# Projectile weapon-physics

[SOLVED] projectile weapon-physics

Are really revolvers, shotguns and rifles powerful enough to toss people backwards? Or is this an image created by Hollywood films?

Thanks for any answers concidering this ;)

A .44 magnum will fire a 15.5 g bullet at about 450 m/s. This would impart enough momentum to accelerate an 80 kg man to 8.7 cm/sec, if it doesn't fly through him. A high-powered rifle will have 2/3 the mass but twice the velocity, imparting a velocity of about 12 cm/sec. A 12-gauge shotgun will fire about 35 g of shot at about 280 m/s - roughly the same momentum as the rifle.

I don't think these velocities are going to send people flying, but there will be knockback.

Njorl

I saw a demonstration on one of those "bloopers" programs. A news reporter was talking about a particular police precinct in which officers would soon begin wearing bulletproof vests (the kind with a metal plates, before Kevlar). He closed his report with the words, "it may be somewhat cumbersome, but in an emergency this(holds up one of the metal plates) could save and officers life." At this point, a revolver (which looked like a .38) points in from offscreen at the plate, which is held in front of the reporter's chest.

"Dirk Witless, Channel 14 action news, reporting." And the weapon was discharged. The reporter's feet were lifted off the ground and he flew backward (just a couple of feet) and landed on his back. What followed was a long series of bleeps, with the words, "that HURT!" somewhere in the middle.

Homework Helper
A bullet CAN knock you down.

Loren Booda
I have heard somewhat the opposite in reference to the Kennedy assasination. JFK's head was supposed to have jerked back toward the bullet impact (no grassy knoll). Of course for momentum conservation there had to be brain matter moving forward. Sorry, Jack.

With JFK, the theory is that the motion was due to the part of the brain hit. There was not much momentum transfer as the bullet passed through with high enough velocity to pass through the front seat and governer Connelly.

Njorl

Loren Booda
Have you ever seen a lightweight after they fire a shotgun the first time? It is not unusual for them to end up on their can.

Originally posted by Loren Booda
I have heard somewhat the opposite in reference to the Kennedy assasination. JFK's head was supposed to have jerked back toward the bullet impact (no grassy knoll). Of course for momentum conservation there had to be brain matter moving forward. Sorry, Jack.

I heard this was due to the "jet effect" that happens as brain matter follows the bullet at high speed due to vacuum pressure, producing a noticable thrust backward.

Mentor
Originally posted by Loren Booda
Have you ever seen a lightweight after they fire a shotgun the first time? It is not unusual for them to end up on their can.
Skeet shooting with the Boy Scouts. A friend of mine (12 years old but probably 150 pounds) though he was too cool to stand sideways and got knocked over the first time he fired a 12 gage.

The thing that surprised me when I first fired one from the hip is that it does not jump out of your hands. It can be a lot of force if you try to stop it cold (against your shoulder) but if there is some travel, it is easier to absorb the energy.

Originally posted by Loren Booda
Have you ever seen a lightweight after they fire a shotgun the first time? It is not unusual for them to end up on their can.

This is a good point which did not immediately occur to me (gettin slow). If a body absorbs the impact of a bullet, the momentum transferred is exactly the same (neglecting air resistance) as the impulse of the kickback when the gun is fired. Ergo, if the kickback can put an unprepared person on his can, so can the shot. In fact the recipient is much less likely to be able to brace for impact and so could easily end up falling over.

Homework Helper
Hollywood's worse when they show no kickback.

Remember in Eraser, when Governor Schwarzenegger is hanging in a parachute and he fires a gun at the oncoming airplane and doesn't budge an inch? Or when Thelma and Louise fire off several rounds into the tanker truck (which blows up) without any kickback at all? This the the fiction. Getting knocked over is real.

Pandemonium
Alternative to Njorl's explanation, think of this. If the momentum of the bullet can send its target flying, then, according to the principle conservation of momentum, the gun/rifle, and hence the shooter, will also experience that same amount of momentum in the opposite direction. So in actual fact, if the target flies off after biting a bullet, so must the firer.

