Recoil Gedankenexperiment...

firefly08

So it doesn't matter whether you've embedded the rifle butt in a mountain or whether it's on an air hockey table with no friction

So where does the force that would normally push the gun backwards go? Is it absorbed by the earth? I don't think so- the force that would normally push out in every other direction is not absorbed by the barrel. If it was the barrel would do no good. Instead what happens is that the gas channelled because it is prevented in going outwards, perpendicular to the bore - it can only push in 2 directions; forwards, pushing the bullet out, and backwards, pushing the rifle back. Surely by reducing the places the gas can go even further, you will increase the pressure? I doubt you could measure the difference in speed, but I think there has to be a difference.

Look at it this way: increasing the mass of the bullet would increase the energy imparted to the shooter. Why should it not work the other way around?

Sparks

firefly08 wrote: »

So where does the force that would normally push the gun backwards go? Is it absorbed by the earth?

Yes. Same as every other time you fire the rifle. I mean, if you grab the rifle, how do you think you stop the rifle moving backwards? By pushing against the planet.

I don't think so- the force that would normally push out in every other direction is not absorbed by the barrel. If it was the barrel would do no good. Instead what happens is that the gas channelled because it is prevented in going outwards, perpendicular to the bore - it can only push in 2 directions; forwards, pushing the bullet out, and backwards, pushing the rifle back. Surely by reducing the places the gas can go even further, you will increase the pressure? I doubt you could measure the difference in speed, but I think there has to be a difference.

Except that by preventing the rifle moving under recoil you do absolutely nothing to the gas pressure. It's pushing out in all directions at once and equally (let's leave aside turbulence and gas fluid pressure waves just for the moment ). The sides of the chamber all resist through the tensile strength of the steel, as does the back of the chamber; the only avenue left is via the barrel, pushing (and pushing around) the bullet in the process. Recoil's irrelevant to this: think about it, if the bullet wasn't in the rifle, would you still get recoil? Yes, and the same amount; which means that the force on the bullet is going to be the same as well.

Look at it this way: increasing the mass of the bullet would increase the energy imparted to the shooter. Why should it not work the other way around?

Actually, it wouldn't; increasing the mass of the bullet would lead to a decrease in speed of the bullet; that's why we measure muzzle energy not muzzle velocity when deciding what the firearm/not-a-firearm cutoff is for airguns, so that you can't go from a toy to a firearm by loading a heavier pellet.

firefly08

if the bullet wasn't in the rifle, would you still get recoil? Yes, and the same amount; which means that the force on the bullet is going to be the same as well.

Are you sure? I have never fired a blank round from a gun, so I admit I'm guessing here - but surely the recoil must be significantly less when there's no bullet? When the gas starts to expand, it meets the bullet; the bullet pushes back - isn't that what causes the recoil?

Sparks

No.
The recoil comes from the gas, not the bullet (the sole force the bullet can put on the gun would be via friction with the lands and that'd drag the gun forwards, not push it back).
Think of the gas cloud behind the bullet; you can model the force as originating in the centre of the cloud.
It's like a rocket ship, basicly; you don't need something outside the rocket for it to push on in order for the rocket to work.

pedroeibar1

Vegeta wrote: »

Yes it's all relative.

:D

Vegeta wrote: »

Consider though the speed relative to the target (chronograph at fixed point near target). If the chamber is moving away from the target at time of firing (say on the back of a very fast truck driving directly away from the target) and the bullet speed is relative to the chamber then it certainly makes sense that the bullet will arrive on target at a lower speed than a setup where the chamber does not move away from the target.

Now from all the high speed shots or super slow motion shots I've seen, it always seems that the round has left firearm before recoil (a force driving the chamber away from the target) takes effect. So I doubt it effects speed in a meaningful way

Sort of contradicting yourself??
but.....



