Interesting physics statement...

[bigmike82]

"So if the target were wearing a kevlar vest and all of that energy were transferred to the target, it should be no worse than what the shooter experienced."
Uhm....I'm going to say no.

http://sipi.usc.edu/~kosko/NAFIPS-04.J04.pdf

I don't know about you, but shooting doesn't leave those kind of bruises on me...

[iareConfusE]

Until a physics professor or someone who uses physics in their everyday job chimes in, I'm going to go ahead and say that all of the conclusions that people are coming up with are wrong. I'm pretty sure it's much more complicated than equal/opposite reaction.

[sevensix2x51]

eh, has nobody in here ever killed something with a gun before? when you shoot a medium/big animal, either it buckles and keels over spouting blood from both lungs, or it turns on the rocket boosters and blasts through whatever is in front of it, in a desparate attempt to save it's own life. of course knockdown power is fake. i mean, is that what the discussion is about? im kind of lost.

[Turbinator]

Until a physics professor or someone who uses physics in their everyday job chimes in, I'm going to go ahead and say that all of the conclusions that people are coming up with are wrong. I'm pretty sure it's much more complicated than equal/opposite reaction.

Yeah. I agree with this thinking. It's more complicated than we're making it out to be.

Turby

[mzimmers]

It's not quite that simple.

No, but it's pretty close.

You can shoot a 500 gr round out of a S&W .500 revolver. Will that knock you down, if you're on the receiving end? Probably.

Not from the terminal ballistics alone.

For those who doubt this, stand up a mannequin (or a cadaver, if you can find one) and prop it up just enough that it doesn't tip from its own imbalance. Shoot it.

It won't move much at all. Don't come back and say I'm wrong; I'll just know you didn't do the test. It.won't.move.much.at.all.

There is so much misinformation about terminal ballistics, knockdown power and the like, I wouldn't know where to begin. But the observation in the OP is essentially correct.

Edit:

Here's a little light reading for those who wish to become more knowledgeable on this subject:

http://www.rathcoombe.net/sci-tech/ballistics/myths.html

[SPaikmos]

When I started shooting, I did a lot of research and thinking about this. IMHO, this reference explains ballistics and human wounding well:

http://www.firearmstactical.com/pdf/fbi-hwfe.pdf

The conclusion, IIRC, is that people generally fall down because they're used to seeing it in the movies. Unless you're hit in the spine or heart, you are likely going to keep moving unless you have the pre-programmed misconception that you're supposed to fall from being shot.

As for physics, there's a lot of misinformation... I am not a physics expert, but I do have some experience with it. To do a proper analysis, you need to understand the difference between kinetic energy and momentum.

There is the law of conservation of momentum, which basically says that Mass x Velocity before and after a collision has to be equal. This is what people think about when they say "equal and opposite".

In the case of a bullet striking a human, it looks like this:

Mass of Bullet x Velocity of Bullet = Mass of Human x Velocity of human

So, when a SMALL bullet, moving very FAST, hits a LARGE person, he will move very SLOW. You can plug in whatever values you want for mass and velocity and do the calculation, just make sure you use the same units. I don't know much about mass of bullets, but let me take a stab:

If a bullet weighs 1 oz and travels 1000ft/s, and hits a human who weighs 200 lbs (which is 3200 oz), then that human will move approximately 1/3 ft/s. Call it 4 inches in 1 second. I don't know how much a real bullet weighs or how fast it moves; I'm just pulling guesses out of my butt.

Is that enough to knock you down? To go from standing still to 4 inches in a second? Well, if the bullet did NO damage (i.e. imagine a rubber bullet or tennis ball or something) then I would say NO. But, if that bullet happened to puncture your lung and you feel like sh*t and collapse onto the floor, then yes.

IMHO, when people talk about wounding, they need to consider MOMENTUM, kinetic energy, and the actual damage the bullet causes.

I haven't discussed kinetic energy... it's related to momentum, but a little different. The way I think of it, if momentum is conserved, and the collision is inelastic (i.e. bullet inserts into human) then you can calculate the loss of energy. That loss of energy is basically the "destructive energy" that the bullet imparts on the person. Tearing of flesh and all that good stuff. Note that two rounds can have the same momentum, but vastly different kinetic energy. That's my first cut at explaining wounding effects. enjoy!

[bruceflinch]

Saw this on a pistol training DVD (Thunder Ranch, I think).

It's one of those things that's so clearly true, but I never thought about it enough to draw the connection.

If any round, fired from any handgun, was strong enough to knock the target down with a single shot, then the kickback would also knock down the shooter.

