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This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons, particularly in larger calibres (12.7+mm).

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Energy vs. Momentum   General Military Discussion

Started 5-Aug by JesseH1234; 3284 views.
QuintusO

From: QuintusO

6-Aug

Having a bunch of experts correct you doesn't mean you're psychic, it means you're wrong.

You think NOBODY here has considered momentum before? I've empirically tested it. There is no correlation to lethality. The theory doesn't even make sense because momentum is conserved.

You're not a poor persecuted genius riling up the academy, you're spouting factually inaccurate half baked thrice-debunked BS and wondering why there's pushback.

QuintusO

From: QuintusO

6-Aug

There's a very large disconnect in your logic where you ally the Newtonian elastic collision dynamics with lethality. They are not allied. The momentum of a .30-06 gunshot that stops in the target will only accelerate a 200lb adult male to a speed of about 0.2 miles per hour, yet that round is highly lethal. It is lethal because of the work it does on the target, and an object's capacity to perform work is expressed by its kinetic energy:

https://www.thefirearmblog.com/blog/2016/07/16/ballistics-201-introducing-new-way-thinking-terminal-effectiveness-force-energy-work/

https://www.thefirearmblog.com/blog/2016/07/17/ballistics-201-introducing-new-way-thinking-terminal-effectiveness-energy-budget/

Also, you make a very serious error in that you mistakenly take "ft-lbs" to mean the ability to move X pounds mass 1 foot. That is incorrect, "foot-pounds" is more properly called "foot-pounds FORCE", as it is an expression of the ability to do work equivalent to to that many pounds force exerted over one foot of distance.

RovingPedant

From: RovingPedant

6-Aug

Energy, momentum and power are all well defined and understood properties. I don’t know why you would feel the need to redefine them and put them in quotes.

nincomp

From: nincomp

6-Aug

QuintusO said...

If that were the case, I should be dead.

 

https://youtu.be/X7EWKiXct48

HA!  I bet that you actually died but are too stubborn to admit it!  :)

JesseH1234

From: JesseH1234

6-Aug

That little "test" is leaving out a key component: time.  The velocity of the .22LR means it is exerting its force against the target in a much smaller window of time; this does not give the receiving substance time to transfer or spread the energy out, or get out of the way.  For instance if you jump into water from 30' up, you will be fine; from 200' up, your bones will turn to powder because the water you are hitting does not have time to disperse/get out of your way so to speak. 

To be clear I actually don't know that I'm "right" and do not even have a clearly defined conclusion. I just want to know WTF "1,000 ft-lbs of energy" really means, because it sure doesn't seem to be the ability to move 1,000 pounds a foot. 

QuintusO

From: QuintusO

6-Aug

JesseH1234 said:

That little "test" is leaving out a key component: time.  The velocity of the .22LR means it is exerting its force against the target in a much smaller window of time; this does not give the receiving substance time to transfer or spread the energy out, or get out of the way.  For instance if you jump into water from 30' up, you will be fine; from 200' up, your bones will turn to powder because the water you are hitting does not have time to disperse/get out of your way so to speak. 

Another element of time... so you're saying the less time there is, the more powerful the round.

As in momentum, that is mass times velocity, divided by time, right?

So perhaps we need an equation that's a bit like, I don't know mass times velocity times velocity... maybe we could divide it in half, too!

JesseH1234 said:

For instance if you jump into water from 30' up, you will be fine; from 200' up, your bones will turn to powder because the water you are hitting does not have time to disperse/get out of your way so to speak. 

You don't know what potential energy is. Got it.
 

JesseH1234 said:

To be clear I actually don't know that I'm "right"

I wouldn't worry about that. You're wrong.
 

JesseH1234 said:

I just want to know WTF "1,000 ft-lbs of energy" really means, because it sure doesn't seem to be the ability to move 1,000 pounds a foot. 

It doesn't mean that, which you'd already know if you'd bothered to read upthread.

QuintusO

From: QuintusO

6-Aug

It's really hilarious how every single human being I've ever had this argument with, WITHOUT exception, did not know their basic high school physics. Every. Single. One.

