Questions about the irrelevance of energy (ft-lbs)

[HandgunHTR]

Yea I should have said rifle not shooter. The point was that the bullet receives more energy due to it having smaller mass.

No it doesn't. The energy is the exact same. The reason the bullet travels at a much higher speed is because of the smaller mass. If you could magically make a rifle have the same mass as the bullet, it would travel the same speed as the bullet, just in the opposite direction.

 

[BigCountry49]

No it doesn't. The energy is the exact same. The reason the bullet travels at a much higher speed is because of the smaller mass. If you could magically make a rifle weigh the same as the bullet, it would travel the same speed as the bullet, just in the opposite direction.

Wait, so you’re saying the rifle receives the same amount of energy as the bullet?

 

[waldo9190]

Wait, so you’re saying the rifle receives the same amount of energy as the bullet?

Well when you think about the fact that object A (bullet) weighs 150-ish grains, and object B (rifle) weighs 70,000-ish grains, its not so hard to fathom how one of them moves significantly faster than the other.

 

[Bluefish]

No it doesn't. The energy is the exact same. The reason the bullet travels at a much higher speed is because of the smaller mass. If you could magically make a rifle weigh the same as the bullet, it would travel the same speed as the bullet, just in the opposite direction.

I think energy is being substituted for momentum. The momentum is conserved, energy is calculated from the velocity of the objects in motion. Since the larger object moves more slowly, energy is significantly less due to the velocity being squared in energy and not in momentum. Energy = 1/2 x mass x velocity ^2, momentum = mass x velocity.

 

[Caseknife]

Wait, so you’re saying the rifle receives the same amount of energy as the bullet?

Newton's Third Law of Motion, for every action there is an equal and opposite reaction. So, yes, @HandgunHTR statement is correct . If the rifle weighed the same as the projectile and powder, wouldn't want to pull the trigger standing behind it.

 

[BigCountry49]

Newton's Third Law of Motion, for every action there is an equal and opposite reaction. So, yes, @HandgunHTR statement is correct . If the rifle weighed the same as the projectile and powder, wouldn't want to pull the trigger standing behind it.

Newton’s third law applies to force and momentum, not energy. The rifle will receive the same amount of momentum, but not the same energy as the bullet.

 

[Dobermann]

I just spend time with a sniper who was deployed 4 times in Iraq. They did shoot .338 L FMJ BT.

Did they say how much knockdown power it had?

 

[HandgunHTR]

I think energy is being substituted for momentum. The momentum is conserved, energy is calculated from the velocity of the objects in motion. Since the larger object moves more slowly, energy is significantly less due to the velocity being squared in energy and not in momentum. Energy = 1/2 x mass x velocity ^2, momentum = mass x velocity.

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.

 

[Stinky Coyote]

and the bullet is only in the tube for a nano second, taking all it's work potential (energy) with it, rifle quits moving pretty quick, why recoil numbers are what they are, a tiny portion of the 62,000 psi explosion goes to the shoulder

 

[Stinky Coyote]

now we take that 62,000 psi and lose some down the tube, some into the shooter, then more through the air, then it arrives at target and whatever is left we translate to ft/lbs although it does provoke some additional thought to the 'energy' at terminal end, ie; if looked at in psi may be easier to visualize than ft/lbs, psi blowing chunks of fragmenting bullets outward in right distance in animal, psi released an easier visual to outward expansion of energy than say ft/lbs,

if you know an eldm of same weight and impact velocity as a mono and the m goes half as deep and weighs half as much as the copper, it has in that distance done twice the 'work', psi or ft/lbs, fragments being driven outward by this 'energy' damaging a lot more material over shorter distance and correspondingly more drt's

edit addition; is there 30,000 psi of the original 62,000 left at a given impact velocity and does that release over 16" or 32", or is ft/lbs the better energy unit as it accounts for bullet mass, can remaining psi be part of that or back calculated from ft/lbs at the terminal end?