Consider a flatbed truck travelling at a constant 100kph on the trailer of which a ball is dropped from a position behind the cab. It does not fall onto the road behind the trailer because the ball has inertia, i.e. the ball is moving at the same speed as the truck. (Inertia being the resistance of any physical object to a change in its state of motion or rest.)* Apply the same logic to a bullet; the ignition speed of the propellant is so fast the bullet’s speed relative to the breech is zero. So there will be no material difference.


However, I do admit that the bullets path, like a ball being dropped off the trailer i.e. outside, will be subject to the much greater forces of friction and gravity. That path can best be described as a load of parabolics.


*a feature common/well-known to some posters on these Boards



I reall have to do some work...............
P.

firefly08

you don't need something outside the rocket for it to push on in order for the rocket to work.

So you are saying that restricting where the gas can go (i.e. having something for it to push against) has no effect on the force it applies to the projectile?

Sparks

Well, it does - in that if the projectile isn't in the barrel in the first place, there'll be no force applied to it
What I'm saying is that the force applied to the rifle is the same as the force applied to the bullet regardless of whether or not the bullet is there.

Look at it on an atomic level. A gas is just atoms flying about. The number of atoms per unit volume is the pressure. So right after the propellant burn, there are propellant atoms flying all over the place behind the round, bouncing off the walls of the chamber and the back of the bullet. Every time one of them bounces, it imparts energy to the surface it bounces off.

Ones that bounce off the back of the bullet push the bullet; ones that bounce off the back of the chamber create the recoil. But if there's no bullet, you still get atoms bouncing off the rear of the chamber.

Hmm. But you wouldn't get so many bounces in a given unit of time, so yes, you'd have less recoil if you'd an open chamber (not thinking about the wad you normally get in a blank, for the moment). But that still doesn't mean that the bullet will get less or more force if the rifle is allowed to recoil or not; the number of bounces the back of the bullet gets is dependant on the speed of the bullet relative to the barrel only; once the bullet leaves the barrel, no more bounces.

Tackleberrywho

Sparks wrote: »

Tack;
(1) constancy is not connected to the dimensionality of the property being measured, but to whether or not it's changing;

(2) you just said that speed is a vector hence it's called velocity. It isn't. We're not talking about velocity at all here, we're talking about speed, the scalar measurement.

We are talking about velocity with acceleration and retardation. it increases in Velocity the longer the barrel. Hence the term Muzzle velocity **not Speed**

Speed is a constant
Velocity is not constant.

An object traveling in space "in theory" is traveling at speed. Although space is curved**

A projectile travelling down a barrel is traveling faster the further down the barrel it can go up to a certain point depending on calibre and powder.

it then is retarding from then on.

"high velocity ammo" not High speed ammo. Muzzle Velocity measured in metres or feet per second.

Weight is also the wrong term for bullets. they are of a mass, which has to be multiplied by gravity to give force in newtons.

if you want to be technical, you have to be very technical.

FISMA

I'll have to get out the auld books. Sparks and Vegeta appear to be saying that the relative speed between the rifle and the bullet is the same in either case, correct?

This is most definitely true for an elastic head-on one dimensional collision, the relative speeds of two particles after the collision has the same magnitude as before and does not depend upon the masses.

I am willing to allow the "collision" herein to be safely assumed as head on and one dimensional, even though there is rotational motion. What concerns me more is that our collision is inellastic and not elastic. I'll have to double check to see if the above paragraph applies to inellastic collisions. The case that we have I colloquially call an un-inellastic collision.:)

How about a little Gedankenexperiment?

In the middle of a large frozen pond, with perfectly frictionless ice, there is the stump of a tree, stuck in to the ground at Pt A. At pt B, there is a stump resting on top of the ice, not stuck to the ground. All things equal, except for the way in which the stumps and anchored.

100 yards downrange is a finish line, stuck in to the Earth, like the stump at A.

Timmy is at A and Jimmy at B. Timmy and Jimmy are going to push off of the stumps and we are to determine the winner, by the one who crosses the finish line first.

The starter pistol fires and both Timmy and Jimmy (initially connected to the stump) push off. Who wins the race?

I think this is like the situation posted by the OP.

The one who wins the race is the faster bullet, as judged by the target - the Earth bound reference frame.

