Would A Bullet Really Knock You Backwards? DEBUNKED

[Andrew Mason]

The matter ejected from the head experienced a change in its motion due to a force supplied by other matter in the head from which it was ejected.  Therefore, that other matter-the remaining head - experienced an equal and opposite change in momentum.

What was the "force supplied by other matter in the head"?
What force did the remaining head use to eject matter?
Where did this force come from?
Because it's not the bullet.
You are saying the remaining head provided a force that ejected the jet of material.
How was this force provided?

The force was provided by pressure that was built up inside the head by the bullet and suddenly released. 

We know that there was pressure built up because the skull opened up and matter exploded out of the head in wide directions.  We can see that in the zfilm.

As you know, the bullet passing through tissue creates a temporary cavity around it.  But in a head, there is no ability to expand the volume.  The brain, like water, is almost incompressible so a very small compression of volume increases the pressure tremendously.  So the energy deposited in the head builds up pressure.  When that pressure is released by the rupturing of the skull (likely initiated by the bullet exiting through the front of the head) matter is propelled out of the opening from the tremendous force (pressure x area).

If the pressure increase was just one atmosphere (1 bar or just over 100 kPa or 100,000 N/m^2) and the area of the skull that opened up was (estimating the size of the opening) was, say, 10 cm x 10 cm (4"x4") or .01 m^2, the total force on the matter expelled (force=Pressure x area) would have been 1000 N. if the matter expelled was 400 grams or .4 kg, then the acceleration would have been:


a=F/m=1000/.4= 2500 m/s².


So in 10 ms. or 1/100th of a second, the speed would have been v=at=2500/100=25m/s  That represents a momentum change of .4 x25 = 10 kg m/s and an equal and opposite momentum change to the head.  Since the total incoming bullet momentum was .01g x 610 m/s =6.1 k m/s, the momentum of the recoiling head exceeds the maximum forward momentum imparted by the bullet. So head goes backward.

[John Mytton]

I think it was a combination of elements that created the back and to the left motion.

Kennedy's back brace stopped him from falling forward.




The jet effect drove him backwards.



And I also agree with Joe's thread opener that a penetrating bullet simply lacks the kinetic energy to throw anyone anywhere.

Mythbusters fired a 50 cal bullet into a steel plated dummy with no penetration and it barely moved.
Also in the following video, a firing squad shoots multiple men and they just drop down and virtually all of them fall back towards the shooters.

@ 0.37

JohnM

[Andrew Mason]

I think it was a combination of elements that created the back and to the left motion.

Kennedy's back brace stopped him from falling forward.



The jet effect drove him backwards.

Great clips of jet-effect.  I will add Chad Zimmerman's turkey shoot:

And I also agree with Joe's thread opener that a penetrating bullet simply lacks the kinetic energy to throw anyone anywhere.

Again it is not about insufficient kinetic energy.  The bullet has more than enough kinetic energy to throw a body.  A .01 kg bullet at 610 m/s has kinetic energy of .005 x (610)^2 Joules = 1860 Joules.  A 100 kg body recoiling at 1 m/s has only 50 Joules of kinetic energy (mv^2/2) or only about 2.5% of the bullet energy..   What is lacking is the mechanism to convert bullet energy into motion of the body.  Jet effect is a rather crude energy converter and uses a small fraction of the bullet kinetic energy.  But the jet effect can impart significant momentum as your clips show - more than the incoming bullet if enough matter explodes from the body.

Suppose a 1800 Joule bullet could be "caught" by a mechanism that compresses a spring in a 100 kg body as the bullet slows down so that the kinetic energy of is all transferred to the spring. The release of that spring against a fixed wall, say, would cause the 100 kg body to be thrown at a speed of:  v= √{2E/m}= √36=6 m/s

[John Mytton]

Great clips of jet-effect.  I will add Chad Zimmerman's turkey shoot:

Again it is not about insufficient kinetic energy.  The bullet has more than enough kinetic energy to throw a body.  A .01 kg bullet at 610 m/s has kinetic energy of .005 x (610)^2 Joules = 1860 Joules.  A 100 kg body recoiling at 1 m/s has only 50 Joules of kinetic energy (mv^2/2) or only about 2.5% of the bullet energy..   What is lacking is the mechanism to convert bullet energy into motion of the body.  Jet effect is a rather crude energy converter and uses a small fraction of the bullet kinetic energy.  But the jet effect can impart significant momentum as your clips show - more than the incoming bullet if enough matter explodes from the body.

