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  • September 23, 2017, 01:59:32 PM
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Author Topic: Do bullets knock you down?  (Read 9777 times)

redcap

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Re: Do bullets knock you down?
« Reply #30 on: October 24, 2007, 05:11:44 PM »

The haunting bit for me was the instant he realised things were going horribly wrong. Hands up those who've been there, felt that?  Wow....tha's a lot of hands.  Redcap.
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“No man knows the hour of his ending, nor can he choose the place or the manner of his going. To each it is given to die proudly, to die well, and this is, indeed, the final measure of the man.” Louis L’Amour

seanross

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Re: Do bullets knock you down?
« Reply #31 on: November 29, 2007, 04:21:18 PM »

A bullet alone cannot knock you down........Newtones law applies here. For every action there is an equal and opposite reaction. For a bullet to have the momentum to knock down a 200 pound man it would kick so hard at firing that it would knock down a 200 pound shooter.

Pain, shock, central nervous system hits, broken bones, etc. cause you to fall.

You are correct, Newton's laws of motion apply here, but your application of Newton's 2nd law isn't quite correct.  Newton's 2nd law applies to forces, but force isn't what knocks a person down unless the force is crushing.  Momentum transfer is what knocks something down.  The correct way to analyze this is using the time integral of Newton's 3rd law F=mA, namely I=delta P, the "impulse momentum" theorem.  The Impulse is the integral of the force over time, F *delta T.  That impulse is equal to the change in momentum imparted to the body.

Shooter:  Bullet leaves very quickly, 900+fps.  Only takes 280 microseconds at that speed to traverse a 3" barrel.  Big force * short time = small momentum transfer.

Shootee:  Bullet enters at 900+ fps but slows so the bullet traverses the human body in a much longer time.  If it takes 12" of human flesh to stop a bullet, then we can assume an average speed of 450 fps over 12".  This takes over 2 milliseconds to traverse the body or about 8 times the time it took the bullet to leave the gun.  Big Force * longer time = greater momentum transfer.  As a thought experiment, you could imagine shooting someone with a sharpened, steel tipped bullet.  It would poke a hole in them and keep on going.  Wouldn't impart very much momentum at all so if they fell down it would be due to a pain reaction or damage, not due to the momentum of the bullet.

Another factor is that the shooter gets to absorb all the recoil on an area of a couple square inches of his palm while the shootee absorbs all the force over only the cross sectional area of the bullet, maybe 1/4 square inch.  Force per area much lower for the guy holding the gun.  This is the same idea with knives:  that sharp edge concentrates all the force on a very tiny thin edge and can thus concentrate the force and so penetrate flesh.  A blunt object spreads out the force.

So the shootee will drop for one of two reasons.  1) Shootee just happened to be off balance and was hit by a force approximately equal to being hit with a baseball bat (shooter recoil * 8) and so they tip over.  2) Bullet damaged something or caused pain which caused shootee to drop.
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rutleddc

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Re: Do bullets knock you down?
« Reply #32 on: November 29, 2007, 08:48:06 PM »

I agree that conservation of momentum applies but not conservation of energy, as the bullet does not have a fully elastic impact with the shootee but rather a fully plastic impact. As the bullet decelerates in the body at a slower rate (longer time of deceleration) the deceleration force is less. The cross sectional area of the bullet does not effect this calculation but it does affect the pressure, i.e. the smaller the bullet the higher the pressure and hence penetration.

The knockdown effect is primarily due to the "pushing" effect of the projectile, which is the momentum (mass x velocity) = Average force x Elapsed time of deceleration. This is interesting as most data bases (even Jane's Weapon Systems!!!) give the mass, velocity, and kinetic energy of projectiles but not momentum.

The bottom line is that the push down power of the projectile on the shootee cannot exceed the pushdown effect on the shooter. As an staff engineer (Dynamics) at a major defense contractor I have more than once had to convince management that the latest super technology is too good to be true, it promises things that violate the laws of Newtonian physics. The last one was purchased earlier this year by the Polish army (insert joke here :D) But there is always someone else hawking snakeoil in the armaments industry.
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seanross

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Re: Do bullets knock you down?
« Reply #33 on: November 29, 2007, 09:55:54 PM »

I agree that conservation of momentum applies but not conservation of energy, as the bullet does not have a fully elastic impact with the shootee but rather a fully plastic impact. ]

SR>>The standard term is inelastic collistion, not plastic.  If one object embeds itself in another, it is called an inelastic collision.  Energy is conserved in either elastic or inelastic collisions.

As the bullet decelerates in the body at a slower rate (longer time of deceleration) the deceleration force is less. The cross sectional area of the bullet does not effect this calculation but it does affect the pressure, i.e. the smaller the bullet the higher the pressure and hence penetration.  The knockdown effect is primarily due to the "pushing" effect of the projectile, which is the momentum (mass x velocity) = Average force x Elapsed time of deceleration.

SR>>Yes.  This is the impulse-momentum theorem,  the integral form of Newton's third law.

The bottom line is that the push down power of the projectile on the shootee cannot exceed the pushdown effect on the shooter.