I am confident that we'll agree that Timmy wins the race. I believe that both Timmy and Jimmy have the same relative velocity with respect to their starting stumps. Although, again, I will have to double check as this is an inellastic collision.

Thus, in theory, for this situation, there is a difference, however, I doubt that in practice using a chrono, one could consistently demonstrate an appreciable difference based upon the mass of the shooter.

So to the OP, No, the mass/weight of the shooter does not have a large effect on the muzzle velocity.

I'll state that the mass of the shooter has a negligible effect on the muzzle velocity of the bullet as measure in an Earth bound reference frame at the muzzle.

Vegeta

pedroeibar1 wrote: »

Sort of contradicting yourself??
but.....

Not at all actually, in one case the gun is being fired from a fast moving vehicle.

The other case the gun is being held by the shooter which is usually pretty much static relative to the target.

Vegeta

Check this video out at like 3 minutes 20 seconds

https://www.youtube.com/watch?v=33JBhnr9lds

firefly08

The ice race example makes sense to me.

Look at it another way:

Suppose the bullet in the rifle is anchored to the earth somehow (let's say it extends out the barrel and is attached to a wall or whatever). Assume the barrel is indestructible. When the shooter fires, what will happen? Surely he will be thrown backwards, off his feet. Doesn't this show that the force is divided between the bullet and the shooter in proportion to the resistance they each offer (just like electricity)?

pedroeibar1

Vegeta wrote: »

Check this video out at like 3 minutes 20 seconds

Sorry Veg, I'm on dongle and outside BB area - 21st C Ireland:mad:
P.

Sparks

Tackleberrywho wrote: »

We are talking about velocity with acceleration and retardation. it increases in Velocity the longer the barrel. Hence the term Muzzle velocity **not Speed**

Speed is a constant
Velocity is not constant.

Tack, that's simply all wrong.
Speed can be constant or can vary. Velocity can be constant or can vary. And we are focussed on the speed of the projectile in this case because its direction is constrained by the barrel and we're uninterested in it after it leaves the barrel.
If you don't keep the basics of problems like these clear in your head, they just fall apart.

"high velocity ammo" not High speed ammo. Muzzle Velocity measured in metres or feet per second.

"velocity" in those cases is a marketing term, nothing more.
Velocity in the physics sense (the vector quantity) cannot be simply measured in metres or feet per second, by definition.

Weight is also the wrong term for bullets. they are of a mass, which has to be multiplied by gravity to give force in newtons.

Indeed, but unless you regularly shoot outside of a gravitational field, it's a moot point; and has nothing to do with this thought experiment anyway.

if you want to be technical, you have to be very technical.

You also have to be correct...

Sparks

firefly08 wrote: »

Doesn't this show that the force is divided between the bullet and the shooter in proportion to the resistance they each offer (just like electricity)?

No. It would just show (if it happened at all) that containing high pressure gas is difficult. If the force really was divided like that, rockets wouldn't work in space because there'd be nothing for them to push against.

firefly08

If the force really was divided like that, rockets wouldn't work in space because there'd be nothing for them to push against.

No, if what I'm suggesting is true, rockets would not work as well in space as they do in air. Equally, they would not work as well in thin air as they would in dense air. I believe this is actually the case.

Otherwise, I'd be suggesting that an explosion cannot move a bullet unless it's in a barrel. Of course it can; but it can move it a hell of a lot faster if it's in a barrel. Why? Because the gas is forced to expand in the direction of whatever will move. The wall of the barrel will not move; therefore all the force that would otherwise push in that direction must now go somewhere else. It is not simply absorbed by the barrel - it is vectored elsewhere. Why should it be different for the force that would push the rifle back if it could? If the rifle will not move, that force will be vectored towards something that will - the bullet.

Another thought experiment: let's say we have a barrel and a special round - this cartridge has a bullet sticking out of each end - they are identical. The propellant is between the two bullets and the primer is buried away in the centre. Don't worry about how we ignite it for now - just assume we can. Put this round in the middle of our symmetrical, non-tapered barrel and fire it. Obviously each bullet will travel the same distance, and at the same speed.

