denton
Member
Exactly so.
Foot Pounds, Momentum, Inertia etc...
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Exactly so.
WestKentucky
Member
Where, in reality, can this scenario be found? We have gravity as a constantly acting force. Yes things can almost always be explained, but in truth, there’s always gonna be some head scratching (and guesswork as to what anomalies caused the head scratching). This is why perpetual motion machines don’t work.
denton
Member
An example of an external force that matters in this situation would be if just before your bullet impacted, your target started to run. So the force of feet/hooves against the ground imparts momentum to the animal. But the law still holds. You just have to add in one more factor, the momentum from running. The additional momentum is just the integral over time of force from feet/hooves.
When the law talks about force, it means the net vector sum of all forces. So if you're sitting on your chair, there is the force of gravity pulling you down, and the equal and opposite force of the chair holding you up (a force couple). The net vector sum of the forces is zero, so it doesn't matter in considering conservation of momentum.
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MachIVshooter
Member
Ehhhh..........not a good example. The chair exerts no force, it just has the structural integrity to resist the force of earth's gravity pulling on a mass it's supporting up to the point where the mass being acted on by gravity exceeds the limits and the structure fails.
A better example would be anything which generates lift equal to the force of gravity, as in a helicopter hovering, or jet of air holding a person up in a skydiving training tube. Or, better yet, maglev.
Thank you! That's what I wanted to express, just couldn't find the words to do so very concisely or eloquently running on a couple hours sleep thanks to a raging toothache.
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denton
Member
Ummmm.... no.
The upward force of the chair equals the downward force of your butt. It's called a force couple. Were it not so, the net force would not be zero and you would accelerate. Since you can obviously still reach your keyboard, it's fair to assume that you have not accelerated.
Don Dayacetah
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At some point you take it all to the range, or to the field. Shoot a given caliber. Watch the results in gel, cinder block, wood, water bottles, or live game.
This gives you a standard of comparison. These numbers all look great, on a ammo box, or in a reloading manual. Once you get a feel for a couple calibers for
comparison, you can get a general idea of how some other caliber may perform by how the numbers stack.
This gives you a standard of comparison. These numbers all look great, on a ammo box, or in a reloading manual. Once you get a feel for a couple calibers for
comparison, you can get a general idea of how some other caliber may perform by how the numbers stack.
MachIVshooter
Member
I don't know about your chairs, but mine have legs or posts, sometimes attached to wheels, which are in contact with the ground. If they were exerting a force that countered gravity, they would rise with my mass removed. They are an inanimate object that apply no force on their own. The only force exerted by the chair itself is downward under gravity pulling on the mass of the chair itself. Absent gravity, they would have no vector without being influenced by another force.
Swifty Morgan
member
Former physicist here. Listen to Denton.
Torque and energy have the same units. Both can be expressed as foot-pounds (foot/pounds is wrong but sometimes you will see it from people who don't know better).
This doesn't mean torque is energy. Another way of saying it is "mass times velocity squared." In imperial units, that would be something like slugs times (feet per second)-squared. It's a bit bizarre that foot-pounds would be used for energy. There is probably some creaky old explanation for it.
Momentum is mass times velocity.
Torque and energy have the same units. Both can be expressed as foot-pounds (foot/pounds is wrong but sometimes you will see it from people who don't know better).
This doesn't mean torque is energy. Another way of saying it is "mass times velocity squared." In imperial units, that would be something like slugs times (feet per second)-squared. It's a bit bizarre that foot-pounds would be used for energy. There is probably some creaky old explanation for it.
Momentum is mass times velocity.
Ok... so setting aside the concept of torque...we fire a bullet. The bullet, having mass, requires a force applied to it to overcome its inertia. The amount of force used to accelerate the bullet over time is work. Once the bullet has accelerated, it carries kinetic energy (measured in ft lbs) the bullet is traveling in one direction (vector) over time, so it has momentum. That bullet strikes at target, its kinetic energy is transferred to the target over the time of deceleration. Assuming the bullet stops in the target, during its deceleration, the bullet applies a force (newtons) upon the target in the direction of its momentum (kilogram meter/ second).
The payoff is how much work that bullet can do in the target...which is a function of how much energy it started with, how much time did it take to decelerate, and how that energy was converted into motion, change of vector, heat, deformation ect. But biological targets, being an open system and terribly inconsistent, makes calculations of terminal ballistics pretty much a nightmare. Is that pretty much the deal?
