Does someone have a good definition for these terms as they relate to ballistics? I understand ftlbs gives us a metric to compare different calibers, or different weights and velocities of projectiles within a caliber. But my understanding of foot pounds is a measurement of torque...which I have a hard time wrapping my head around how that relates to linear motion of a projectile.
Foot Pounds, Momentum, Inertia etc...
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Many times people will express kinetic energy in foot-pounds - in that context, a foot-pound is the energy needed to exert one pound of force over one foot of distance. But don't worry about that. Far more important is understanding the difference between what kinetic energy is measuring versus what momentum/inertia is measuring. We could name the units whatever we want... what matters is understanding the difference between the different things they are measuring.
For a bullet that hits with 1000 ft lbs of energy imagine that there were 1000 lbs behind the bullet as it is dropped from a height of 1'.
Ft. lbs. energy CAN be effective to help predict how a bullet will behave after impact IF you are comparing very similar rounds with projectiles of the same construction. It can also be very misleading. If you double a bullets weight and velocity stays the same you double the energy. But if you keep the weight the same and double the speed you quadruple energy. More speed doesn't necessarily mean more effectiveness. None of this considers bullet diameter or construction. If comparing a 223 with a fragile bullet it may appear to be more effective than a 45 caliber moving much slower. But if you are comparing 2 different 30-06 loads with bullets from the same manufacturer and of the same type then the one with more energy is probably going to be more effective.
The greater a bullets momentum the harder it is to stop it once it is in motion. This method always shows the heavier, slower bullet as being more effective. For guys playing shooting games where they have to knock over steel plates, bowling pins or other reactive objects then more momentum is an advantage. In living animals momentum doesn't tell us much.
There is no magic math formula that will accurately predict how well a bullet will perform in living animals. The most accurate tests, and they are far from perfect, involve shooting bullets into ballistic gel and measuring how much they expand and how far they penetrate. This takes into consideration all things including bullet construction.
Quite often we find that too much speed means some bullets over expand and don't penetrate well. Slowing down impact speed results in less expansion, and better penetration. Some bullets need to impact at some pretty fast speeds or they don't expand. Because of factors like this both energy and momentum calculations can be very misleading.
Ft. lbs. energy CAN be effective to help predict how a bullet will behave after impact IF you are comparing very similar rounds with projectiles of the same construction. It can also be very misleading. If you double a bullets weight and velocity stays the same you double the energy. But if you keep the weight the same and double the speed you quadruple energy. More speed doesn't necessarily mean more effectiveness. None of this considers bullet diameter or construction. If comparing a 223 with a fragile bullet it may appear to be more effective than a 45 caliber moving much slower. But if you are comparing 2 different 30-06 loads with bullets from the same manufacturer and of the same type then the one with more energy is probably going to be more effective.
The greater a bullets momentum the harder it is to stop it once it is in motion. This method always shows the heavier, slower bullet as being more effective. For guys playing shooting games where they have to knock over steel plates, bowling pins or other reactive objects then more momentum is an advantage. In living animals momentum doesn't tell us much.
There is no magic math formula that will accurately predict how well a bullet will perform in living animals. The most accurate tests, and they are far from perfect, involve shooting bullets into ballistic gel and measuring how much they expand and how far they penetrate. This takes into consideration all things including bullet construction.
Quite often we find that too much speed means some bullets over expand and don't penetrate well. Slowing down impact speed results in less expansion, and better penetration. Some bullets need to impact at some pretty fast speeds or they don't expand. Because of factors like this both energy and momentum calculations can be very misleading.
Kinetic energy has the same units as "work" and provides the following information:
1. It tells how much work was done accelerating the projectile to the velocity it currently has.
2. It tells how much work the projectile can do if decelerated back to zero.
3. In conjunction with the distance it took to accelerate the projectile to the velocity it currently has, it tells how much total force was applied to the projectile.
4. In conjunction with the distance it will take to decelerate the projectile to zero, it tells how much force will be applied to the target medium. It's worth noting that the force will not all be applied in the direction of projectile travel.
Note that "work" has a specific scientific definition. "Work" is done when a force is applied to move something.
Momentum provides the following information:
1. In conjunction with the time it took to accelerate the projectile to the velocity it currently has, it tells how much force was applied to the projectile.
2. In conjunction with the time it will take to decelerate the projectile to zero, it tells how much force will be applied to the target medium. It's worth noting that not all the force will be applied in the direction of projectile travel.
