was leaning left at an angle of 10 degrees and his head was leaning forward 40 degrees. I will assume the head mass is 5 kg. which appears to be the average mass of a human head (google).
2. The bullet was travelling at a right to left angle of 5 degrees and a downward angle of arctan(60/275)=12 degrees relative to the horizontal. The car was on a downward 3 degree slope. So the downward angle relative to the car was 9 degrees.
3. The 10 gram bullet entered the head travelling 1900 fps or 580 m/s, passed through the head and exited at 360 fps or 110 m/s and did not change direction.  So the change in momentum was .01 x 470 =4.7 kg m/s which imparted a forward momentum of 4.7 kg m/s to the head at an angle of 5 degrees right to left and 9 degrees down.
4. The bullet struck above the centre of mass of the head.  I would put the centre of mass of the head halfway between the ears, so the bullet path was about and inch or 2.5 cm above the centre of mass.
5. The bullet struck JFK just before z313 was exposed but after the end of exposure of z312. There is 30 ms. between frames where film is not being exposed. I will assume that it struck 30 ms. before the exposure of z313 began.
6. So 4.7 kg m/s impulse to the 5 kg head would result in a 5 degree right to left and 9 degree downward velocity of just under 1 m/s: 940 mm/s or .94 mm/ms.  So in the 30 ms before exposure of z313 began, the centre of mass of the head would have moved 30x.94= 28 mm or just over an inch in a direction 5 degrees to the right and 9 degrees down, assuming it was free to pivot from the neck.
7.  Average speed through the head would have been  (580+110)/2=345 m/s. Assuming a path of 20 cm or .2 m, the transit time through the head, assuming uniform stopping force was applied to the head would have been t=d/v=.2/345=.0006 s = .6 ms.  So we don't really have to take into account the fact that this impulse took some time to impart momentum.
8. Since the head pivots from the neck and since the bullet struck above the centre of mass, one has to determine the angular impulse to the head.  The head centre of mass (middle of the ear to base of the neck) is about 17 cm above the neck, so the angular impulse is r x p=.17 x 4.7= .8 kg m^2/sec which is the angular momentum of the head. To determine the angular speed, we divide by the moment of inertia of the head. Assuming it to be approximately a 5 kg sphere of radius 10 cm rotating about a pivot 17 cm. from the centre of mass, the moment of inertia is .02 + 5x(.17)^2=.16 kg m^2.  So the angular speed would be .8/.16=5 radians/sec or about 280 degrees per second or .28 degrees per millisecond. In the space of 30 ms (the time between end of frame 312 and beginning of frame 313) the head would pivot 9 degrees forward.
9. There is a limit to how far forward the head can move because the chin cannot go past the chest.


Here are my estimates of the motion of the head and body due to the jet-effect:

1. The jet effect did not begin until the bullet exited the head. At that point there was no longer any forward momentum being transferred to the head from the bullet.
2. Frame z313 appears to show a streak from the motion of a piece of matter, possibly a skull fragment moving at an upward angle of about 65 degrees above car horizontal.  It appears to be about 1 metre long and it starts about a metre from the head. This gives us information not only about the speed of the ejected matter but also the timing of when it started.  Since the exposure is 25 ms duration, 1 m of travel in 25 ms is 40 m/s.  Since it had already travelled 1 m before z313 began to be exposed, this means that the head erupted about 25 ms. before z313 was exposed or about 5 ms. after the end of exposure of z312.  Since z312 does not provide any indication of the head being hit, this suggests that the bullet hit less than 5 ms after the exposure of z312 ended.
3. There appears to be a substantial amount of brain, skull bone and blood erupting from the head wound. Doctors reported that a substantial portion of the brain was missing. If the mass of matter exploding from the head was only 300 g, travelling initially at 40 m/s, the total momentum would be 12 kg m/s or almost 3 times the momentum of the incoming bullet. But it does not all explode in the same direction. It forms what appears to be a hemispheric cloud around the exit wound.  So the average would be about cos 45 x total momentum = 70% of 12 kg m/s = 8.4 kg m/s/ 
4. So the head and the body to which it is attached would, under those assumptions, recoil to the body left with 8.4 kg m/s of momentum. Unlike the forward momentum imparted by the bullet, however, there is nothing really to stop the upper body from moving to the left so it keeps going. 
5. If the torso received an impulse of 8.4 kg m/s applied to the right side of the upper skull, which would be a distance of approximately 90 cm above the seat or pivot point of the torso, the angular impulse, r x p would be .9 x 8.4 kg m^2/s = 7.5 kg m^2/s.

