Debunking the "Jet Effect"?

[Dan O'meara]

The dangers of sucking up to someone just because they've got a shiny prize:

Chapter 9 covers the involvement of Nobel prize winner Dr. Luis Alvarez, who also assumed he had a PhD in assassination “science.” Alvarez, a blatant Warren Commission apologist, is known for shooting melons, thus trying to create a reverse jet effect to explain the rearward component of JFK’s double head motion. Alvarez is one of many scientists, like Vincent Guinn, in the governmental and academic circles to have used their prestige when approaching the assassination from their individual field of expertise. Thompson recounts a long period of contentious personal communication between he and Alvarez, mainly over Alvarez’s “jiggle analysis” of the Zapruder film and “reproducing” the reverse jet effect. Critics had immediately pounced on Alvarez’s claim that a single frame horizontal blur seen at 313 reflected Zapruder’s reaction to a rifle shot, as a muzzle blast from the TSBD would not have even reached his ears yet. Ironically later in Chapter 14, a same horizontal blur will be viewed as a reaction to a shot from the Grassy Knoll, with a similar lack of success based upon similar principles. Alvarez’s attempts at shooting various objects, plus his publications, are revisited. During the writing of the book, Paul Hoch provided the author with photos and notes from the actual melon shooting sessions, which almost invariably showed objects moving forward in the direction of the bullet as had the Warren Commission tests. Thompson details the intellectual dishonesty and despicable behavior exhibited by this Nobel prize winner. I do not think the author adequately describes the enjoyment he found after obtaining Alvarez’s materials, provided by Hoch, which are now conserved at the Sixth Floor Museum.

Proof that Alvarez faked his results?
D'oh!

[Andrew Mason]

I've read somewhere that in his calculations Alvarez never included the dominant acting force - that of the bullet as it flattened and fragmented against the skull driving JFK's head forward. He never accounted for a piece of the fragment travelling along the skull in a tangential manner imparting even more forward momentum to the head and he never accounted for the second interaction with the skull as the bullet fragments exited, creating even more forward momentum. He completely forgot to include the dominant acting force, the silly Nobel prize winner doing a study funded by the government.

Alvarez certainly included the forward momentum of the bullet in his analysis.  On page 819-820 of his article in Am. J. Phys Vol. 44, No. 9 Sept. 1976 he states:

• When I studied the graph
  showing the changing position of the President's head relative
  to the moving car's coordinate system, I was finally
  convinced that the assassination buffs were right; there had
  to be a real explanation of the fact that the President's head
  did not fall back, but was driven back by some real
  force.
• I concluded that the retrograde motion of the President's
  head, in response to the rifle bullet shot, is consistent with
  the law of conservation of momentum, if one pays attention
  to the law of conservation of energy as well, and includes
  the momentum of all the material in the problem. The
  simplest way to see where I differ from most of the critics
  is to note that they treat the problem as though it involved
  only two interacting masses: the bullet and the head. My
  analysis involves three interacting masses, the bullet, the
  jet of brain matter observable in frame 313, and the remaining
  part of the head. It will turn out that the jet can
  carry forward more momentum than was brought in by the
  bullet, and the head recoils backward, as a rocket recoils
  when its jet fuel is ejected.
• ... As we shall now see, the momentum carried
  forward in this way can be much larger than the momentum
  brought in by the bullet.

This is what causes JFK's head to shoot forward, as you've so ably demonstrated:



But note, in none of the images you've posted showing the Jet Effect in action do we see this initial rapid forward movement.
So, they don't really show us what you would like to believe they do.
They're not really equivalent to what we see in the Z-film:





It's all a bit of smoke and mirrors really.
But if you want to carry on believing that JFK's head acted like a balloon released then you crack on.

The forward motion results from the time delay between the bullet impact in the back of the head and the outward ejection of matter from the head.

The rear-and-to-the-left reaction is due to the explosive ejection of blood and brain tissue from the head.  But that ejection occurs as the skull opens up which is a finite amount of time after the bullet strikes the back of the skull.  So the head goes forward from the bullet momentum imparted to the head before the head opens up and spews matter from the head in a forward-right cone which then imparts rear-left momentum to the head. This explosive ejection then slows, stops and then reverses the forward movement of the head. 

The Bell & Howell camera ran at 18.3 frames/sec (55 ms/frame) with an exposure time of 1/40th of a second (25 ms).  So the time between the bullet impact and the beginning of exposure of z313 could be as much as 30 ms. 

If the moment impact occurred immediately after the end of the exposure of z312 there would be 30 ms for the head to move forward before the beginning of exposure of z313.  For the centre of mass of a 5 kg head to move 3 cm in 30 ms. (average speed .03/.03=1 m/s) the head acquired an average momentum of 5kg x 1 m/s =5 kg m/s. We know the momentum of the incoming bullet: p_b=m_bv_b=.01kg x 610 m/sec. = 6.1 kg m/sec or 6.1 N.sec.  So if the bullet imparted most of its forward momentum to the head which then erupted sending a jet of matter forward and to the right, that would explain why the head moved forward first.

In the examples of firing at a light plastic membrane milk jug or thin membrane balloon, there is almost no delay for the exit wound to open up to expel matter from the container. So the rearward impulse begins before the target has any time to move forward.  It is different with the container is strong, rigid bone that has mass. It takes time for that matter to move away to allow the pressurized brain matter to explode out.