3. In conjunction with the weight of the firearm, it tells how fast the firearm (or the recoiling mass, if the firearm is not the same as the recoiling mass) will be moving when the projectile exits the muzzle.
1. It tells how much work was done accelerating the projectile to the velocity it currently has.
2. It tells how much work the projectile can do if decelerated back to zero.
3. In conjunction with the distance it took to accelerate the projectile to the velocity it currently has, it tells how much total force was applied to the projectile.
4. In conjunction with the distance it will take to decelerate the projectile to zero, it tells how much force will be applied to the target medium. It's worth noting that the force will not all be applied in the direction of projectile travel.
Note that "work" has a specific scientific definition. "Work" is done when a force is applied to move something.
Momentum provides the following information:
1. In conjunction with the time it took to accelerate the projectile to the velocity it currently has, it tells how much force was applied to the projectile.
2. In conjunction with the time it will take to decelerate the projectile to zero, it tells how much force will be applied to the target medium. It's worth noting that not all the force will be applied in the direction of projectile travel.
3. In conjunction with the weight of the firearm, it tells how fast the firearm (or the recoiling mass, if the firearm is not the same as the recoiling mass) will be moving when the projectile exits the muzzle.
Thanks for the replies. Starting to get an idea of it. So looking at it from the other end, say we have a bullet in motion with 1000 ftlbs of energy. Would it be accurate to think of it as that bullet would move a 1000lb mass 1 foot? (All things being equal, in a vacuum, on a frictionless surface, yada yada yada)
This is theoretical of course, I realize there is a time factor, friction, density of target...all that stuff. Just trying to get an idea of how this translates from torque to inline force.
This is theoretical of course, I realize there is a time factor, friction, density of target...all that stuff. Just trying to get an idea of how this translates from torque to inline force.
[Note: I am not a physicist nor an engineer. Someone with particular training in this may come along to offer a different understanding, and I would defer to those with relevant professional training.]
No, you generally wouldn't use an energy calculation to predict movement of the target object. Momentum is the calculation for that (think of a Newton's cradle toy). Energy is better at predicting disruption/displacement... how big of a splash a bullet will make.
If you're trying to kill something with a big splash (hydrostatic shock/stretch), then energy is a very important calculation. If you're trying to physically topple or push some object, momentum is the calculation to use.
In this reply we strive for a minimum momentum of .75 or minimum KE of 400# - desire a "modern" bullet meeting at least one of those calculations.
A 180 gr. 40 S&W @ 950 fps is about .76 momentum / 361# KE - this bullet meets the momentum criteria.
Other examples:
Glock 19: Federal HST 124 gr. +P @ 1,210 fps / 403# KE
Glock 32 (357 Sig): Federal HST 125 gr. @ 1,358 fps / 512# KE
Glock 22: Speer Gold Dot 180 gr. @ 975 fps / 380# KE - .78 momentum
Glock 21: Federal HST 230 gr. @ 863 fps / 380# KE - .88 momentum
Spoiler: 380 (and less) aint likely gonna make either criteria.
MachIVshooter
Member
It doesn't. Kinetic energy is not force, and you do yourself a great disservice trying to understand KE by thinking in terms of applied force or inelastic collisions.
Energy, as John explained, is the ability to do work. That work as it relates to a projectile includes energy lost in deformation of the bullet, mechanical energy converted to heat through friction, etc.
JMR40 already gave you the simplest way to conceptualize/visualize ft.lbs. of KE. But to be clear, that is a tool to understand, not an equivalent way to demonstrate the energy of a bullet.
In this reply we strive for a minimum momentum of .75 or minimum KE of 400# - desire a "modern" bullet meeting at least one of those calculations.
A 180 gr. 40 S&W @ 950 fps is about .76 momentum / 361# KE - this bullet meets the momentum criteria.
Other examples:
Glock 19: Federal HST 124 gr. +P @ 1,210 fps / 403# KE
Glock 32 (357 Sig): Federal HST 125 gr. @ 1,358 fps / 512# KE
Glock 22: Speer Gold Dot 180 gr. @ 975 fps / 380# KE - .78 momentum
Glock 21: Federal HST 230 gr. @ 863 fps / 380# KE - .88 momentum
Spoiler: 380 (and less) aint likely gonna make either criteria.
What is all that nonsense, and how does it answer the OP's question?