[Correction: I was using the wrong axis of rotation for the torso.  The moment of inertia of a cylinder of length Len, mass M and radius R about an axis at one end is: MR^2/4 + M(Len)^2/3.    If the centre of mass of the torso is midway, 45 cm above the seat (Len=.9), and the torso is approximately a solid cylinder of mass=40 kg and radius 20 cm (moment of inertia, I = MR^2/4+M(Len)^2/3= 40x(.2)^2/4+40x(.9)^2/3=.4+10.8=11.2 kg m^2/s).  So the angular speed of the torso would be:  L/I=7.5/11.2=.7 radians/sec or 40 degrees/second In 55 ms or one frame, it would rotate a bit more than 2 degrees to the body left.

[Joe Elliott]

Do you mean rearward momentum of the head and body relative to the body or to the car? There is a difference because JFK was turned to the left and leaning to his left and forward. Also, the head was leaning forward more than the body.  I don't see the head being driven back relative to the body. It was driven to the body left which was to the car rear-left and this caused the whole body to move rear-left.

This is going to be a rough estimate, but here goes:

The head and body pivot. The head pivots on the neck. The upper body, being seated, pivots from the seat.  So we really need to find the angular speed imparted to the head by the bullet and the angular speed of the head and body from the jet effect.  The angular speed (ω) of a body can be determined from the angular impulse or angular momentum (L) and moment of inertia (I) of the body by:
L=Iω.  We are going to use the moment of inertia of a sphere for the head pivoting about the neck and the moment of inertia of a cylinder pivoting on the seat for the head/torso.

My estimates of the motion of the head due to the bullet imparting momentum due to its impact to the head:

1. JFK was turned 15 degrees to the left. He was leaning left at an angle of 10 degrees and his head was leaning forward 40 degrees. I will assume the head mass is 5 kg. which appears to be the average mass of a human head (google).
2. The bullet was travelling at a right to left angle of 5 degrees and a downward angle of arctan(60/275)=12 degrees relative to the horizontal. The car was on a downward 3 degree slope. So the downward angle relative to the car was 9 degrees.
3. The 10 gram bullet entered the head travelling 1900 fps or 580 m/s, passed through the head and exited at 360 fps or 110 m/s and did not change direction.  So the change in momentum was .01 x 470 =4.7 kg m/s which imparted a forward momentum of 4.7 kg m/s to the head at an angle of 5 degrees right to left and 9 degrees down.
4. The bullet struck above the centre of mass of the head.  I would put the centre of mass of the head halfway between the ears, so the bullet path was about and inch or 2.5 cm above the centre of mass.
5. The bullet struck JFK just before z313 was exposed but after the end of exposure of z312. There is 30 ms. between frames where film is not being exposed. I will assume that it struck 30 ms. before the exposure of z313 began.
6. So 4.7 kg m/s impulse to the 5 kg head would result in a 5 degree right to left and 9 degree downward velocity of just under 1 m/s: 940 mm/s or .94 mm/ms.  So in the 30 ms before exposure of z313 began, the centre of mass of the head would have moved 30x.94= 28 mm or just over an inch in a direction 5 degrees to the right and 9 degrees down, assuming it was free to pivot from the neck.
7.  Average speed through the head would have been  (580+110)/2=345 m/s. Assuming a path of 20 cm or .2 m, the transit time through the head, assuming uniform stopping force was applied to the head would have been t=d/v=.2/345=.0006 s = .6 ms.  So we don't really have to take into account the fact that this impulse took some time to impart momentum.
8. Since the head pivots from the neck and since the bullet struck above the centre of mass, one has to determine the angular impulse to the head.  The head centre of mass (middle of the ear to base of the neck) is about 17 cm above the neck, so the angular impulse is r x p=.17 x 4.7= .8 kg m^2/sec which is the angular momentum of the head. To determine the angular speed, we divide by the moment of inertia of the head. Assuming it to be approximately a 5 kg sphere of radius 10 cm rotating about a pivot 17 cm. from the centre of mass, the moment of inertia is .02 + 5x(.17)^2=.16 kg m^2.  So the angular speed would be .8/.16=5 radians/sec or about 280 degrees per second or .28 degrees per millisecond. In the space of 30 ms (the time between end of frame 312 and beginning of frame 313) the head would pivot 9 degrees forward.
9. There is a limit to how far forward the head can move because the chin cannot go past the chest.