Equal and Opposite...

All I know about ballistics, I learned from Hollywood...

[sawchain]

SPaikmos, I think you mean conservation of energy. Momentum is the concept that objects in motion tend to stay in motion. They're related, but different.

As for whether a bullet will knock someone down, that is an extremely complex mechanical process. For example, if someone were leaning in the direction of the impact, some kinetic energy of the bullet would be passed down their legs and into the ground. And no, I'm not saying they'll piss themselves, although that's probable.

[j4strngr]

There is so much more involved for example density of target and bullet... Also energy transferred to shooter over a larger area ie the gun it's self absorbs energy and spreads it out over a larger surface area. And so much more

[guns4life]

I guess it depends on what you're shooting....

[SPaikmos]

SPaikmos, I think you mean conservation of energy. Momentum is the concept that objects in motion tend to stay in motion. They're related, but different.

As for whether a bullet will knock someone down, that is an extremely complex mechanical process. For example, if someone were leaning in the direction of the impact, some kinetic energy of the bullet would be passed down their legs and into the ground. And no, I'm not saying they'll piss themselves, although that's probable.

No, I do mean conservation of momentum. This is one area where people tend to get very confused. During a collision, momentum is ALWAYS conserved. This is a fundamental law of nature. I.e. when two masses collide (bullet hits person) momentum before the collision must equal momentum after the collision.

http://en.wikipedia.org/wiki/Momentum

Conservation of energy comes from thermodynamics. In a closed system, all energy is conserved. This is also true for a collision, but the issue is that energy can be turned into heat, as well as the destruction of tissue. So, before impact, all the energy in the system is the kinetic energy of the bullet. After impact, the person + bullet has some kinetic energy, but it is not equal to the kinetic energy of the original bullet. Where did the rest of the energy go? It went into destroying tissue + heat.

Some classic examples of this:

1) When two billiard balls collide, they usually "bounce" off each other. This is ideally an ELASTIC collision, where both momentum and kinetic energy is conserved.

2) Now, consider if both balls were sort of sticky, like silly putty or clay. When they collide, they won't bounce, but they may "glob" into a single giant ball. In this case, this is an INELASTIC collision. Momentum is still conserved, but kinetic energy is not. Some energy is lost into heat (stuff sticking together, changing shape, etc).

This link gives a simple one page description of what I'm talking about:

http://hyperphysics.phy-astr.gsu.edu/hbase/elacol.html

http://hyperphysics.phy-astr.gsu.edu/hbase/inecol.html#c1

In wounding ballistics, I'd say most collisions of bullet + body is inelastic. Of course, there are other forces at work (hydrostatic, fluid, etc) but at a macroscopic scale, the simple analysis of momentum and kinetic energy can give you an idea of how much energy can be doing "damage" to a body. That's my simplistic explanation of it.

Adding wind resistance and terminal velocities and what-not are just unnecessary complications. For a first-order approximation, the only things you really care about are the mass and speed of the objects involved at the moment of impact. Then we can go into second order effects, like how deep a wound channel and how much tissue is destroyed. I'm also assuming the bullet stays in the person, and doesn't exit him; otherwise a lot of these calculations go out the window.

I read somewhere that the impact of a bullet onto a person is the equivalent of dropping a 10lb weight a couple of inches. The calculations I've seen in terms of energy and momentum support this. It really isn't much force, as evidenced by the recoil you feel from the gun (although you need to keep in mind that the springs in the gun absorb a fair amount of recoil). And, reading the FBI study, I'm inclined to accept their conclusion that people fall down due to psychological factors (pain) more than actual physical "knock down power".

[pinwheels]

SPaikmos got it right. Energy is dissipated by heat etc. on impact, but momentum (mass x velocity) is conserved. A 230 gr bullet (1 grain = 1/7000 of a pound) at 1000 fps has the same momentum as a 200 pound body moving at 1.97 inches per second. This assumes the bullet is stopped (does not penetrate). This is roughly the same momentum as if someone tossed you a 12 lb bowling ball (I don't really know how much bowling balls weigh) at about 2 mph. Not likely to knock down someone.

[rojocorsa]

At least the Mythbusters tells you how much serious business 00 buck is.

[Turbinator]

I guess it depends on what you're shooting....

Awesome video! I like how people feel backwards into the chair, hearing protection came off, the spotting scope got knocked over.. just incredible. Of course, the last guy is the manliest, he simply took the recoil then went back on target.

Turby

[bussda]

Yes, but what is happening when the bullet strikes the target is an unbalancing of forces that causes the potential energy of a standing person to become kinetic energy.