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So here's why I have trouble understanding ftlbs, and it may just be a terminology problem. Take a situation where you have a rusted bolt that is seized up. Put a 12" wrench on it at put nearly your entire body weight into it (about 200lbs in my case), and it doesn't budge. Now you either get a longer handle...or when that wasn't available I have taken a hammer and smacked the end of the 12" wrench handle (I know, not recommended, but sometimes you gotta do what you gotta do). Now, personal experience shows, I can come up from the floor with a an 8oz hammer and smack that wrench handle...nothing happens. But i pop it with a 2lb hammer, and it breaks free. So figure I can swing an 8oz hamer at about 40mph, and a 2lb hammer at 20mph. With that math, those hammers should have the same kinetic energy (check my math, but i think that's right)
Right, either way...I was wrong. Ok, so...the ftlbs is the total energy of the projectile, momentum is the amount of force required to start (or stop) the projectile over a given time. I think that right...I just need a refresher on energy vs work vs force. Thank you guys
denton
Member
It's very confusing because we often use the same name for two entirely different things.
Torque is the length of a lever arm times the force applied to it. So if you have a 1 foot long wrench, and apply 10 pounds of force to the end of it to rotate a bolt, you have 10 foot pounds of torque.
Kinetic energy is something quite different, but it also comes out in foot pounds.
There has been an effort to keep this a bit more straight by referring to one as foot pounds, and the other as pound feet. I forget which is which.
Kinetic energy and momentum are really just different faces of the same thing. With a little calculus, you can convert one to the other. The big difference is that when a bullet impacts, kinetic energy is not conserved but momentum is. At impact, some kinetic energy is used to smash and tear tissue, so following the KE trail is hard. But momentum of bullet + target before impact = momentum of bullet + target after impact, so that trail is a lot easier to follow.
Torque is the length of a lever arm times the force applied to it. So if you have a 1 foot long wrench, and apply 10 pounds of force to the end of it to rotate a bolt, you have 10 foot pounds of torque.
Kinetic energy is something quite different, but it also comes out in foot pounds.
There has been an effort to keep this a bit more straight by referring to one as foot pounds, and the other as pound feet. I forget which is which.
Kinetic energy and momentum are really just different faces of the same thing. With a little calculus, you can convert one to the other. The big difference is that when a bullet impacts, kinetic energy is not conserved but momentum is. At impact, some kinetic energy is used to smash and tear tissue, so following the KE trail is hard. But momentum of bullet + target before impact = momentum of bullet + target after impact, so that trail is a lot easier to follow.
MachIVshooter
Member
Ft.lbs. is a measure of KE, while lb-ft is a measure of force, which includes rotational torque. Keep those units straight for one.
Momentum is the resistance of an object in motion to deceleration; inertia is an object's resistance to acceleration.
Energy is the ability to do work, force is force. Force has a vector, while available energy being converted into work may produce force, heat, pressure.
As for this:
Therin lies an example of energy being converted into work, but there's more in play there than just KE. AS Dave mentioned, momentum is related but different. And there is shock; I can have the full 20 tons of force on my hydraulic press trying to push a hub out of a bearing not budge it, but a hammer impact to the part, even perpendicular to the applied force, will break it loose.
Physics is fun, but there's no simple way to conceptualize much of it in a visible or tangible manner. Take horsepower, for instance, which really is a unit of measure derived from equines. It has a direct conversion to Watts (Indeed, engineer James Watt coined the term horsepower), and is used to describe work done/time. 10 horsepower can accomplish the same work as 100 horsepower, it just takes 10 times as long to do it. Where it gets confusing is in it's relationship to force; 1 HP=550 foot pounds force for 1 second. So does a 240 HP engine produce 132,000 foot pounds force? Of course not. That's where the time component comes in.
John can probably explain it better than I can, but you can't conflate energy, force and power when talking physics they way they are often interchanged in informal speech.
MachIVshooter
Member
There's certainly conservation of energy, it's just not the same conservation as a closed system and very difficult to quantify, especially in terminal ballistics post impact with biological targets. None of the bullet's KE is lost, but converted into work producing penetration though medium, heat, pressure, deformation of target & bullet.
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WestKentucky
Member
Conservation of anything when looking at effect on target is laughable.
Bullets shed weight quickly upon impact. They may retain 90%, or the biggest chunk you find may be 9%. Since mass is a huge part of any calculation on energy the whole point is moot.
Momentum is conserved? Momentum is vectored energy. So if a bullet deflects off of a rib then the vector changes and a lot of energy is lost by that sudden change of direction. If the bullet stops moving it has used all of its energy, so there is none left to conserve.
