Knockdown Power

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svtruth

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From time to time someone posts about the unreality of peoples bodies being thrown backwards by a bullet strike.
Well, I dusted off the old high school physics. Momentum equals mass times velocity, P=mv.
Thus: mass times velocity of the bullet equals some value of momentum which (momentum being conserved) must equal the mass times velocity of the poor unfortunate SOB who gets hit with the round.
So, a 230 grain .45 apc @ 1000fps has 230,000 grain feet per second.
If it hits a 200lb person (and we assume complete "momentum dump") the equivalent momentum is 200 times 7000 (grains per pound) or 1,400,000 grains times about 0.16 fps, call it 2" per second.
Critiques welcome.
 
You forgot to factor in the 21,000,000 grain station wagon that's behind the scenes at the other end of a rope.:D
 
How DO they get that fantastic knock down effect in the movies? I doubt it's really done with a station wagon and a rope.
 
It's very simple actually, if the shooter wasn't knocked down by firing the gun, then the shootee won't be knocked down by the bullet, unless he's off balance to begin with. Falling down from wounds is another matter.
 
I saw a vid of a fellow who had to defend himself against a charging elephant. He was using s double rifle with a really big bore.... don't know the cartridge, but his 1st shot from very close (too close) range smacked the bull just off-center on his forehead. It was violent, jerking the animal's head. The bull went right down and he finished him off with the 2nd round. I suspect he then went into the bush to finish his Charmin moment.
 
the reality to REALLY stop someone like as INSTANT drop would be a round hitting the spinal column, Medula oblongata, or Cerebrum. I have the old W.A.W box set and theres alot of graphic footage. i will say that it is possible for people to turn to silly putty after getting shot
 
How DO they get that fantastic knock down effect in the movies? I doubt it's really done with a station wagon and a rope.
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Yes, it has been done with a station wagon and a rope. I'm pretty sure I saw that on a TV show about special effects simulating a shotgun hit. The Jerk Ram looks like more fun, but in bit I saw, they were only jerking the guy back a few feet, not making him fly through the air. It was shot from the front so you couldn't even see the rope.
 
Mythbusters did an episode on this and in fact even a 12 guage slug wont throw a person backwards, They may fall backwards is all.
This is all from memory on the ammo info, But im positive being shot wont throw you backwards. Unless its a rocket propelled grenade, Then its more about what parts of you fly backwards.
 
DasFriek said:
Mythbusters did an episode on this and in fact even a 12 guage slug wont throw a person backwards, They may fall backwards is all.
This is all from memory on the ammo info, But im positive being shot wont throw you backwards. Unless its a rocket propelled grenade, Then its more about what parts of you fly backwards.
Click to expand...
Beat me to it.


The Mythbusters did some good testing with a wide range of firearms (handguns, rifles, shotgun) and basically proved that you the knockdown/knockback power is a myth.


However, they did not (and really couldn't) test how the human body reacts to getting shot.
Hit the right place and the person could react in a way that would look like a knockback / knockdown. But the classic movie, fly-back-five-feet-through-a-window scene isn't going to happen.
 
Nothing will absolve a person of the belief in "knockdown power" faster than actually taking a firearm out and using it to make something alive not be (HUNTING)

With high vel rifle rounds I've turned deer OFF from pure hydrostatic shock. But they always fell straight down after the initial reflexive jerk. Lower velocity hits almost always result in some tracking even with vital organs shot away. Handguns are exceedingly worthless at anchoring game immediately. I once burnt up an entire box of Cor-Bon 380 eliminating nuisance armadillos one afternoon. Not once did a hit prevent one of these self propelled rocks from getting into it's burrow
 
Any bullet with enough force to theoretically "knock you down" will mostly likely penetrate you all together and or shatter your body before it will throw you anywhere anyway.

So I've always thought: who cares?
 
OP's calculation is correct. But suppose we assume that the same bullet is brought to rest in 10 inches of penetration. The energy of the bullet (0.5mv^2) is 511 ft lbf. If the resistance of flesh is constant during the deceleration of the bullet, the force will be the energy divided by the penetration, which equals 613 lbf. The deceleration time will be the momentum divided by the force, which equals 1.667 milliseconds. So the force would actually be rather large but it is applied to the target for only a very short time.
 
Taylor Knockout Value is the best way to calculate knockdown power

Invented by John Taylor, an african hunter

He realized the following things are needed for proper knockdown power of large dangerous game

1. Bullet size
2. Bullet diameter
3. Speed

TKO = mass of bullet x velocity of bullet x diameter of bullet / 7000
 
I believe one thing equal force on your shoulder is not the same as equal force of the impact of the bullet. If you stopped up your gun to the point it didn't actually blow up but at that last moment released all that force at the bullet exited, it would likely push you backwards from all the pressure build up that's way beyond normal. That's where mythbuster's is wrong. Does a bullet actually knock something off it's feet NO. But I've seen plenty of animals pushed right over onto the ground from solid hits on bone where all of the bullets energy is displaced right there.