JesseH1234

From: JesseH1234

6-Aug

I do know what potential energy is; but it's meaningless for that example, because the water doesn't know how high you jumped from, only your impact velocity (which in this case would be determined by the acceleration of gravity x time (determined in this case by height) minus the friction of the atmosphere).  I could calculate the velocity difference if you want but I'm not sure that would help.

Different thought experiment:

Two bullets of equal caliber/cross section, say .30; same nose shape, same material construction, just different lengths.  Forget spin and stabilization; all we care about is velocity.  They are both going to impact plywood boards, as many as possible, at point blank range, and then get all the way through an equally sized and positioned pig behind them. 

Bullet A: 50 grains, impact velocity 2000 fps.

Bullet B: 200 grains, impact velocity 1000 fps.

These have the same kinetic energy of 444 ft-lbs.  However B has double the momentum. Which do you think might be able to chew through more pine boards to get at through the pig behind? 

Now, put an additional 300 yards between launch and impact; which projectiles ability to penetrate X number of pine boards will drop off more quickly? 

I'm not saying velocity is meaningless, I just suspect relying on kinetic energy figures overstates the significance of impact velocity within the context of comparing firearms. 

QuintusO

From: QuintusO

6-Aug

JesseH1234 said:

I do know what potential energy is; but it's meaningless for that example, because the water doesn't know how high you jumped from, only your impact velocity (which in this case would be determined by the acceleration of gravity x time (determined in this case by height) minus the friction of the atmosphere).  I could calculate the velocity difference if you want but I'm not sure that would help.

This implies you do not know what potential energy is.

JesseH1234 said:

Two bullets of equal caliber/cross section, say .30; same nose shape, same material construction, just different lengths.  Forget spin and stabilization; all we care about is velocity.  They are both going to impact plywood boards, as many as possible, at point blank range, and then get all the way through an equally sized and positioned pig behind them.  Bullet A: 50 grains, impact velocity 2000 fps. Bullet B: 200 grains, impact velocity 1000 fps. These have the same kinetic energy of 444 ft-lbs.  However B has double the momentum. Which do you think might be able to chew through more pine boards to get at through the pig behind? 

The error you're making here is assuming that penetration is a proxy for lethality. It's not.

By the strict premises of the example, they will penetrate exactly the same. In real life, you could get any number of results depending on how either bullet held together, and many other factors. Something you're missing is that the lower velocity bullet is less likely to disintegrate on impact, which greatly assists its penetration, and that the higher velocity bullet is much more likely to be shaped in such a way that it destabilizes, which negatively impacts penetration.

JesseH1234 said:

Now, put an additional 300 yards between launch and impact; which projectiles ability to penetrate X number of pine boards will drop off more quickly? 

Ah, but this isn't the question you asked. You asked about lethality, not external ballistics.

JesseH1234 said:

I'm not saying velocity is meaningless, I just suspect relying on kinetic energy figures overstates the significance of impact velocity within the context of comparing firearms. 

No, I read what you said. You said momentum is a much better approximate of a bullet's "power" than energy is. I proved you wrong, then you made excuses.

At some point in this conversation, you will have to admit that you're far, far out of your depth. The only question is how hard I will have to beat you upside your head before you do.

roguetechie

From: roguetechie

6-Aug

Velocity is extremely important for a large number of reasons having to do with not only trajectory, time of flight, drift, drop, AND most importantly when it comes to people especially people wearing anything resembling body armor... Velocity is extremely important.

Humans and military equipment aren't pine boards. Pig carcasses are not very good people equivalents either. Pigs are built substantially different than we are and them walking on four legs results in FAR DEEPER organs as well as time for momentum to do it's work.

Your entire argument is based on extremely bad examples which aren't analogous to what military ammunition is meant to deal with.

Even in general though, your argument is just plain off period.

TL;DR Big and slow is not the way to go. Your entire idea is based on very faulty understanding of the basics and an almost complete lack of knowledge of the specifics.

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