The payoff is how much work that bullet can do in the target...which is a function of how much energy it started with, how much time did it take to decelerate, and how that energy was converted into motion, change of vector, heat, deformation ect. But biological targets, being an open system and terribly inconsistent, makes calculations of terminal ballistics pretty much a nightmare. Is that pretty much the deal?
Uh, no. Perpetual motion machines don’t fail because of head-scratching anomalies. They fail because of thermodynamics - usually friction is the specific problem.
You keep talking about this stuff as though it’s deeply mysterious. It’s not.
MachIVshooter
Member
The SI unit is also distance-force/weight-time; the Newton is a force measurement just like the pound (or poundal or pound-force, depending on context & application).
MachIVshooter
Member
Are you deliberately mixing EE and SI units?
But yes, that's basically it, although I would caution against using the term "energy transfer", as it's really energy conversion.
Terminal performance is something theoretically calculable, but there are innumerable variables where living organisms in a real-world environment are concerned, which is why we pretty much just accept that there's no way to know exactly what a bullet will do inside a critter.
denton
Member
LOL.... OK, one more try.
Force = mass x acceleration, right? If we can't agree on that, then any other discussion is futile.
And in that equation, force is defined as the vector sum of all forces operating on an object or collection of particles. Once again, if we don't have agreement on that, any other discussion is futile.
If we agree on that, then the question is, are you accelerating away from your computer? If you can still reach the keyboard, the answer is probably no.
If you are not accelerating away from your keyboard, then by the F=ma law, either your mass must be zero, or the vector sum of all forces acting on you must be zero. Or maybe both.
Your mass is not zero, or you couldn't type.
The only remaining possibility is that the vector sum of all forces acting on you is zero.
You already realize that gravity is acting on you. If it is not as I have described, what force is counterbalancing gravity?
Not deliberately, admittedly, this whole post is born of my confusion of the terminology
Swifty Morgan
member
I have no idea what SI is. When I was doing physics, we used cgs and mks, and that's what everyone called those systems. Cgs units turn up in electrodynamics, and I was told it was because of the short distances involved. For all I know, engineers use a system called SI, or maybe SI means "imperial." I am too lazy to Google it. Physics and engineering are not the same thing, and the conventions are different. Physicists and engineers don't even use the same letters for unit vectors in 3-space. I didn't study engineering.
No physicist would use a unit that looked like "distance-force/weight-time." That's a new one on me.
If you were to put a unit of weight under a unit of force in a quotient, they would cancel, because they're the same thing, and you would end up with distance/time, which is the magnitude of velocity. There would be no reason to put identical units in a numerator and a denominator. It's a waste of time to divide Newtons by Newtons.
Physicists don't consider "weight" a unit or dimension. Weight is expressed in terms of force, and in mks, that's Newtons. Weight is not an important concept in mechanics, and instructors discourage students from thinking in terms of weight, because they confuse it with mass, which has nothing at all to do with weight. An object that weighs a thousand tons on earth weighs nothing in space, even though it has the same mass. Physicists talk a lot about force and mass, but they don't say much about weight.
Just listen to Denton.
Swifty Morgan
member
As long as I'm here, energy increases with velocity much faster than momentum. Momentum varies with velocity, while energy varies with the square of velocity. A bullet moving twice as fast has twice the momentum but four times the energy. This is why little bitty rifle bullets moving at 3000 fps are nastier than great big pistol rounds doing 800 fps. They have huge amounts of kinetic energy.
That's...not right. At all. Foot-pounds can be torque or energy, but it is impossible to express force in foot-pounds. Force is part of torque, but force is not torque. The phrase "rotational torque" doesn't really make sense, since torque itself implies a center of rotation.
The real definition of torque is the cross product of two vectors. The first is the displacement vector from the chosen center of rotation, and the second is the momentum vector, which is generally mass times the velocity vector. You have to have a center of rotation in order to have torque.
In case anyone asks, the rotation of a bullet is not considered when the energy is calculated.
It's very unfortunate that ballistics people chose foot-pounds to express energy. It suggests a lack of understanding of the basics of physics. The kinetic energy of an object in translation is 1/2 (mass*(velocity squared)). To anyone familiar with second-semester calculus, this is obviously the result of performing an integral. It's the integral of momentum with respect to velocity. That makes sense to anyone who does physics. The use of "foot-pounds" does not.
Torque is expressed in units like foot-pounds, and torque has nothing whatsoever to do with the kinetic energy of moving bullets. The imperial unit for mass is the slug, and velocity is expressed in feet per second. It would make more sense to talk about energy in these units.