and this what we do, we don't need to know more than what we know, get enough sd, construction, and impact velocity for game intended and preference to speed of kill and we're good to go

as Mac said, until we can measure work in gel to be able to start computing a terminal coefficient we're stuck working it all out like the past 38 pages, not the end of the world but we're also not done, it's a lot easier understanding what .523 bc does differently than .697 bc in how we choose to our desires, the TC would do the same thing to choosing for our desires as we'd understand what performance level we're after for an intended goal, we would see the picture between an eldm TC vs a mono TC and this ability would change the game in how we discuss this stuff and assist all shooters of live targets around the world for any size game etc.

till then, eldm construction type, .25 sd or higher, 1800 fps.....and that covers off about the peak of NA big game for those that prefer to walk right up to everything after they shoot, energy does not need to be relevant although it's there and would be part of the equations likely to get to the TC

in meantime we can look at an eldm in gel and if they give impact, final bullet weight/diameter and inches of penetration we can math that somewhat to get a better visualization of how that mono will damage internals vs an eldm on a 'per inch' basis, little eldm vs big eldm lower ft/lbs per inch, higher ft/lbs per inch, both kill quickly but there's still a difference and work potential that may or may not be relevant to most goals, the mono's transfer easily explainable by going twice as deep with half the ft/lbs per inch along the way, where needing more penetration than normal broadside this becomes important and all the way up to dg where you want all the energy applied to penetration and use solids so that can be achieved, the TC would just make talking about all this an equivalent to discussing BC on the inflight side....easy and fast, much better than say cup/core or match vs bonded or mono....get it in objective number and all the noise goes away

 

[AZ_Hunter]

Newton's Third Law of Motion, for every action there is an equal and opposite reaction. So, yes, @HandgunHTR statement is correct . If the rifle weighed the same as the projectile and powder, wouldn't want to pull the trigger standing behind it.

Or if shot in a 0 G vacuum…. That would be fun ride. Take the 30-06 to space and fire a round then accelerate backwards at the same speed of the bullet going forwards. Yeee haw!

 

[HandgunHTR]

now we take that 62,000 psi and lose some down the tube, some into the shooter, then more through the air, then it arrives at target and whatever is left we translate to ft/lbs although it does provoke some additional thought to the 'energy' at terminal end, ie; if looked at in psi may be easier to visualize than ft/lbs, psi blowing chunks of fragmenting bullets outward in right distance in animal, psi released an easier visual to outward expansion of energy than say ft/lbs,

if you know an eldm of same weight and impact velocity as a mono and the m goes half as deep and weighs half as much as the copper, it has in that distance done twice the 'work', psi or ft/lbs, fragments being driven outward by this 'energy' damaging a lot more material over shorter distance and correspondingly more drt's

edit addition; is there 30,000 psi of the original 62,000 left at a given impact velocity and does that release over 16" or 32", or is ft/lbs the better energy unit as it accounts for bullet mass, can remaining psi be part of that or back calculated from ft/lbs at the terminal end?

and this what we do, we don't need to know more than what we know, get enough sd, construction, and impact velocity for game intended and preference to speed of kill and we're good to go

as Mac said, until we can measure work in gel to be able to start computing a terminal coefficient we're stuck working it all out like the past 38 pages, not the end of the world but we're also not done, it's a lot easier understanding what .523 bc does differently than .697 bc in how we choose to our desires, the TC would do the same thing to choosing for our desires as we'd understand what performance level we're after for an intended goal, we would see the picture between an eldm TC vs a mono TC and this ability would change the game in how we discuss this stuff and assist all shooters of live targets around the world for any size game etc.

till then, eldm construction type, .25 sd or higher, 1800 fps.....and that covers off about the peak of NA big game for those that prefer to walk right up to everything after they shoot, energy does not need to be relevant although it's there and would be part of the equations likely to get to the TC

in meantime we can look at an eldm in gel and if they give impact, final bullet weight/diameter and inches of penetration we can math that somewhat to get a better visualization of how that mono will damage internals vs an eldm on a 'per inch' basis, little eldm vs big eldm lower ft/lbs per inch, higher ft/lbs per inch, both kill quickly but there's still a difference and work potential that may or may not be relevant to most goals, the mono's transfer easily explainable by going twice as deep with half the ft/lbs per inch along the way, where needing more penetration than normal broadside this becomes important and all the way up to dg where you want all the energy applied to penetration and use solids so that can be achieved, the TC would just make talking about all this an equivalent to discussing BC on the inflight side....easy and fast, much better than say cup/core or match vs bonded or mono....get it in objective number and all the noise goes away

Please, just stop. Your complete misunderstanding of what PSI is and how it is applied is just muddying the water even more.

