ôMuzzle velocityö 8

[Veemax]

I have a question about the “muzzle velocity” of projectiles fired from a gun barrel! I’ve always assumed that the projectile is still accelerating- providing the rate of burn and energy of the charge is sufficient to continue to overcome friction and inertia as it leaves the muzzle- that the projectile velocity is still increasing, even when it’s left the barrel. Am I correct in saying that only the rate of acceleration decreases, ultimately of course- the velocity decreases.

I’ve read several articles in various shooting magazines, that this is not the case! They appear to suggest that the velocity decreases as soon as it leaves the barrel and that maximum velocity is at the muzzle!

Are we to say that the charge and weight of projectile are perfectly balanced- so that the velocity is constant as it leaves the last section of barrel? If this is not the case, then the projectile could still be accelerating at many tens of g’s, as it leaves the barrel.

 

[Ltwine]

Somp:

You don't need to explain the "uncomfortable" phenomenon but my question - motion/acceleration after force is stopped.

 

[Ltwine]

Another amusing question:

"In fact, acceleration would not even occur if you instantaneously removed friction."

??? Really don't get it. Can anyone explain?

 

[FeX32]

Some of you are trying to simplify a problem that should not be so.
Interesting how this turned into a physics lesson.
I suggest not poking each other.



Fe

 

[handleman]

When I said "force over distance," I meant that the force is applied over a distance. Not force divided by distance. Work is f(dot)d. Force divided by distance is undefined, whether or not the distance is zero. ;-)

As far as a force being instantaneously removed, as I said I agree with you that it's impossible in the real world due to elasticity. Whether or not you need to account for these infinitesimals or they can be ignored depends on your analysis needs.

Another amusing question:

"In fact, acceleration would not even occur if you instantaneously removed friction."

??? Really don't get it. Can anyone explain?

I'm guessing you misunderstood my context. This thread is branching away from the bullet and into elementary physics.

What I intended to say was that if a body is in free motion and is losing velocity due to friction, if you remove that friction the body will not accelerate, it will simply cease to lose velocity.

-handleman, CSWP (The new, easy test)

 

[ione]

After the impact your mass is subjected to a new force which is the difference between the force which produced the acceleration before the impact and the reaction force produced by the impacted mass on the impacting mass.

 

[SNORGY]

FeX32:

Star.

We're all here to help each other in instances where we don't fully understand something - for whatever reason - regardless of how complex or how simple.

We all went to college or university, we all graduated in engineering and got a degree, and we all have had at least some success in staying employed and getting at least some things right. It stands to reason, therefore, none of us are stupid - or whatever adjective or innuendo one wants to read in place thereof.

"Poking" is counterproductive and doesn't help anything or anybody. I know from experience in *every* aspect of life that the best way to start an argument or to have someone disagree with you is to say or do something.

I am assuming that all of the "pokers" out there all went through scoring 100% on every exam, never needing technical advice from a mentor, never having been faced with a drawing revision due to an oversight during detailed design, and so forth. If that is true, they have every right to continue to poke away with impunity.

For myself, I simplify things:

(1) Try to help ehere I can;
(2) Try to learn where I need.

Well...OK...I still might bash the odd MBA, but that's a separate issue.

Regards,

SNORGY.

 

[SomptingGuy]

I suppose not everyone "gets" dynamics. It's too easy to assume that what seems natural to you will also seem natural to someone else.


- Steve

 

[Ltwine]

It is difficult to cover a deep involved problem like this in a short but concise manner. Misunderstanding is inevitable, therefore, I would say chasing for clarification rather than poking should be considered normal.

This is indeed a discussion on basic physics pertaining the opening question. What a refreshment for many of us whose college days were hidden in remote memory.

 

[MintJulep]

Why do two rise from the other side? Why not one, twice as far?

Damn. Never thought about that.

 

[aafuni]

It seems like you guys need to start a new thread on basic physics/engineering. The debate over instantaneous acceleration (acceleration being a mathmatical term describing the change in velocity) is a seperate issue from the bullet problem. Science says acceleration can change instantly (with an instant aplication of force, which is possible unless you are looking on a molecular level with most materials), so for the point of the bullet analysis that is the truth.

If you want to argue with science, why not start a seperate thread for that.

 

[Ltwine]

I guess it can be explained by conservation of mass & energy.

 

[aafuni]

The newtons cradle? That is momentum and energy.

m1*v1=m2*v2
and
(1/2)*m1*v1^2=(1/2)*m2*v2^2

The same amount of mass (same number of balls) must move and they must have the same kenetic energy (minus sound and other nonconservitive losses).

 

[SNORGY]

A similar thread occurred a while ago - on a totally unrelated topic - wherein energy, inertia and elasticity (in various forms) all got tied into the discussion. Of the phenomena discussed was the "flywheel". At some point in the thread, I think, a landing was arrived at in which "instantaneous change in acceleration" might not necessarily occur in an elastic system, but that some elasticity would have to exist in order for there to *not* be instantaneous change in acceleration upon release of an applied force.