Suppose a 1800 Joule bullet could be "caught" by a mechanism that compresses a spring in a 100 kg body as the bullet slows down so that the kinetic energy of is all transferred to the spring. The release of that spring against a fixed wall, say, would cause the 100 kg body to be thrown at a speed of:  v= √{2E/m}= √36=6 m/s

The bullet has more than enough kinetic energy to throw a body.

Of course you are right, I should have said that the amount of movement would be limited to an inch or two, just like we see in the Zapruder film and the Mythbusters clip.

JohnM

[Joe Elliott]

The force was provided by pressure that was built up inside the head by the bullet and suddenly released. 

We know that there was pressure built up because the skull opened up and matter exploded out of the head in wide directions.  We can see that in the zfilm.

As you know, the bullet passing through tissue creates a temporary cavity around it.  But in a head, there is no ability to expand the volume.  The brain, like water, is almost incompressible so a very small compression of volume increases the pressure tremendously.  So the energy deposited in the head builds up pressure.  When that pressure is released by the rupturing of the skull (likely initiated by the bullet exiting through the front of the head) matter is propelled out of the opening from the tremendous force (pressure x area).

If the pressure increase was just one atmosphere (1 bar or just over 100 kPa or 100,000 N/m^2) and the area of the skull that opened up was (estimating the size of the opening) was, say, 10 cm x 10 cm (4"x4") or .01 m^2, the total force on the matter expelled (force=Pressure x area) would have been 1000 N. if the matter expelled was 400 grams or .4 kg, then the acceleration would have been:


a=F/m=1000/.4= 2500 m/s².


So in 10 ms. or 1/100th of a second, the speed would have been v=at=2500/100=25m/s  That represents a momentum change of .4 x25 = 10 kg m/s and an equal and opposite momentum change to the head.  Since the total incoming bullet momentum was .01g x 610 m/s =6.1 k m/s, the momentum of the recoiling head exceeds the maximum forward momentum imparted by the bullet. So head goes backward.

I'm not certain I agree with these calculations. But putting this aside, you say that after 10 ms, the head started to move backwards. If so, how is it, that according to the study by Physics Graduate Student William Hoffman, in his work for Josiah Thompson for the book "Six Seconds in Dallas" measured JFK's head at frame 313, two inches, or 5 cm, forward of the 312 position, by frame 313?

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

Provide me with a possible scenario. Don't use the excuse "Well, we can't tell exactly what happened". Just give us a possible scenario. That would be good enough.

We can use the following time system. z312.0 is when the shutter opens on frame 312. z312.5 is when the shutter closed on frame 312.

So, I need to see something like the following:

z312.5 - the bullet strikes.
during z312.5-z312.7, the head moves forward 2.5 inches, or 6.25 cm, at a rate of 12.5 mph.
z312.7 - the head explodes and the head starts moving backwards, at a rate of 0.6 inches per frame, or about 0.6 mph.
z313.5 - the head has moved back to a position where the head is 2.0 inches ahead of the position in frame 312.

This scenario is clearly impossible, for the bullet to push the head forward at 12.5 mph.

But what possible scenario, not a scenario you can 'prove', but a possible scenario, that would allow the head to start moving backwards after 10 ms, and still have the head 2 inches ahead of it's frame 312 position by the time the shutter closed at frame z313.

So, to summarize:


Questions:

Is there a possible scenario that fits the William Hoffman data?

Is this scenario in accordance with Classical Physics?

Is so, what is that scenario?

[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?