Your continued assertion that a "work curve" can be developed for a given bullet continues to have more holes in it than Form's evaluation targets as there are so many variables that you cannot account for that it is not even remotely feasible. That is why practical tests are carried out and educated observations are made based on them. Each animal is different, each hit is different, and each bullet will behave differently based on those variables.

 

[eric1115]

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.

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.

 

[HandgunHTR]

Or if shot in a 0 G vacuum…. That would be fun ride. Take the 30-06 to space and fire a round then accelerate backwards at the same speed of the bullet going forwards. Yeee haw!

Unfortunately, that is not how it works. Just because there is no gravity it doesn't reduce the mass of the object. The weight may be zero, but its mass is the same. Therefore, the amount of force applied to the bullet and the rifle will be the same as it is when it is at sea level. Addtionally, if you were behind the rifle in space, your mass would have to be added to the calculation.

My guess is that if you are a person of average mass and you were able to somehow get your mass lined up exactly in line with the direction of rearward force, that when firing that 180 grain bullet out of a 30-06 you and the rifle would probably be moved backwards at about .3 feet per second.

 

[Lou270]

The energy comes from the burning powder. Some of it turns into heat/frictiom. Some of it moves the rifle. The majority of it moves the bullet down the barrel. You can figure out the amount of energy used up by conservation of momentum of the rifle and bullet. A bullet will be potentially several hundred or thousands of ft-lbs. The energy used in moving the rifle will be few tens of ft-lbs.

Think of energy as charge on a battery or gas in a tank. It is the potential to do work. As work gets done energy is used. For ex, A small diameter bullet will use less energy penetrating tissue or gel or whatever to the same depth as a wider diameter bullet. It will just do less work/damage along the path construction being equal since there is less resistance to overcome. A softer bullet that fragments uses up energy more quickly doing work than one that holds together and penetrates more deeply.

Lou

 

[HandgunHTR]

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.

It's all good.

This is why these types of discussions are usually such crap-shows. It is also why KE is such a shit unit to use in the context of what we are trying to accomplish. KE doesn't actually do anything. It has to be converted to work in order to affect anything.

In response to your post, I would say that I think where we are getting crossed up is that acceleration is inversely propotional to mass. I think that is what you were trying to say in that last sentence that I highlighted, but I want to make sure.

As for interia, even if a force is applied, if the object doesn't move, no energy was transferred. It was still converted to force (i.e. "used up") but if the force applied doesn't impart motion it wasn't transfered. In the context of this discussion there are other forces that need to be overcome besides just inertia, but for the sake of making it easier for most to understand, interia is a good "catch all" term.

If that doesn't clear it up, I am happy to share my number via PM and am willing to get on a call to discuss this more dynamically.

 

[Stinky Coyote]

Please, just stop. Your complete misunderstanding of what PSI is and how it is applied is just muddying the water even more.

Your continued assertion that a "work curve" can be developed for a given bullet continues to have more holes in it than Form's evaluation targets as there are so many variables that you cannot account for that it is not even remotely feasible. That is why practical tests are carried out and educated observations are made based on them. Each animal is different, each hit is different, and each bullet will behave differently based on those variables.

oh calm yer boobs

because we don't have ability to work curve to then create the TC means everyone has to visualize this their own ways from everyone else's word of mouth of the same, it's hilarious, with TC you don't have to understand each animal and each hit is different, you'll understand the expected performance a lot more accurately than a picture of result of work at whatever impact velocity it was or work transferred, don't need pics, you can just describe it, like taking a picture of the wind and using that to tell you what the wind speed is...or do we just measure the wind speed?