Fuego
Originally posted by Pandemonium
Alternative to Njorl's explanation, think of this. If the momentum of the bullet can send its target flying, then, according to the principle conservation of momentum, the gun/rifle, and hence the shooter, will also experience that same amount of momentum in the opposite direction. So in actual fact, if the target flies off after biting a bullet, so must the firer.

but the shooter should be in a strong steady position and prepared to fire. the target is probably not ready for it and not in a very strong stance. the shooter can absorb recoil by the positioning of their body.

Originally posted by Fuego
but the shooter should be in a strong steady position and prepared to fire. the target is probably not ready for it and not in a very strong stance. the shooter can absorb recoil by the positioning of their body.

Right. Exactly the way one man can punch another and knock him down, without knocking himself down in the process.

Pandemonium
Even if the shooter is in a prepared position, the force acting on him due to the recoil will still be great... it'd be as if catching hold of the 'flying' target. I personally believe it is quite impossible to remain in a position to continue to fire if I've just caught hold of a person being shot by a Hollywood bullet.

The difference between what happens to the shooter and the shot is often a matter of angular, rather than linear momentum. The shooter, with one foot ahead of the other and weight well distributed, will have his weight rock onto the back foot. The shot guy, unprepared, may well have his feet positioned in a line perpendicular to the flight of the bullet. If his feet don't move, he will be whipped into the ground.

Njorl

Mentor
Military assault rifles used to be fully automatic, but they have switched to 3 round burst because even with a counterweight to absorb much of the recoil, they are virtually impossible to hold steady. Watching someone try to fire an old M-14 in fully-auto mode is something to see.

What if.

Haven't any of you seen the actual physics in action? No way would a person be sent back five feet by a .50 caliber projectile, no way.

I have a question for you guys. Do you know what the impact is of a .45 caliber bullet or a 5.56 nato round? I'm curious. At school they didn't teach us the formula for the amount of impact for a regular bullet, so I'm curious. How many pounds of force can a bullet inflict if going at a supersonic speed?

Gold Member
Hi, Mr. Mad. You can find all of the ballistics information that you want from the ammunition manufacturers or in various gun magazines. My .44 magnum Super Blackhawk produces 1,150 ft/lbs of muzzle energy from a 7 1/2" barrel. Strangely enough, both that and the mid-range ballistics are identical to a .30-30 Winchester fired from the same barrel length.

Mentor
I have a question for you guys. Do you know what the impact is of a .45 caliber bullet or a 5.56 nato round? I'm curious. At school they didn't teach us the formula for the amount of impact for a regular bullet, so I'm curious. How many pounds of force can a bullet inflict if going at a supersonic speed?
Pounds of force is entirely dependent on the way the impact is absorbed and is largely irrelevant anyway. The relevant issue is momentum. Momentum is just mv.

Regarding knocking someone down, I didn't mean to imply a bullet can knock someone back much. If you stand with your feet square to where you are shooting, your center of gravity is only a couple of inches in from the back of your heel. That means the shotgun only needs to knock you back a few inches to knock you over.

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Bubble99
It takes very little to knock you off-balance. It takes MUCH more to send you flying through the air. Bullets fired from handguns and rifles simply cannot send you flying through the air.
(Doesn't anyone watch Mythbusters...they covered this.)
Perhaps if you were hit by an artillery round, AND your body stayed together, AND you "caught" the shell (it didn't go through you), you could be thrown off your feet. But I think we all know that won't happen.

Gelsamel Epsilon
I wouldn't be supprised if getting shot with a gun with bullet proof armor on would knock you back. Impulse from that would be massive since the bullet stops almost immediately in the armor. However getting shot without armor would impart considerably less impulse on the target due to the longer stoping time.

Staff Emeritus
Gold Member
pervect said:
The amount of recoil is not very great. I would recommend watching the Mythbusters episode #38 (or the previous one).

http://kwc.org/mythbusters/2005/10/episode_38_mythbusters_reviste.html

They couldn't even knock a pig carcass off a hook with a .50 caliber rifle.

First of all, there's a lot of politics involved with the .50 cal due to Geneva Conventions, so it's design and use can be argued (killing people vs. stopping vehicles). It's not made for stopping power, like say the .44 or the shotgun. If you want to break it down to physics, I'd make the assumption that it's fast and heavy enough for it's cross-sectional size, so it's more likely to burst through a human (it's momentum is localized, and tears a hole in them).