Penetration is dependent upon so many factors that it is hard to figure out beyond saying that a round is usually going to do X. USUALLY. Everything changes when hits are to bone, soft tissues like organs, or heavy muscle on a shoulder.
What can be said is that upon impact energy is used to impart damage to the target. That can be a hole in a piece of paper, or that can be a wound channel on a mythical beast. Big, heavy, slow typically stays together and goes deep. Small light and fast typically shreds on impact and does a lot of damage in a small, comparatively shallow area. The way a bullet reacts is due to design which is dependent upon energy transfer.
Bullets shed weight quickly upon impact. They may retain 90%, or the biggest chunk you find may be 9%. Since mass is a huge part of any calculation on energy the whole point is moot.
Momentum is conserved? Momentum is vectored energy. So if a bullet deflects off of a rib then the vector changes and a lot of energy is lost by that sudden change of direction. If the bullet stops moving it has used all of its energy, so there is none left to conserve.
Penetration is dependent upon so many factors that it is hard to figure out beyond saying that a round is usually going to do X. USUALLY. Everything changes when hits are to bone, soft tissues like organs, or heavy muscle on a shoulder.
What can be said is that upon impact energy is used to impart damage to the target. That can be a hole in a piece of paper, or that can be a wound channel on a mythical beast. Big, heavy, slow typically stays together and goes deep. Small light and fast typically shreds on impact and does a lot of damage in a small, comparatively shallow area. The way a bullet reacts is due to design which is dependent upon energy transfer.
denton
Member
If momentum is not conserved, then the physics texts are all wrong together.
A bullet impacting an animal is an inelastic collision. Momentum of bullet + target before the impact = momentum of bullet + target after the impact. If it were not so, ballistic pendulums would not work.
It's very hard to model a bullet impact via kinetic energy, as some energy goes into crushing and tearing tissue and breaking bones, some goes into elastic stretching of tissue, and so on. Figuring out the splits and where the energy goes is not easy.
Momentum, on the other hand, is much simpler. The momentum before the collision = the momentum after the collision. This is the basis for calculating recoil.
Tissue has a yield strength. Exert more pressure on it than that, and it will crush and tear. The pressure that a bullet exerts on tissue is proportional to the rate at which it is shedding momentum. When the bullet is no longer shedding momentum above the yield strength of tissue, it will come to rest.
Since momentum and kinetic energy are different faces of the same thing, matter in motion, the one we choose for calculations is simply a matter of which one is easier to handle in the given situation.
A bullet impacting an animal is an inelastic collision. Momentum of bullet + target before the impact = momentum of bullet + target after the impact. If it were not so, ballistic pendulums would not work.
It's very hard to model a bullet impact via kinetic energy, as some energy goes into crushing and tearing tissue and breaking bones, some goes into elastic stretching of tissue, and so on. Figuring out the splits and where the energy goes is not easy.
Momentum, on the other hand, is much simpler. The momentum before the collision = the momentum after the collision. This is the basis for calculating recoil.
Tissue has a yield strength. Exert more pressure on it than that, and it will crush and tear. The pressure that a bullet exerts on tissue is proportional to the rate at which it is shedding momentum. When the bullet is no longer shedding momentum above the yield strength of tissue, it will come to rest.
Since momentum and kinetic energy are different faces of the same thing, matter in motion, the one we choose for calculations is simply a matter of which one is easier to handle in the given situation.
MachIVshooter
Member
Not wrong, just that elastic or inelastic collisions at the atomic level in an isolated system doesn't translate well to bullets hitting animals, where the collision is neither totally elastic or perfectly inelastic, so you end up with partial conservation of both KE and momentum. And because it's an open system subject to external variables, both are lost almost completely in the end when all bodies cease to have any movement or stored energy. Theoretically, some energy and momentum would be conserved as it is converted into heat & pressure outside the animal, the air molecules and particulates having been accelerated by the momentum & conversion of energy into heat & pressure, and being a mass in motion, possesses KE and momentum, but that wouldn't be a measurable amount outside of a controlled laboratory experiment.
That "nonsense" is specific examples of the OP's terms in handgun calibers.
Carry caliber fail to meet desirable minimums?
MachIVshooter
Member
OP is trying to understand the correlation & application of units and measurements in physics, not what calibers meet some arbitrary minimum requirement using a convoluted formula.