Here are my estimates of the motion of the head and body due to the jet-effect:

1. The jet effect did not begin until the bullet exited the head. At that point there was no longer any forward momentum being transferred to the head from the bullet.
2. Frame z313 appears to show a streak from the motion of a piece of matter, possibly a skull fragment moving at an upward angle of about 65 degrees above car horizontal.  It appears to be about 1 metre long and it starts about a metre from the head. This gives us information not only about the speed of the ejected matter but also the timing of when it started.  Since the exposure is 25 ms duration, 1 m of travel in 25 ms is 40 m/s.  Since it had already travelled 1 m before z313 began to be exposed, this means that the head erupted about 25 ms. before z313 was exposed or about 5 ms. after the end of exposure of z312.  Since z312 does not provide any indication of the head being hit, this suggests that the bullet hit less than 5 ms after the exposure of z312 ended.
3. There appears to be a substantial amount of brain, skull bone and blood erupting from the head wound. Doctors reported that a substantial portion of the brain was missing. If the mass of matter exploding from the head was only 300 g, travelling initially at 40 m/s, the total momentum would be 12 kg m/s or almost 3 times the momentum of the incoming bullet. But it does not all explode in the same direction. It forms what appears to be a hemispheric cloud around the exit wound.  So the average would be about cos 45 x total momentum = 70% of 12 kg m/s = 8.4 kg m/s/ 
4. So the head and the body to which it is attached would, under those assumptions, recoil to the body left with 8.4 kg m/s of momentum. Unlike the forward momentum imparted by the bullet, however, there is nothing really to stop the upper body from moving to the left so it keeps going. 
5. If the torso received an impulse of 8.4 kg m/s applied to the right side of the upper skull, which would be a distance of approximately 90 cm above the seat or pivot point of the torso, the angular impulse, r x p would be .9 x 8.4 kg m^2/s = 7.5 kg m^2/s. If the centre of mass of the torso is midway, 45 cm above the seat, and the torso is approximately a solid cylinder of mass=40 kg and radius 20 cm (moment of inertia = MR^2/2= 40x(.2)^2/2=.8 kg m^2/s), the angular speed would be angular momentum/moment of inertia = L/I=7.5/.8=9.3 radians/sec or 530 degrees/second or .53 deg/ms.  In 55 ms or one frame, it would rotate about 25 degrees to the body left.

We are in agreement on a lot of issues.

The bullet struck at about the time the shutter closed on z312.