SR>>All depends what you mean by "pushdown" effect.  If a shooter at rest on roller skates shoots a completely absorbing target at rest on roller skates, they will both have the equal and opposite momenta after the shot.  In that sense the "pushdown" effect is the same.  However, the shooter gets the luxury of bracing for the shot and takes all the force on a large area of his hand holding a gun made to spread out the force.  The target takes it wherever it lands and all the force is concentrated on a small area so that the bullet penetrates and causes damage.  In this sense, the "pushdown" effect is not equal.
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fire

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Re: Do bullets knock you down?
« Reply #34 on: January 16, 2008, 01:12:20 AM »

  first off, I have never shot anyone, I have never been shot-

  In the interest of continuing to be able to make the latter statement, I have watched a lot of youtube video of police and other shootings, in the hope of seeing what it looks like in the real world, and thus what I am training for- from what I have viewed, a whole lot of the time you cannot even tell if someone has been shot, and they certainly do not fly back like in the movies, often the only sign that they are hit is that they just drop, or sort of slump- I also remember a few years ago when CNN covered a lawyer who was shot outside a courthouse, it was caught on film by a crew there film something else- said lawyers angry client walked up to him and emptied a revolver into him, as he tried to dodge from one side to another of a  sapling (it would have been comical in other circumstances) - he took 5 shots and walked about 30 feet, gesturing to the camera crew, before he just fell over onto the ground. so from what I have seen I would not count on someone dropping from a handgun round, or at least count on them having a few seconds to try to end my life, unless the shot shatters a support limb, or it destroys the brain...... so I suppose that I would agree with the statement that one never knows what a shot will do to a given person....
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Hock

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Re: Do bullets knock you down?
« Reply #35 on: January 16, 2008, 07:04:05 AM »

Note that on the other new thread, the SAS double-tap, the SAS sais getting hit twice was like being hit with TWO sledge-hammers. There would be a score of gun people quick to leap up and explain the science and complain that -

"if the bullet landed like a sledge hammer, the shooter would also feel a sledge hammer in their hand." 

Hock

Hock

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Re: Do bullets knock you down?
« Reply #36 on: April 06, 2017, 06:47:15 AM »

I. John Farnam on “Deanimation”: A dicey challenge where anything can happen
Officers have long been advised to “expect the unexpected” in armed confrontations. That admonition certainly holds true when it comes to “deanimation,” a threatening subject’s cessation of movement after he or she has been shot.
The venerable firearms trainer John Farnam addressed the issue of “rapid and permanent deanimation” in a recent issue of his popular newsletter Dtiquips.
Even with shots to the heart, Farnam wrote, “most cardiologists agree with the ‘five-second rule.’ When blood pressure drops [suddenly] to near zero, most people will still remain animated for at least five more seconds before becoming comatose. And ‘five seconds’ is the minimum. Some cardiologists insist the real figure is closer to 10 seconds or more”—an eternity in a gunfight.
Individual physiological and psychological factors enter the equation, Farnam noted. Some people fall down when shot (even in non-vital places) for no reason other than that they want to. They literally ‘act out’ what they think they’re supposed to do, absent any external physical compulsion.”
Then, too, there’s “the nebulous issue of ‘neural-shock paralysis.’ Sometimes it’s there,” Farnam said, “and sometimes it’s not, all for reasons no one really understands. [It] cannot be predicted nor produced on demand.”
Readers responded with dramatic anecdotes attesting to the erratic nature of “shootee reactions.”
A private investigator and firearms consultant recalled a case he’d handled that involved a 6-ft. 4-in., 220-pounder who, thanks to his constant workouts in state prison, “was built like an action-figure doll.” He was shot in the upper shoulder with one .25-cal. semi-auto round. “Witnesses reported that upon the single shot being fired this giant fell as if he had been pole-axed!”
In contrast came this, from an attorney and frequent expert witness in shooting cases:
In one of his cases, police fired over 60 rounds at a PCP suspect, “and the autopsy detailed 45 separate wound paths through [his] body. The suspect, with pistol in hand, took 11 steps toward police, while being simultaneously struck by a hail of police handgun bullets, until a shotgun slug that struck his spine between T6/T7 dropped him to the pavement.
“Even then, his upper body remained functional, as he tried to point his handgun at police with his right hand, while he held a cigarette [in his left]. It took a 40 S&W round to the brain stem to finally stop this threat.
“In another recent case, the [suspect], shot through his heart with a 9mm and also hit in the thigh and arm, subsequently walked down a hall, down a flight of stairs, across the stair landing, and halfway down another flight of stairs before he collapsed, and thereafter died.
“The medical examiner and I, without speaking with one another, both noted in our reports that a man shot through the heart can subsequently remain upright, mobile, and aggressive for 30 seconds or more!
“While stopping effects [of ammunition] seem to be better now than a few decades ago, there is still no certainty, and two suspects of the same size and physical condition, hit in the same part of the body with the same rounds, may well behave dramatically differently.
“We must train to keep firing accurately, creating distance, using cover and obstacles, reloading, and getting out of the kill zone when possible, until the threat is stopped.”
Farnam added: “We must be mentally prepared to confront nearly any eventuality, from the felon turning and running away, to the felon falling down immediately (albeit sometimes reanimating seconds later), to the felon continuing his attack while displaying scant discomfort.”
And “we need to be cautious about believing glowing reports about ‘wonder bullets.’ ”
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