Now we try again, but this time we coat one of the bullets with heat resistant ,unbreakable glue, so that bullet cannot move once it's in place in the barrel. We fix the barrel to the very earth, so it's not going anywhere either. Now when we fire the round, will the bullet that can move go faster and farther than the last time? Yes! It will because there is nowhere else for the energy to go. To suggest that the resistance offered by the blockage in the barrel doesn't affect the energy imparted to the moving bullet is the same thing as saying the barrel itself has no such effect.

Sparks

firefly08 wrote: »

No, if what I'm suggesting is true, rockets would not work as well in space as they do in air. Equally, they would not work as well in thin air as they would in dense air. I believe this is actually the case.

NASA disagrees:

...rockets actually work better in space than they do in air. As the exhaust gas leaves the rocket engine it must push away the surrounding air; this uses up some of the energy of the rocket. In space, the exhaust gases can escape freely.

Otherwise, I'd be suggesting that an explosion cannot move a bullet unless it's in a barrel. Of course it can; but it can move it a hell of a lot faster if it's in a barrel. Why? Because the gas is forced to expand in the direction of whatever will move. The wall of the barrel will not move; therefore all the force that would otherwise push in that direction must now go somewhere else. It is not simply absorbed by the barrel - it is vectored elsewhere. Why should it be different for the force that would push the rifle back if it could? If the rifle will not move, that force will be vectored towards something that will - the bullet.

It's different because the gas is trying to escape from the chamber, and is pushing the bullet out of the way to do it. That's the motive force involved; the pressure on the rear of the chamber doesn't affect the speed of the bullet any more than the pressure on the side of the chamber.

Another thought experiment: ... Now we try again, but this time we coat one of the bullets with heat resistant ,unbreakable glue, so that bullet cannot move once it's in place in the barrel. We fix the barrel to the very earth, so it's not going anywhere either. Now when we fire the round, will the bullet that can move go faster and farther than the last time? Yes! It will because there is nowhere else for the energy to go.

*ahem* Actually, there is.
I'm not saying you're going to spin the Earth very much faster, you understand, but that's where the energy is dumped when you absorb the recoil normally anyway, unless you're exempt from Newton's first law...

firefly08

NASA disagrees

NASA SCHMASA

Seriously though, fair enough...I can't argue with that. I still don't really understand, but it wouldn't be the first time...:)

FISMA

When it comes to Rockets, true they are happy in Space. However, since there's no oxygen to cause combustion, you have to bring it with you - which is desperately expensive.

At some point you're chasing your tail. You have to bring like oxygen to combust the fuel. But now you need more fuel, to carry that oxygen. But then, you need more fuel, to accelerate the fuel to Mach 20 something... You get the idea. I think it costs $10,000 to get a gallon of water into low earth orbit.

Anyhow, it's best to say accelerate in Physics. The "m" word - move, is a bad word in Physics. Since all motion is relative, all Physics cares about is whether or not you are accelerating.

Anywho...

Here's a simple image

When the fuel ignites it expels gasses for propulsion. Remember, that objects accelerate in the direction of the unbalanced Force.

There are Forces everywhere inside of the bell. Up and down Forces pretty much cancel out. In the diagram above, the gasses push on the left hand side of the bell/motor/whatever it is called. However, the gasses that go out into space do not push on anything. Thus, the LHS force is unbalanced and the rocket accelerates to the left. Cool huh?

Tackleberrywho

FISMA wrote: »

When it comes to Rockets, true they are happy in Space. However, since there's no oxygen to cause combustion, you have to bring it with you - which is desperately expensive.

At some point you're chasing your tail. You have to bring like oxygen to combust the fuel. But now you need more fuel, to carry that oxygen. But then, you need more fuel, to accelerate the fuel to Mach 20 something... You get the idea. I think it costs $10,000 to get a gallon of water into low earth orbit.