I believe the common belief among LNers, that the 'Jet Effect' caused JFK's head to move backwards is false. Because the head moved forward by two inches between frames 312 and 313.
The expected momentum the head should receive from a WCC/MC bullet fired from behind is 2 mph, or 2 inches per Zapruder frame.
The two inches of forward movement means:
a. The bullet struck around z312.5, just as the shutter closed. Because we need a full Zapruder frame, 55 ms, for the head to reach two inches forward by the time the shutter closes at z313.5.
b. The is no strong 'Jet Effect' pushing the head back, because the head would not have reached anything close to two inches ahead of the z312 position, with a full 45 ms of time for this alleged momentum change to affect the movement of the head.

The one disagreement is that you seem to claim the 'Jet Effect' definitely pushed JFK's head to the left.

I say, this may be true. Or might not be true. It may be the head was pushed strongly to the left. As you pointed out, a 'Jet effect' does not have to be related to the original direction of the bullet. It mainly has to do with the direction organic material erupted towards, which may be in almost any direction. If there was a strong 'Jet effect' push to the left, it is hard to see, but it would be hard to see, because Zapruder was filming from the side so we can easily see and measure movement that is forward, or backward, or upward, or downward, but not to the left or the right.

But is equally possible, in my view, that the 'Jet Effect', while it did push the head in some direction, might not have pushed the head very much. Unlike taped melons. Possibly because too much energy is used to break apart a human skull and the is not enough energy left over to propel organic material at a high enough velocity to affect the movement of the head very much.

Without a film view from a different direction, like from behind, like from David Powers position, we will never know if there was an immediate move of the head to the left, in the next frame after the bullet struck.

[Andrew Mason]

We are in agreement on a lot of issues.

The bullet struck at about the time the shutter closed on z312.

I believe the common belief among LNers, that the 'Jet Effect' caused JFK's head to move backwards is false. Because the head moved forward by two inches between frames 312 and 313.
The expected momentum the head should receive from a WCC/MC bullet fired from behind is 2 mph, or 2 inches per Zapruder frame.

The bullet, although moving right to left relative to the car by about 5 degrees, passed from the middle of the back of he skull to the right side of the head.  This means that JFK was turned to the left so that his head was 30 degrees to the car direction. A leftward impulse from the explosive ejection of matter from the head will move the head to the car left and car rear. That is exactly what we see.

Also, please note that I corrected my calculation of the movement of the body.  I did this late yesterday and as I was looking at it this morning I immediately saw that my figure of 25 degrees in one frame for the body made no sense.  I quickly realized that I used the moment of inertia of a cylinder (torso) about its long axis when I should have used the moment about an axis at its end.

The moment of inertia of a cylinder of length Len, mass M and radius R about an axis at one end is: MR^2/4 + M(Len)^2/3.    If the centre of mass of the torso is midway, 45 cm above the seat (Len=.9), and the torso is approximately a solid cylinder of mass=40 kg and radius 20 cm (moment of inertia, I = MR^2/4+M(Len)^2/3= 40x(.2)^2/4+40x(.9)^2/3=.4+10.8=11.2 kg m^2/s).  So the angular speed of the torso would be:  L/I=7.5/11.2=.7 radians/sec or 40 degrees/second In 55 ms or one frame, it would rotate a bit more than 2 degrees to the body left, which is about 1/4 of the rate of rotation of the head from the incoming bullet.  This is a left and rearward rotation of the body including head, so there will not be much change in the position of the head for several frames.

The two inches of forward movement means:
a. The bullet struck around z312.5, just as the shutter closed. Because we need a full Zapruder frame, 55 ms, for the head to reach two inches forward by the time the shutter closes at z313.5.
b. The is no strong 'Jet Effect' pushing the head back, because the head would not have reached anything close to two inches ahead of the z312 position, with a full 45 ms of time for this alleged momentum change to affect the movement of the head.

The one disagreement is that you seem to claim the 'Jet Effect' definitely pushed JFK's head to the left.

You cannot ignore the different directions of the forces.  We can see that a huge flap of skull on the right temple opens up but the forehead remains intact.

