In movies, a person who is hit by a large gun blast, such as from a shotgun, is usually pushed backwards a few feet. Does that happen in real life? Wouldn’t the person firing be pushed backwards as well? Im sure the body moves some, but not as greatly exaggerated in a movie, right? What is the physics behind these forces?
You are quite right. The physics of it are that the momentum imparted by the bullet to the victim after the strike is (for this one we can probably neglect the bullet slowing down due to air friction) essentially the same as the momentum imparted to the shooter by the kick-back of the gun. Of course, the bullet has far far higher energy as it is lighter and faster, which is why it damages the victim far more than the kickback damages the shooter. And, of course, the shooter braces himself where the victim does not. And there may be some muscle spasming going on due to damage by the bullet, but there’s no reason why this would invariably propel the victim backwards. That is justinsultingly bad movie physics
A fellow I knew since childhood was in his twenties when he sold a pile of illegal herbal substance to a police informant. He drew a gun while running away from police officers. Out of several of their shots, one hit him in the triceps of his right arm. It spun him around and knocked him down. He elected to stay down.
In those days, the gendarmes were using .38 Special revolvers, for those of you with ballistics charts.
My old chum is working in the transportation industry now, in a high tech position. He has sworn off illicit career paths.
I am almost sick from laughing from reading that site. WAY cool… What a blast… he he he
Thanks for the link Viking
Have you ever seen people get shot? Have you ever been shot? I have on both. On OCCASION people do get blown the hell out of their shoes. Not all the time, but depending on the weapon and the situation, it happens.
Alot depends on the ballistics used and type of gun. A shotgun blast will stop a normal man. It doesn’t really matter if you use 10, 12, or 20 gauge. It doesn’t matter if you use slugs, buckshot, or even birdshot. If you hit a normal person with a shotgun they will feel it and will most likely move a few feet if they don’t fall over. Rifle and handgun bullets are different, however. These bullets fire at a much higher muzzle velocity, and often pass right through the target. If a bullet passes through somebody, they’ll feel the pain, but none of the momentum will be transferred, so they could easily keep right on moving. In handguns and rifles for defense, most shooters use JHP (Jacketed Hollow Point) rounds because these rounds tend to expand and not pass through the body of the target. That way, the target takes the entire momentum of the bullet and his body could easily be thrown down by the force. To be confident that you could stop anyone coming after you or running from you, you need a decently powerfull caliber. A .38 revolver would do nicely. A 9mm (usually Parabellum rounds) would be O.K. A .22, however, would not be as reliable.
While watching a special on the bank robbery in LA (I think) a few years ago where the two guys came out wearing body armour and shot the hell outta anything in their way, one lawman described when he was hit in the knee while running across the street. He said it was like getting struck in the knee with a sledge hammer, and the force flung his leg to one side which subsequently took him off his feet.
I would have thought the rounds fired by these guys (armour peircing, no?) would have punched a nice hole straight through anything as soft as a knee joint, but I’d prefer to take the guy’s word for it over asking someone to shoot me so I can see for myself 
It is interesting that you mention the Holywood bank robbery, because it helps clarify a point that is in question here. The bad guys were hit repeatedly with all kinds of handgun bullets and, later, rifle bullets - they took the full force of each hit on their body armor, and yet they did not get knocked off their feet. As depicted in the video, “Deadly Weapons,” it is not always the kinetic force of the bullet that causes the reaction - it is the body reacting to the intrusion of the bullet. As is said in the video, “when you jab someone in the butt with a needle, the violent reaction has nothing to do with the energy inherent in the needle,” or some such. There are several demonstrations of body armor, including one in which the narrator, wearing body armor, is shot by a .30 caliber rifle from a distance of about three feet. To show that a rifle bullet will not knock a person off his feet, the narrator stands on one foot while being shot a second time!
Hollywood, even
Thanks for the doses of sanity! Yes, it seems the body’s reaction to a sudden, strong impact will account for much of the person’s reaction. If the shooter doesn’t move much, the person getting shot can’t move much more.
Every action is acted upon by an equal or opposite reaction, right?
I posted this thread originally because i was watching the ‘Heist’ with gene hackman and thought one part was stupid. In the dock scene, the black guy jumps up, fires his shotgun and hits a bad guy. The movie gives the illusion that the man is literally pulled off his feet and pushed back and up into the air at least several feet.