Anyhow, it's best to say accelerate in Physics. The "m" word - move, is a bad word in Physics. Since all motion is relative, all Physics cares about is whether or not you are accelerating.

Anywho...



When the fuel ignites it expels gasses for propulsion. Remember, that objects accelerate in the direction of the unbalanced Force.

There are Forces everywhere inside of the bell. Up and down Forces pretty much cancel out. In the diagram above, the gasses push on the left hand side of the bell/motor/whatever it is called. However, the gasses that go out into space do not push on anything. Thus, the LHS force is unbalanced and the rocket accelerates to the left. Cool huh?

I always was very interested in rockets and propulsion
so can we now state that SPEED is not the correct word and Velocity is

And that a round is accelerating in a barrel until it is retarding & is a vector Quantity as it has magnitude

This has to be the weirdest thread in a long time

Mr.Flibble

Tackleberrywho wrote: »

so can we now state that SPEED is not the correct word and Velocity is

And that a round is accelerating in a barrel until it is retarding & is a vector Quantity as it has magnitude

No.

Sparks is correct; to participate meaningfully in this thread you need to know what you're talking about. In this case you don't.

Tackleberrywho

Mr.Flibble wrote: »

No.

Sparks is correct; to participate meaningfully in this thread you need to know what you're talking about. In this case you don't.

I beg to differ.
speed is a constant everything in a rifle is a dynamic from whence the trigger is squeezed.

"meaningfully participation"

Mr.Flibble

Tackleberrywho wrote: »

speed is a constant

O.k. If it's a constant, what's its value?


(Other than 42, obviously.)

Tackleberrywho

Mr.Flibble wrote: »

O.k. If it's a constant, what's its value?


(Other than 42, obviously.)

If what is a constant?
As far as I remember we are talking about whether carrying a few pounds will make one a better shot or not.
Or has the shooters size got any affect on the accuracy of the strike.

So you may rephrase your question please so I can attempt to answer "if I can answer"

Sparks

Tack, constant just means "is not changing". And its opposite is not "dynamic", but "variable";
Speed is what we're interested in here, rather than velocity, since we have fixed the direction;
The word is not "retarding" but "decelerating";
And a rifle round is never decelerating in any normal rifle barrel, and we're not considering an abnormal barrel here.

housemap

Could I ask a question regarding rifling and spinning bullets being far more accurate , if the bullet wasn't spinning any even tiny irregularity would generate lift and throw it off target, if the bullet is spinning I would imagine any lift would be cancelled out.

Is this a factor in the accuracy of spinning bullets and if so is it a large factor ?

I know spinning objects are more stable just because they are spinning, satellites spin and there is no aerodynamic considerations, I don't know the why behind it though and is the why (that I am not aware of) a bigger factor than aerodynamic consideration ?

Thx

Tackleberrywho

Sparks wrote: »

Tack, constant just means "is not changing". And its opposite is not "dynamic", but "variable";
Speed is what we're interested in here, rather than velocity, since we have fixed the direction;
The word is not "retarding" but "decelerating";
And a rifle round is never decelerating in any normal rifle barrel, and we're not considering an abnormal barrel here.

Dynamics are variable
retarding means to slow down (many variations of use in english language,from Old French retarder, from Latin retardāre, from re- + tardāre to make slow, from tardus=sluggish)

So if a "bullet" is not decelerating in a rifle barrel it is accelerating as it's movement is not constant unless it has stopped accelerating; which has been pointed out that a 30" barrel gets the optimium velocity for 1000yard comps.

as the trajectory is curved of a round the point where a constant "speed" may be achieved may only be for an instant in time, when it is not accelerating or decelerating .

"fixed in direction is only inside the barrel" as when it leaves the barrel it is under "dynamic or variable" conditions.

the op asks a question about the "mass" of the Shooter having an effect on the Bullets actions

And I "think" the answer is No, however i do not have a phD yet in physics so I don't "know for sure"

Sparks

Tackleberrywho wrote: »

Dynamics are variable

Tack, that sentence makes no sense at all - it's the very definition of the word nonsense.

retarding means to slow down (many variations of use in english language,from Old French retarder, from Latin retardāre, from re- + tardāre to make slow, from tardus=sluggish)

And if we were speaking old French, you might have a point; but in physics, the word is decelerate. Not retard.