I say, this may be true. Or might not be true. It may be the head was pushed strongly to the left. As you pointed out, a 'Jet effect' does not have to be related to the original direction of the bullet. It mainly has to do with the direction organic material erupted towards, which may be in almost any direction. If there was a strong 'Jet effect' push to the left, it is hard to see, but it would be hard to see, because Zapruder was filming from the side so we can easily see and measure movement that is forward, or backward, or upward, or downward, but not to the left or the right.

But is equally possible, in my view, that the 'Jet Effect', while it did push the head in some direction, might not have pushed the head very much. Unlike taped melons. Possibly because too much energy is used to break apart a human skull and the is not enough energy left over to propel organic material at a high enough velocity to affect the movement of the head very much.

Energy is used when work is done. Work is Force x distance.  Breaking a skull requires a huge force but it is not applied over much distance - a few molecular diameters to break molecular bonds. The work required to break apart a skull is also made easier after the bullet has already penetrated the skull and caused fractures. Ejection of 400 grams of matter at 40 m/s only requires .4x(40^2)/2=320 Joules or only about 1/6th of the total bullet energy.

Without a film view from a different direction, like from behind, like from David Powers position, we will never know if there was an immediate move of the head to the left, in the next frame after the bullet struck.

Because the body-leftward rotation of the body is only 2 degrees/frame where as the car-forward rotation of the head would be 9 degrees/frame, one does not see an immediately reversal of the head movement despite the greater impulse from the head explosion. The different directions have a completely different effect upon the motion of the body. 

We know that jet effect occurs because of the explosive nature of the exit wound. The only disagreement should be over its magnitude and direction and its effect on the body, not whether it occurred.

[Joe Elliott]

. . .

We know that jet effect occurs because of the explosive nature of the exit wound. The only disagreement should be over its magnitude and direction and its effect on the body, not whether it occurred.

Agreed. The Jet Effect is real. The Jet Effect did occur. But I suspect it was of weak magnitude. It did not seem to effect the motion of JFK's head, either forward or backwards, a significant amount.

But it is possible the Jet Effect was stronger than I suspect. If the jet of material just happened to erupted to JFK's right, it would push the head to the left. This would be hard to measure in the Zapruder film, since that motion would be directly away from the camera.

When I state the 'Jet Effect' is a false explanation, the 'Neurological Spasm' is the true explanation of JFK's backwards motion, I am only referring to the effect on the backwards motion, which leads many people to think the shot had to have come from the front. The Jet Effect clearly had minimum effect on the backwards motion. The effect of the Jet Effect on motion to the left is unknown, since it cannot be easily measured. Although Dale Myers may have some information on that.

[John Iacoletti]

The “neuromuscular spasm” isn’t a great explanation either, but it’s all they have in order to preserve the narrative.

[Joe Elliott]

The “neuromuscular spasm” isn’t a great explanation either, but it’s all they have in order to preserve the narrative.

It is a very good explanation. Because it is the only explanation that explains JFK's head continuous acceleration backwards from z313 through z317. The only other explanation, which no one believes, is that there was a series of bullets during z313 through z317, each striking the head in at least 4 consecutive frames, striking at the same rate the Zapruder film was made, one bullet every 55 milliseconds. Plus another bullet fired from under the street, through the asphalt, through the bottom of the limousine and into JFK's right elbow.

The "neurological spasm" or the "neuromuscular spasm" is also a good theory because it has been demonstrated with animals. It has not been demonstrated with humans. It cannot be demonstrated with humans. Not because it has been tried and tried again and shown that it just doesn't happen. It's because this experiment is forbidden, even for condemned criminals because it is an afront to human dignity. Even film of murders, which I understand seem to show a neurological spam in humans, like the murder of a reporter in Nicaragua in 1979 cannot be shown as an afront to human dignity.

Any explanation that can be shown to occur in animals, and is the only plausible explanation, is a good explanation, except to the hopelessly biased.