I found a good essay on kinetic energy v. momentum, and it's written on a level lay people can understand if they try. It even includes material on firearms.
https://www.real-world-physics-problems.com/difference-between-momentum-and-kinetic-energy.html
That's...not right. At all. Foot-pounds can be torque or energy, but it is impossible to express force in foot-pounds. Force is part of torque, but force is not torque. The phrase "rotational torque" doesn't really make sense, since torque itself implies a center of rotation.
The real definition of torque is the cross product of two vectors. The first is the displacement vector from the chosen center of rotation, and the second is the momentum vector, which is generally mass times the velocity vector. You have to have a center of rotation in order to have torque.
In case anyone asks, the rotation of a bullet is not considered when the energy is calculated.
It's very unfortunate that ballistics people chose foot-pounds to express energy. It suggests a lack of understanding of the basics of physics. The kinetic energy of an object in translation is 1/2 (mass*(velocity squared)). To anyone familiar with second-semester calculus, this is obviously the result of performing an integral. It's the integral of momentum with respect to velocity. That makes sense to anyone who does physics. The use of "foot-pounds" does not.
Torque is expressed in units like foot-pounds, and torque has nothing whatsoever to do with the kinetic energy of moving bullets. The imperial unit for mass is the slug, and velocity is expressed in feet per second. It would make more sense to talk about energy in these units.
I found a good essay on kinetic energy v. momentum, and it's written on a level lay people can understand if they try. It even includes material on firearms.
https://www.real-world-physics-problems.com/difference-between-momentum-and-kinetic-energy.html
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WestKentucky
Member
Perhaps poorly worded. Gravity pulls two parts together which move, therefore friction is an issue. And some people keep repeating the same drivel that is taught as fact which is absolutely true but is also absolutely worthless. Energy is conserved...ok you want X to happen so whatever system you derive makes X happen, but there is ALWAYS a trade off somewhere in the mix. Even in chemistry, often a thermal piece of the equation uses up some of the potential. Ok you will argue that, so let’s look at electronics...wires heat up, and you argue that... my point isn’t that these things aren’t real or that they aren’t present, my point the whole time is that they just don’t make a difference in real world scenarios. There is ALWAYS an outside force messing up at least some of whatever is being done. Whack a golf ball to test the theory of gravity and yes your ball falls back to earth, but wind resistance also plays its part. Fire a projectile through air at a deer or a piece of paper and your bullet has so many variables acting on it that there truly is no “constant” because you can’t actually control the real world. And the dead scratching comes in when some wiseacre tries to explain every little bit of minutia by grasping at random theories and other such malarkey when they should just chalk up a minuscule difference to factors that they don’t understand or that aren’t well enough controlled.
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MachIVshooter
Member
So your contention is that the chair exerts a force equal to earth gravity but variable based on the mass upon it. Which means that in zero gravity, my chair will accelerate me at 32 FPS per second. Good to know if I ever find myself in space with exhausted thrusters in my suit but happen to have a chair!
Snarky levity aside, I get the subatomic physics behind an object having force applied in one vector remaining stationary has to have an equal force in the opposite vector acting upon it, as those subatomic particles can't be supported, blocked or otherwise prevented from moving by any physical object since they quite literally slip between atoms. But I disagree that an object which is able to resist acceleration has to have any force of it's own. If you walk up and try to push a 5 ton road divider, it's not going to move, but that isn't because something is pushing back on the other side; it's because us puny humans can't overcome the inertia of such a heavy object, especially when we consider the friction of it's mass at earth gravity. Similarly, something supporting a mass, be it my bones, my chair, the floor or an overpass pylon, isn't acting against gravity with a load-adaptive variable force; it's able to resist deformation up to a certain amount of force, which would be the weight of a mass under the force of gravity. That's why we rate things as being able to support X amount of weight, or so many pounds per square inch, etc. The legs of this chair can support probably 300-350 lbs, which is no problem resisting the 190 pound weight of my body under the gravitational force of earth. If I try to it on this chair on Jupiter at 2.5 times earth gravity, the legs will snap. I don't know of anyone but you who would say that it's because the legs can't exert enough force; we'd just say it was too much weight, exceeded the structural limit.
So, yes, we agree that F=ma, but I still do not consider resistance to acceleration under force to be the result of equal and opposite force without exception.
Systeme International; international system of units. The metric system.
That's exactly what MKS is (meters-kilograms-seconds). The EE (Imperial) equivalent is FPS, feet-pounds-seconds.