[John Mytton]

Alvarez certainly included the forward momentum of the bullet in his analysis.  On page 819-820 of his article in Am. J. Phys Vol. 44, No. 9 Sept. 1976 he states:

• When I studied the graph
  showing the changing position of the President's head relative
  to the moving car's coordinate system, I was finally
  convinced that the assassination buffs were right; there had
  to be a real explanation of the fact that the President's head
  did not fall back, but was driven back by some real
  force.
• I concluded that the retrograde motion of the President's
  head, in response to the rifle bullet shot, is consistent with
  the law of conservation of momentum, if one pays attention
  to the law of conservation of energy as well, and includes
  the momentum of all the material in the problem. The
  simplest way to see where I differ from most of the critics
  is to note that they treat the problem as though it involved
  only two interacting masses: the bullet and the head. My
  analysis involves three interacting masses, the bullet, the
  jet of brain matter observable in frame 313, and the remaining
  part of the head. It will turn out that the jet can
  carry forward more momentum than was brought in by the
  bullet, and the head recoils backward, as a rocket recoils
  when its jet fuel is ejected.
• ... As we shall now see, the momentum carried
  forward in this way can be much larger than the momentum
  brought in by the bullet.

The forward motion results from the time delay between the bullet impact in the back of the head and the outward ejection of matter from the head.

The rear-and-to-the-left reaction is due to the explosive ejection of blood and brain tissue from the head.  But that ejection occurs as the skull opens up which is a finite amount of time after the bullet strikes the back of the skull.  So the head goes forward from the bullet momentum imparted to the head before the head opens up and spews matter from the head in a forward-right cone which then imparts rear-left momentum to the head. This explosive ejection then slows, stops and then reverses the forward movement of the head. 

The Bell & Howell camera ran at 18.3 frames/sec (55 ms/frame) with an exposure time of 1/40th of a second (25 ms).  So the time between the bullet impact and the beginning of exposure of z313 could be as much as 30 ms. 

If the moment impact occurred immediately after the end of the exposure of z312 there would be 30 ms for the head to move forward before the beginning of exposure of z313.  For the centre of mass of a 5 kg head to move 3 cm in 30 ms. (average speed .03/.03=1 m/s) the head acquired an average momentum of 5kg x 1 m/s =5 kg m/s. We know the momentum of the incoming bullet: p_b=m_bv_b=.01kg x 610 m/sec. = 6.1 kg m/sec or 6.1 N.sec.  So if the bullet imparted most of its forward momentum to the head which then erupted sending a jet of matter forward and to the right, that would explain why the head moved forward first.

In the examples of firing at a light plastic membrane milk jug or thin membrane balloon, there is almost no delay for the exit wound to open up to expel matter from the container. So the rearward impulse begins before the target has any time to move forward.  It is different with the container is strong, rigid bone that has mass. It takes time for that matter to move away to allow the pressurized brain matter to explode out.

Thank you Andrew, having someone who actually understand physics explain some of these concepts is refreshing, because arguing with Dan like trying to argue with Moon Landing Hoaxers or the Flat Earthers, even though they mean well, their knowledge is minimal and their evaluation of the visual record is frankly embarrassing.

JohnM

[Dan O'meara]

Thank you Andrew, having someone who actually understand physics explain some of these concepts is refreshing, because arguing with Dan like trying to argue with Moon Landing Hoaxers or the Flat Earthers, even though they mean well, their knowledge is minimal and their evaluation of the visual record is frankly embarrassing.

JohnM

"...their evaluation of the visual record is frankly embarrassing."

Let's see whose evaluation of the visual record is embarrassing.
It was you who posted this:

This high contrast image of Zapruder frame 313 shows a clear expulsion of matter out the front.



A child can see that you're wrong.
Your evaluation of this image is genuinely embarrassing.
I don't have the slightest doubt that you will avoid this observation like the plague as it demonstrates your true grasp of the issues.
You can pretend to "understand the physics" all you want but your inability to understand a basic image tells us all we need to know.
Alvarez also believed this matter was being expelled out the front. It makes me cringe that you have simply swallowed this down (as, it seems, has Andrew).
If you can't even understand the basics how trustworthy do you think your assessment of the physics is.

Just for laughs - Andrew, do you think the image above shows matter being expelled out of the front of JFK's head?
Let the  begin.

[Andrew Mason]

Just for laughs - Andrew, do you think the image above shows matter being expelled out of the front of JFK's head?.

From the front half of his head?  Yes.
From the front surface of the head?  No

What matters is the direction of the matter exploding from the head exit wound at the moment it separates from the body. That matter spewed over a wide area that included the inside the car, on either side of the car, on the windshield and over onto the hood of the limo. 

The resulting kick from that explosive ejection of that matter will be equal in magnitude and opposite in direction to the vector sum of the momenta of all ejected particles at the moment they separated from the body.

[Dan O'meara]

From the front half of his head?  Yes.
From the front surface of the head?  No

What matters is the direction of the matter exploding from the head exit wound at the moment it separates from the body. That matter spewed over a wide area that included the inside the car, on either side of the car, on the windshield and over onto the hood of the limo. 

The resulting kick from that explosive ejection of that matter will be equal in magnitude and opposite in direction to the vector sum of the momenta of all ejected particles at the moment they separated from the body.

Form the front surface of the head?

??
??
??
I'm not even going to ask what this means.
So, just to be clear, you're saying the expulsion of matter highlighted below is coming from the front half of JFK's head.
It's a yes/no thing.

[Andrew Mason]

Form the front surface of the head?

??
??
??
I'm not even going to ask what this means.
So, just to be clear, you're saying the expulsion of matter highlighted below is coming from the front half of JFK's head.
It's a yes/no thing.

That is what I said.  But I also said that what matters is the direction of the ejected matter.   The point on the head from which it ejected does not affect the direction of the recoil.