Coming back to the OP, in short, it is my belief (right or wrong) that maximum bullet velocity occurs at some point in space away from the exit from the muzzle of the gun, not *right at* the exit from the muzzle of the gun.

But...I think I have contributed to the decay of this thread, and offer my apologies for that.

Regards,

SNORGY.

 

[SNORGY]

I also apologize for obviously completely blowing it in my attempt to attach the thread.

Regards,

SNORGY.

 

[aafuni]

The point of max velocity can not be general said to be at any specific point in general. There are too many variables. If you dont have enough grain in your shell, the bullet will be slowing before it even exits the barrel, not to mention the possiblity of it not even making it out. Conversely if you pack the bullet casing with a high explosive and have a short barrel, the shock wave could easily accelerate the bullet for a distance after the muzzle.

I would guess for a well designed gun with the proper grain in the shell, the max velocity will be just before exiting the barrel. If upon exiting the bullet was not faster than the shock expansion out of the muzzle, it would not be as accurate, even with its rotational inertia stabalizing it.

That is my take on this.

 

[CH5OH]

if you charge with high explosive, creating a shock wave, even if it travels at mach 5 , when it hits the bullet, the bullet need to accelerate first. so the initial shockwave comes to a halt, remaining energy deforms the bullet, deforms the barrel and creates a shock wave traveling back into the barrel.what doesn't happen is a mach 5 shock hitting the bullet and therefore slows down to the speed of the bullet,and then reaccelarate to mach 5 after bullet leaves the barrel

 

[aafuni]

CH5OH, good call. Not to mention the horrible shock reflections in the barrel, even without a bullet.

 

[btrueblood]

"I would guess for a well designed gun with the proper grain in the shell, the max velocity will be just before exiting the barrel."

Almost all modern firearms have significant excess pressure left in the barrel before the projectile exits. Carrying the excess weight around to achieve the condition you describe would be pointless (why purposely make a gun with a built-in braking mechanism for the bullet), and even if built...

" If upon exiting the bullet was not faster than the shock expansion out of the muzzle, it would not be as accurate, even with its rotational inertia stabalizing it."

The friction loss for the bullet being squeezed down the rifled barrel means you must have enough pressure in the barrel to drive and accelerate the projectile. The speed of sound in the driving gases must be significantly higher than the speed of the projectile, or you won't be able to accelerate the bullet at all (the pressure wave(s) from burning propellant wouldn't catch up to the bullet). In a typical firearm, the speed of sound of the gas mixture in the chamber are around 4,000 to 5,000 fps. Typical firearm muzzle velocities are about 60% of those sound speeds, and that ratio of speeds holds for almost all guns.

The net result is that even if the bullet's acceleration had dropped to near zero just prior to exiting the gun, the blast wave from escaping gas will still be there, and possibly add a slight net thrust and acceleration as the chamber empties of gases moving at speeds higher than that of the bullet itself.

In most high speed video and film of guns firing, you cannot see the bullet due to blow-by gases escaping the muzzle before the bullet exits. The cloud of smoke obscures the muzzle for several frames, and the bullet then appears out of the expanding smoke cloud.

Maybe a high-speed xray would discern the slight nudge given the bullet by escaping gas. It would need a frame rate somewhere in excess of 100000 per second to do so, which might be pushing current technology.

 

[SNORGY]

I think I have come to a realization of my errors in thought.

Fire the gun - or throw an object - in space in the absence of fluid friction and in the absence of gravity (for all intents and purposes). So, if an astronaut throws a baseball, there comes a point in time or, easier to conceptualize, a position in space where the baseball is no longer incontact with his hand, which is the only source of unbalanced applied force. Beyond that point in time / space, acceleration must immediately be zero; otherwise, the ball would gain speed indefinitely. Relativistic mechanics aside, that won't happen - can't happen, so the acceleration *must* instantaneously become zero.

One could describe the firing of a gun in space in similar fashion. There will come a point in time coincident with a position in space where the "thing" providing the propelling force pushing on the bullet is no longer in contact with the bullet, at which point acceleration must immediately be zero.

Do the same thing on Earth firing the gun from left to right. The propelling force on the left driving the bullet to the right is counteracted by the retarding forces on the right pushing to the left back on the bullet, with the imbalance producing acceleration. There comes an instant in time coincident with a position in space where the propelling force on the left is equal to (in balance with) the retarding forces on the right. It is at that point in time / position in space where acceleration has been reduced to zero and velocity reaches a maximum. Beyond that point, velocity decreases and acceleration becomes deceleration.

So, the problem was never one involving instantaneous rate of change of acceleration. It was one of trying to pinpoint the position in space where the summation of forces being applied to the projectile reach equilibrium.

Feeling dumb now.

Regards,

SNORGY.

 

[IRstuff]

There is a significant difference between a hand and an expanding gas. The hand is finite object with defined boundaries, while the gas expanding from the shell has no boundaries of its own, although it can be contained by the barrel of the gun.



TTFN

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