So if a "bullet" is not decelerating in a rifle barrel it is accelerating as it's movement is not constant unless it has stopped accelerating

Again, this is nonsense. A bullet can be accelerating or decelerating or neither. In a rifle barrel however, unless something has gone horribly wrong, it is either accelerating or at rest. It does not decelerate in the barrel, ever.

as the trajectory is curved of a round the point where a constant "speed" may be achieved may only be for an instant in time, when it is not accelerating or decelerating .

We're not considering the entire trajectory though, but only the initial few microseconds between the ignition of the primer and the bullet leaving the barrel.

"fixed in direction is only inside the barrel" as when it leaves the barrel it is under "dynamic or variable" conditions.

Okay, again, that is complete gibberish. You appear to be choosing random words from the dictionary and sticking them together with no heed to meaning or context. Please don't do that.

Tackleberrywho

Sparks.
Gibberish is not my chosen language, nor do I try to randomly pick "big words out of the dictionary".

An object "bullet" at rest.
A mechanically induced (a firing pin hitting a primer) then a chemical chain reaction causing gases to form, expand and have to escape through the barrel by pushing the "bullet" out of it's way and as a bi-prod cause it to spin as a result of the rifling.

(I hope this is not considered gibberish)

referring back to the question

Does the "mass" of the shooter affect the round.
I still say NO

FISMA

Speed and Velocity - either work for me here. There's only one direction in which the bullet goes - parallel to the barrel. Hence, I really have no objection to either being used.

If you want to consider the rotational velocity of the bullet, we could do that to and come up with a vector. It may be in the direction of the barrel too, depends upon the grooves in the barrel.

The direction of a vector does not always have to be in the direction of the object's motion. This is most easily demonstrated with angular displacement which, unless very small, generally does not have a vector in the direction of motion. Also, the direction of the angular velocity vector is perpendicular to the direction of the angular velocity.

House - not sure about the lift. The shape of the bullet (not spinning) is symmetrical, unlike a wing. If part of the bullet causes an "up" Force, than there should be a "down" force caused by the mirror image part of the bullet.

However, some imperfection could, theoretically, cause an up force - lift.

Also, I rarely use the word deceleration. Deceleration means slowing down, so why make a new word?

Acceleration by definition is a change in velocity. Velocity changes when you speed up, OR slow down, OR change direction. When a Physicist says acceleration, you should not automatically assume they mean speed up.

Most correctly, a Physicist should say something like: positive acceleration and speeding up, positive acceleration and slowing down, negative acceleration and speeding up, or negative acceleration and slowing down.

However, this would make little sense to most people in normal situations.

To correctly state acceleration, you must give the direction (+ or -) and whether you are speeding up or slowing down. Positive acceleration could imply speeding up or slowing down, done in whatever direction you have called positive.

It's pretty confusing when learning first that you can slow down with positive acceleration and speed up with negative acceleration. That's why many Intro Physics teachers will try to minimize the verbiage and omit deceleration.

Also, there are many uses of retarding, such as Forces. We speak of retarding forces such as Friction and Frictional Torques that retard rotational motion.

Most bullets should continue to speed up along the length of the barrel, if the powder is packed properly. Indeed, if you ever see a flame out of the barrel, you are definitely accelerating as the gasses haven't finished burning.

Finally, the instant the bullet clears the barrel, even though it has no velocity towards the ground (y-velocity), it is in free fall - like space ships whose acceleration is probably slightly less, say, 9.00m/s^2. Strange as this sounds, this has to happen, as if there is no acceleration towards the ground the instant the bullet clears the barrel, it would never speed up towards the ground.

Spin does stabilize objects. Something to do with gyroscopes. Never studied it... Spin does not cause lift. However, I am not sure about drift.

Funny where this thread has gone .