Guns/bullets designed for stopping power try to distibrute the momentum. They want to reduce the piercing affect and try to transfer the momentum to traverse momentum in the whole person (instead of creating icky fluid dynamics in their vital organs).

I'm not saying a gun will always knock someone back. Remember that torque comes into play if you're getting hit at a single point, but that torque can be broken into several torques, depending on how your muscles react to the hit. There's factors, are namely:

where you hit them:

the higher you hit them, the more torque. The denser the spot where you hit them (thick bone), the more momentum gets transferred to traverse (since it pierces less),

their stance and current traverse momentum: (russ commented on this a bit)

even if someone's running towards you, as they swing their leg forward, they're helping put the same torque that you want to put on their shoulder (think about which shoulder would be best). You essentially flip them onto their back.

the bullet's mass, cross-secitonal area, and speed

I'd assume you'd want lots of mass, bigger cross-sectional area, and a lower speed (to avoid piercing)

edit: is mythbuster's peer-reviewed?

Gold Member
The .475 A&M magnum generates 70 ft/lbs of recoil. That can set you back a couple of feet if you're light and not very well braced. The muzzle energy is 10,000 ft/lbs; basically, it was designed just a tad too late for hunting Tyranasaurus Rex.

Gold Member
Danger said:
The .475 A&M magnum generates 70 ft/lbs of recoil. That can set you back a couple of feet if you're light and not very well braced. The muzzle energy is 10,000 ft/lbs; basically, it was designed just a tad too late for hunting Tyranasaurus Rex.

good for grizzlies and polar bear too, but not so much for hunting (accuracy isn't that great at long ranges) more for self-defense when they're coming at you. Shotguns are good for that too.

I don't hunt for sport, but I'm all about self-defense.

dingpud
Pythagorean explained it best. It all depends on the situation, the type of bullet, the composition of the target in questions, etc.

Excellent point on whether Mythbusters is peer reviewed or not. I really question some of there stuff. Maybe they test all aspects, and just have to cut some of the footage to make up for time. Must say though...that's a pretty cool job.

-Every action has an equal and opposite reaction-
In the situation of a big fat fast moving bullet moving towards a target with a bulletproof vest on, where a good portion of the energy is transferred to a small cross section on the vest, I would believe that the reactive force from the bullet would be pretty high...and most likely unexpected.

...but then again, Hollywood doesn't use bullets...

R.I.P. Brandon Lee

Tclack
Airsoft Gun

Ok, I'm a bit confused. My friend bought an airsoft gun and two different types of pellets. The pellets are the same diameter (6mm), but different masses. (.2g and .12g). Just for fun, I wanted to predict the velocity of one after getting the other, but my calcultations were way off. We have one of those little radar guns that the highway patrol has.

the .2 g about 160mph so I predicted that the second would go 270 mph (with the m1v1=m2v2. After all, it would make sense that the spring in the airsoft gun would hit the pellet with the same momentum, so I set the momentum of the pellets equal to each other. However when we fired the .12 g pellet it when about 185 mph.

We decided to try the same experiment with a different gun. It fired the .2g pellet at about 100 mph, so I calculated the result for the .12g pellet to be about 165mph, but the result was 133 mph.

So I'm not sure what could be wrong, Is it air resistance? I wouldn't think so because I thought air resistance was dependant on surface area, and the pellets have the same surface area. Another possibility is that the spring load doesn't always have the same momentum, but I did shoot each pellet type about 3 times, and the result was generally the same within 5mph. Or it could be the accuracy of the speed gun. I don't know, do you guys have any ideas?

misnoma
this is simply a case of conservation of momentum - recoil/kickback is not a term liked in education- you should use the term reaction. For momentum to be conserved (as it is always, negating any external effects) then this reaction on the person would depend on the masses and velocity. as a simple analogy, consider a tank firing a shell compared to a pistol firing a bullet. Simply put, the momentum is the product of mass x velocity. For each system in the scenario the equality is always upheld and remember that it is a vector quantity and thus has magnitude and direction.