32 ft lbs from a 300 win mag on your shoulder is alot differnt than 3500 ft lbs of bullet energy displacement on the same shoulder. You can call it reaction if you want to but your being shoved backwards as that bullet smashes into your shoulder bone.

Myth busters shot into a rubber body. It's not the same if the bullet strikes a bone or unmoveable object. But I agree it won't knock you backwards through a window. It's going to drop you right in your tracks.

But like I mentioned earlier I'd seen animals knocked over but not knocked off their feet.

I think Knockdown power is used in terms of an average amount of power needed to take down a certain size of animal with the best effective ft lbs of energy from a bullet. Hollywood always over does animations.

12ga 3" buckshot distance of 10' on a human chest your going over on your back. Just too much force to stop it from happening. Call it reaction if you want.
 
black_elk said:
Myth busters shot into a rubber body. It's not the same if the bullet strikes a bone or unmoveable object. But I agree it won't knock you backwards through a window. It's going to drop you right in your tracks.
Click to expand...

MythBusters often shoots into ballistic gel dummies. Sometimes with real human bone, sometimes not.


However, for the knockdown myth, they were shooting into pigs.
 
Chemist said:
It's always 'nice' how in the books you can assume things like perfect transfer, no frictional forces, etc.
Click to expand...

It usually doesn't matter in casual settings because you could always give the benefit of the doubt to something that you're trying to disprove, and if it's still disproved, then all the better for it (cuts to the chase in proving a point with less quibbling over details).

Chemist said:
I dislike physics because I had to teach it...
Click to expand...

And I dislike students because they made you dislike physics. ;)
 
Manco, the students were great but the situation wasn't. My degree is in Chem, so I actually like Schrodinger's equation but I got burned out on physics 101 so to say.

I'd rather be doing P. Chem or identifying compounds from their NMR signatures...so thankfully my husband made me a stay at home mom.
 
I think it is largely psychological. I once saw a man literally knocked off his feet by a blank gun. It was a foolish prank but it was set up well enough that the victim actually believed that he had been shot and appeared to be flung violently backwards. A little bit of the wadding hit him in the chest and even though he was not hurt, he believed so strongly that it was a real bullet that he behaved accordingly. People have been conditioned by Hollywood to believe that bullets knock people down. When people get shot they often react the way that they have seen hundreds or thousands of others do on the screen.

There is a curious psychological reverse to this. The British military had a problem with this phenomena during colonial times when the army had to engage primitive tribesmen who had never seen a gun before. Often these people did not react at all to being shot because they did not realize the connection between a sudden sharp pain somewhere in their body and that curious loud bang way over there.

Ever notice that when you hear stories of people accidently shooting themselves that very often they say something like “At first I did not realize that I was hit, then I started to notice a burning pain in my leg” This is because they were so startled by the loud bang that they did not even feel it, this is called sensory override. Then when they realize what happened and start to get over being startled, only then do they become aware of the pain. That is exactly what the British were up against when facing people who did not know that they were supposed to fall down when they were shot.
 
It's very simple actually, if the shooter wasn't knocked down by firing the gun, then the shootee won't be knocked down by the bullet, unless he's off balance to begin with.
Click to expand...

That's the one thing I never understood. Sure, there's the equal and opposite reaction but the bullet starts from rest and accelerates down the barrel, although it happens very fast.
The M33 .50BMG pushes a 668gr fmj down the tube to 2910fps hitting 12,550ft.lbs energy at the muzzle, but the recoil energy from it is somewhere in the 70ft.lb. neighborhood. Is the rest soaked up by the rifle? How does the muzzle energy (or downrange energy) relate to recoil energy because if a .50bmg hit your shoulder with over 12,000 lbs of force, even distributed over a good sized recoil pad I don't think it would be too popular.

I watched an elk get knocked off her feet when hit with a .375 H&H (there was air under all 4 hooves anyway). Like Owen mentioned it was probably psychological and she was startled/jumped upon being hit.
 
Chemist said:
Manco, the students were great but the situation wasn't. My degree is in Chem, so I actually like Schrodinger's equation but I got burned out on physics 101 so to say.

I'd rather be doing P. Chem or identifying compounds from their NMR signatures...so thankfully my husband made me a stay at home mom.
Click to expand...

I understand. Sorry about the joke I made (and it was just a joke)--reading it again, I have no idea why I thought it was even remotely funny. :scrutiny: And no, I wasn't drunk when I posted it. ;)
 
The M33 .50BMG pushes a 668gr fmj down the tube to 2910fps hitting 12,550ft.lbs energy at the muzzle, but the recoil energy from it is somewhere in the 70ft.lb. neighborhood.
Click to expand...

The balance of the energy is converted into heat. Gunpowder, like Congress, works by generating a large volume of hot gases.
 
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