anyhow, Forms pics are very revealing, so are everyone else's, but Form shoots lots more with much greater variety of boolits and cartridges, so you get to see what 240 ft/lbs/inch over 14" looks like, the entire rear ham of a deer vaporized, the hide and meat could not contain that 'energy' lol, most of us shooting milder formula's most of the time the ribs and hide around it contain the say ~100 ft/lbs/inch over 18" (say eldm example) while the soft gooey stuff in the middle gets the damage needed for quick kill (aka drt), and same 100 ft/lbs/inch mono with a 32" potential does half the work through 16" of deer then the other half into the hillside (aka 100 yard runner)

carry on, you know where we need to go to make it into the 21st century, this 20th century focus on result of work is just painting same thing over with a different color and no masterpiece ever realized, you're welcome

 

[Bluefish]

Or if shot in a 0 G vacuum…. That would be fun ride. Take the 30-06 to space and fire a round then accelerate backwards at the same speed of the bullet going forwards. Yeee haw!

Um, no. You would go backwards, but not as fast as the bullet. F=ma. Force is case area X chamber pressure. Your mass is so much higher than the bullet, you would accelerate, but slowly.

 

[AZ_Hunter]

Unfortunately, that is not how it works. Just because there is no gravity it doesn't reduce the mass of the object. The weight may be zero, but its mass is the same. Therefore, the amount of force applied to the bullet and the rifle will be the same as it is when it is at sea level. Addtionally, if you were behind the rifle in space, your mass would have to be added to the calculation.

My guess is that if you are a person of average mass and you were able to somehow get your mass lined up exactly in line with the direction of rearward force, that when firing that 180 grain bullet out of a 30-06 you and the rifle would probably be moved backwards at about .3 feet per second.

I know. It was funny in my mind.

 

[eric1115]

It's all good.

This is why these types of discussions are usually such crap-shows. It is also why KE is such a shit unit to use in the context of what we are trying to accomplish. KE doesn't actually do anything. It has to be converted to work in order to affect anything.

In response to your post, I would say that I think where we are getting crossed up is that acceleration is inversely propotional to mass. I think that is what you were trying to say in that last sentence that I highlighted, but I want to make sure.

Eh, sort of. I was saying that the rate of acceleration of the rifle compared to the bullet is directly proportional to the ratio of their masses. If the mass of the rifle is, say 500 times that of the bullet, it will accelerate at 1/500th the rate of the acceleration of the bullet. Ratio of masses and ratio of acceleration (and therefore velocity since the force acts on each for the same amount of time) being equal but inverse explains how/why conservation of momentum happens.

When you said that the bullet ends up with more momentum than the rifle, how did you arrive at that? Were you taking the mass of the shooter and/or friction with whatever the rifle is resting on into account? Free body diagram of rifle and bullet should give them equal momentum if we hand wave away the powder/gases and assume we're talking about a free recoil condition, and more momentum for the rifle if we assume powder/gases are moving in the same direction as the bullet.

As for interia, even if a force is applied, if the object doesn't move, no energy was transferred. It was still converted to force (i.e. "used up") but if the force applied doesn't impart motion it wasn't transfered. In the context of this discussion there are other forces that need to be overcome besides just inertia, but for the sake of making it easier for most to understand, interia is a good "catch all" term.

I think that's where some folks are getting hung up (and part of where I'm quibbling with you). Energy can be transferred in the form of heat/sound/light/etc even if the object doesn't move. Force that doesn't move an object is not too weak to overcome its inertia, it's just not enough to overcome the force of static friction. It's been a minute since I last did a FBD for anything other than making my kids' eyes glaze over, but a very small force on a very large mass just has small acceleration (not none). I realize I'm being a bit pedantic and hopefully not too assholeish about it, and partly just want to make sure I am not being one of those confidently wrong people. I never took anything above 200 level physics in college, and it's been a long damn time since I used it seriously so I could be due for a refresher.

As an aside, I'm with you (if I understand your position correctly) on KE being a completely useless metric for anything happening forward of the muzzle. I think it has value describing recoil characteristics though, since it gets transferred fairly consistently to the shooter's shoulder. So in some ways this really doesn't matter all that much other than to satisfy my own curiosity.

If that doesn't clear it up, I am happy to share my number via PM and am willing to get on a call to discuss this more dynamically.

I appreciate that, if we're still not connecting after this exchange I may take you up on it.