I have a question bearing on this; does it strike anyone as unrealistic that, in The Terminator (Stop laughing! I’m serious!), the only more-or-less effective weapon is a shotgun stolen from a police squad car? In the police station siege scene, they’re hitting him with rifle fire (What would that be? M-16?) and he brushes it off, but hit him with a shotgun and down he goes.
A stypical 12 gauge shotgun round has 1,100-1,200 feet per second velocity but a projectile weight of maybe 1-1/8 ounces. The bullet from an M-16 is moving along at a very speedy 2,900 fps of so but only has a mass of 1/8 ounce. I did a few back of the envelope calculations and it gives a significant edge to the shotgun in kinetic energy and at least three times as much inertia.
On top of those differences my experience shooting steel plates leads me to believe that heavy shotgun rounds would have more effect on the Terminator. In action shooting shotgun targets are half inch thick steel plates that must be knocked down. Shotguns do a good job of this, tranferring a lot of energy into pushing the target. Shooting plates with 5.56mm as all the energy goes into blasting holes in the target. Since we can presume the Terminator to be made of an ultra strong future alloy nearly all the energy would go into vaporizing the bullet.
Um, not quite. At least not unless a firearm enables one to circumvent that pesky law about creating and destroying energy…
The bullet leaving the barrel of the gun carries precisely the same amount of energy imparted to the shooter. The bullet striking its target will carry somewhat less energy, depending on distance downrange, do to air friction losses.
The difference between the effect on the victim and that on the shooter is not the quantity of energy applied, but rather how it’s applied.
On the shooter’s end, the energy is dissipated several ways. First, it is expended imparting motion (“kick”) to the weapon itself. In addition, with conventional firearms, some of the energy is lost in literally lifting the firearm itself (“muzzle climb”). Consider that a Smith & Wesson N-frame revolver with a 6" barrel weighs 3 lbs. empty (unloaded), and a hunting rifle or shotgun can weigh 10 lbs. or more. Then the firearm itself carries the remaining energy to the shooter’s body, but it is applied fairly evenly over a relatively large area, a minimum of 4-5 sq. in. even in the case of a handgun.
On the shooter’s end, however, all the energy is applied to a tiny area very quickly. Using a .38 caliber bullet as an example (~.357 in. diam.), and assuming a perfectly flat-nosed configuration, the terminal energy is expended over an area of 1/10 of a square inch.
Thus, if we use a total energy of 1,000 ft.-lbs. on both ends, the shooter receives a force of 250 ft.-lbs./sq. in. or less, while the shootee is treated to a localized 10,000 ft.-lbs./sq. in. OUCH!
Well I, for one, am hugely disappointed to learn that the biggest, baddest gun around won’t even knock someone back a few inches. Let alone several feet. And “stopping power” is a myth too? Dang. I hate facts. 
Peace,
mangeorge
Are you confusing energy with momentum?
Hummmmm We take a rifle and throw it up in the air and when it is in a level and upright position it fires. Now the bullet say goes one mile and weighs xx oz. and the gun which weighs xx pounds will go the other way precisely the same amount in relation to size, weight, air resistance etc.?
No. Are you?
That’s pretty close.
It takes energy to exert force, force applied over time to do work, work done to impart momentum. But for the sake of this discussion, factors like time, acceleration, etc. can be ignored, since they’re essentially equal on both sides of the equation at the point when the bullet leaves the barrel. The bullet leaving the barrel will carry X energy and Y momentum; the firearm will carry X energy and Y momentum in the opposite direction. What we have here is a fairly pure example of the basic, “For every action, there is an equal and opposite reaction.” We can get tangled up in the mathematics and the terminology, but the gun industry’s practice of using kinetic energy as a measure is quite sufficient, for the reasons I’ve stated.
Has anyone ever done a true study of the different velocity of a bullet from a gun that is locked from moving and one that is free completely free to move from the moment of cartridge ignition?
What should the difference be, one foot / second or less? Substantial?
Is it really measurable without using MIT?
TBone2: I don’t think this is correct:
True, the shooter does receive energy when firing a gun. But there’s no reason this energy must be the same as the bullet’s kinetic energy.
Yes, it does. For every action there is an equal and opposite reaction. The recoil is actually stronger than the force imparted to the bullet, because some of the energy being projected forward is gasses.