Questions about the irrelevance of energy (ft-lbs)

[NorthIdahoDude]

Most of the numbers that get involved in a discussion of terminal ballistic potential end up being used in discussions in terms of comparing cartridge one to cartridge two (which is kind of fair use), or (far more common) are used in isolation from all the other numbers which should be considered in conjunction with them to make them meaningful. However, that doesn't make such things useless, it just means we have data we aren't usually using for what it's good for.

What kills animals, is sufficiently damaging something that yee olde animal cannot live without. To do that, shot placement (hitting a thing that cannot be lived without) and penetration (putting a hole in that thing) are hard requirements. Everything else is just trade-offs we make for (sometimes) good reasons.

You can make trade-offs to get either more penetration (low expansion/high weight retention such as Barnes/Mono's/etc), or to a wider wound tract (expanding and/or fragmenting bullets like the ever-popular Accubond, ELD-X, etc, etc).

To go at one aspect of that in particular, we can hedge our bets with regards to marginal shot placement by widening our wound tract with a fragmenting and/or expanding bullet, but we will loose penetration depth by doing this. Too little penetration, and we loose one of our 'must-have's, but, we can easily hedge our bets against having too little penetration by using a bullet with sufficient mass to allow some reasonable percentage of the bullet to fragment/expand a lot and still leave enough mass traveling at enough velocity for sufficient penetration.

Of course, if you're familiar with physics, you will instantly recognize that Mass and Velocity are how we calculate Kinetic Energy - so yeah, true story, we do want sufficient 'energy'. However, among hunters, the term 'energy' usually gets kicked around in context of 'you need X amount of energy to hunt elk (or moose, or deer, or whatever)', which is a totally irrelevant way to look at it. So yeah, in that kind of discussion, energy is utterly and totally irrelevant.

But energy is relevant (required even) if you wanted to fully calculate out that you need X amount of energy, to cause bullet of mass Y, with expected retained mass/fragmentation ratio of Z, with an impact velocity of A, and an expanded diameter of B, to penetrate C inches of critter D, through E inches of bone, and F inches of muscle, with G margin for error, and...

You get the idea - mass and velocity (aka: energy) are indeed part of the math by which we could ultimately come to a fairly accurate guesstimation of the answer to the question, "is this gonna git er dun, or do I need More Gun(tm)?", but used outside of that context, particularly when used as a standalone measure of killing ability as hunters almost always do, calling it irrelevant in that context is spot-on.

 

[Thegman]

I feel like maybe I'm not understanding the way you're wording this but it seems essentially backwards. Momentum is conserved (meaning that the combined momentum of the bullet/gases/powder and the momentum of the rifle are equal magnitude, opposite direction) but in real world inelastic interactions like this kinetic energy is not conserved. In fact, two objects of different mass by definition cannot have both equal momentum and equal KE. Definitionally, a lower mass object will have more KE than a higher mass object with equal momentum.

The bullet ends up with slightly less momentum than the rifle (not more) in a free recoil scenario since some of the "equal and opposite" momentum with the rifle is shared between the bullet and the powder/gases (though I suppose that by the end of the event some of the gas at the breech end of the barrel is headed rearward with the rifle).

The force imparted on the bullet and the rifle are nearly the same (at the beginning of the burn the force is being applied to rifle in one direction and the bullet plus remaining powder column in the other). Obviously equal force accelerates the less massive object at a higher rate. That doesn't mean the KE gets "used up" "overcoming inertia" for the heavy rifle but not for the light bullet. An object has the same inertia whether it's in motion or stopped. The force applied to the base of the bullet accelerates it in one direction, and the (equal) force applied to the bolt face accelerates the rifle in the opposite direction, at a rate directly proportional to the ratio of the masses of the two objects.

If I'm misreading what you're saying (and/or not being clear in what I'm describing), I apologize.

Edit to add: Your posts (especially as they relate to physics) are generally pretty right on from what I can tell, and my assumption is that there's a communication breakdown and not a fundamental understanding problem causing me to think your description of what's happening is not correct. Not sure if that breakdown is on my end or yours, or maybe some of both.

And, we have a winner! This is the actual physics, folks.

 

[BigCountry49]

I am well aware of the differences between energy and momentum. This is ultimately why it is difficult having this conversation with people who's understanding of physics comes from gun rags and ammo boxes (comment not aimed at you, just folks on gun forums in general).

The statement that I refuted was "the bullet receives more energy due to it having a smaller mass". That is simply not correct. Accuracy of wording is crucial when having these discussions, because as you have pointed out, there is a significant difference between energy, momentum, mass, weight, etc.

The amount of KE imparted to the bullet and the rifle are the same. However, in order to overcome inertia a lot more of it is used up to convert that energy to motion when talking about the rifle. So the amount of acceleration for the bullet is much higher than it is for the rifle (Newton's 2nd law). The result of this is that the bullet ends up with more momentum than the rifle, but energy imparted to both of those objects is the same.

Can you explain why the top response to this incorrect?

Why does the bullet have greater KE than the rifle?

A rifle is fired, and the bullet moves much faster than the rifle, and momentum is conserved. My question is whether the kinetic energy of the bullet is greater than the kinetic energy of the rifle

physics.stackexchange.com

 

[nm.otter]

But energy is relevant (required even) if you wanted to fully calculate out that you need X amount of energy, to cause bullet of mass Y, with expected retained mass/fragmentation ratio of Z, with an impact velocity of A, and an expanded diameter of B, to penetrate C inches of critter D, through E inches of bone, and F inches of muscle, with G margin for error, and...

So we have ten variables and what, one or two equations? Literally unsolvable.