WestKentucky
Member
Not true. When all other factors are controlled and the only thing being tested is this particular point, then yes this law of physics is true. Real world, no dice. Same goes for almost all scientific theories/laws. Objects in motion tend to stay in motion...ok, in a vacuum removed from gravity, and not ever touching anything else. So we fire a bullet at 45 degrees angle relative to the horizon and we know it’s going to hit the ground somewhere a mile or three away. Why does it slow down in real life? Speed is a part of energy and it’s supposed to be conserved. Why does direction change to make the bullet come back to earth when we stick satellites into orbit for decades, and the moon has yet to fall? Law of conservation of energy and law of gravity right? But in real world scenarios their practicality is not exactly unlimited. That’s like saying that a 150 grain jsp from a 30-30 is never as good as a 170 gr A frame from a 30-06. Sounds right, but if that 06 is a mile away and the 30-30 is point blank then my money rides on the 30-30 being more effective.
Some things look nice on paper. Some things work on paper. Sometimes that’s where those things need to stay.
The ordinary laws of physics have absolutely no difficulty explaining why a bullet eventually falls to the ground, nor why its velocity declines during its time in the air. This is pretty simple stuff that can be predicted to a very high level of precision.
Terminal ballistics, OTOH, are quite difficult to mathematically model. But the difficulty doesn’t come from the physics, it comes from the biology.
Terminal ballistics, OTOH, are quite difficult to mathematically model. But the difficulty doesn’t come from the physics, it comes from the biology.
denton
Member
This important result is call the law of conservation of momentum. It tells us that, if no external forces act on a system of particles, the total momentum of the system remains constant.
Like the law of conservation of energy that we met in Chapter 8, conservation of momentum is a more general law than Newtonian mechanics itself. It continues to hold in the subatomic realm, where Newton's laws fail. It holds for the highest particle speeds, where Einstein's relativity prevails; it is only necessary to use Eq. 20 rather than Eq. 18 for momentum.
Halliday and Resnick, Fundamentals of Physics
On the land, on the sea, in the air, in space, at all times, in all places, for billiard balls on tables (elastic collisions) or bullets passing through elk (inelastic collisions), conservation of momentum applies and works. It is one of the grand, fundamental laws of physics.
If it did not always work, ballistic pendulums would not be able to give us accurate measurements of bullet momentum.
This is not the only thing in physics that sometimes offends my sensibilities, but it is true nevertheless.
Like the law of conservation of energy that we met in Chapter 8, conservation of momentum is a more general law than Newtonian mechanics itself. It continues to hold in the subatomic realm, where Newton's laws fail. It holds for the highest particle speeds, where Einstein's relativity prevails; it is only necessary to use Eq. 20 rather than Eq. 18 for momentum.
Halliday and Resnick, Fundamentals of Physics
On the land, on the sea, in the air, in space, at all times, in all places, for billiard balls on tables (elastic collisions) or bullets passing through elk (inelastic collisions), conservation of momentum applies and works. It is one of the grand, fundamental laws of physics.
If it did not always work, ballistic pendulums would not be able to give us accurate measurements of bullet momentum.
This is not the only thing in physics that sometimes offends my sensibilities, but it is true nevertheless.
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Just one caution about discussing conservation of momentum and energy. In either case, if the analysis of a collision/impact is sufficiently thorough, ALL of the energy and ALL of the momentum can be shown to have done SOMETHING. In other words, whether we're talking about energy or momentum, none of it simply "disappears" without leaving any traces.
It is certainly much easier to keep track of "where the momentum goes" during a collision since momentum doesn't get converted to other things while energy does, but it's important to understand that although it's harder to keep track of energy, that doesn't mean some of it just "goes away" without doing anything at all.
Power is the rate that Work is done, or the rate that Energy is generated.
Said a slightly different way, Power is the amount of Work done during over a given Time interval or the amount of Energy generated over a given Time interval.
It is certainly much easier to keep track of "where the momentum goes" during a collision since momentum doesn't get converted to other things while energy does, but it's important to understand that although it's harder to keep track of energy, that doesn't mean some of it just "goes away" without doing anything at all.
It's not hard to say various things about power but it is hard (at least for me) to visualize exactly what it means.
Power is the rate that Work is done, or the rate that Energy is generated.
Said a slightly different way, Power is the amount of Work done during over a given Time interval or the amount of Energy generated over a given Time interval.
Energy and torque are related and it is interesting to see how they are related. But for the purposes of understanding projectile motion it's going to be more productive to just ignore the fact that the units of Torque and Energy are similar and realize that they are two different concepts and quantities.
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