Which is one reason I have always wondered why, in general, the metric system uses a mass unit for weight measurements. I guess lucky for us it's a pretty constant value on earth and not many scales floating around in space to confuse people.
denton
Member
Oh dear....
Yes, SI stands for System International, what us old guys used to call the metric system. And within that, you had the choice of working in centimeters-grams-seconds, or in meter-kilogram-seconds. That's the cgs and mks systems correctly mentioned earlier.
Try this link if you like: https://www.livescience.com/46561-newton-third-law.html
Pushback
Forces always occur in pairs; when one body pushes against another, the second body pushes back just as hard. For example, when you push a cart, the cart pushes back against you; when you pull on a rope, the rope pulls back against you; and when gravity pulls you down against the ground, the ground pushes up against your feet.
Yes, SI stands for System International, what us old guys used to call the metric system. And within that, you had the choice of working in centimeters-grams-seconds, or in meter-kilogram-seconds. That's the cgs and mks systems correctly mentioned earlier.
Try this link if you like: https://www.livescience.com/46561-newton-third-law.html
Pushback
Forces always occur in pairs; when one body pushes against another, the second body pushes back just as hard. For example, when you push a cart, the cart pushes back against you; when you pull on a rope, the rope pulls back against you; and when gravity pulls you down against the ground, the ground pushes up against your feet.
It is a standard concept in physics and is a consequence of Newton's first law.
If an object isn't moving there are only two options. Either there are no forces acting on it or the forces acting on it are balanced.
Since gravity exerts a downward force on any object with mass, if that object is not moving downward there must be an upward force being exerted on it that exactly counters the force of gravity.
https://www.physicsclassroom.com/class/newtlaws/Lesson-1/Balanced-and-Unbalanced-Forces
https://study.com/academy/lesson/balanced-forces-definition-examples.html
http://www.csun.edu/~psk17793/S9CP/S9 Balanced_and_unbalanced_forces.htm
Passive forces (such as the "Normal" force applied by a chair to resist the downward gravitational force of the object resting on it) are not as intuitive as active forces but they do exist.
http://people.virginia.edu/~ecd3m/1429/Spring2011/manual/Lab06.pdf
Swifty Morgan
member
There is force, and then there is net force. If your chair holds you up, the net force is zero. That means the chair is applying force to you, to counteract the force you exert with your weight. When the net force is not zero, there has to be movement.
The better question is why the imperial system equates weight with mass. It works here on the earth's surface, but it doesn't make a lot of sense. When you buy strawberries, you want to know how much mass you're getting. If metric people were like us, they would buy strawberries by the Newton.
A pound of strawberries isn't really a pound of strawberries. It's 453.6 grams of strawberries.
The better question is why the imperial system equates weight with mass. It works here on the earth's surface, but it doesn't make a lot of sense. When you buy strawberries, you want to know how much mass you're getting. If metric people were like us, they would buy strawberries by the Newton.
A pound of strawberries isn't really a pound of strawberries. It's 453.6 grams of strawberries.
denton
Member
Bersaguy....
I think you deserve a short synopsis that answers your question a little more briefly than this thread has done:
Pounds (English) and Newtons (SI or metric) are measures of force. Something that weighs a pound has a downward force of one pound acting upon it. Similarly, in the SI system you might say that an object weighs a Newton. As has been pointed out, weight is a poor standard for measurement. There is roughly a 1% variation in what an object will weigh at different points on Earth, let alone off the Earth. That's half a grain in a 50 grain powder charge. That's why scientists (and European reloaders) normally use mass, which is the same everywhere. Weight = mass x acceleration of gravity, about 9.8 m/s/s or 32 f/s/s. Mass is measured in grams and Kilograms. Balance scales measure mass. Digital scales measure weight.
Energy can be measured in foot-pounds. If you slowly lift a 1 pound object 1 foot, you have done 1 foot pound of work, and the object has 1 foot-pound of potential energy. If you then drop the object, it will land with 1 foot-pound of kinetic energy (conservation of energy). The SI unit of energy is the Joule, which is 1 Newton-meter, which is also 1 watt-second (one watt of power for one second).
Torque happens to come out in very similar units, but it's not at all the same thing as energy. Torque is defined as lever arm length x force applied at right angles to the lever. A 1 foot lever, with a 1 pound force, does obviously come out as pounds x feet. And a lot of people call it foot-pounds. There are those that simply put up with this confusion, and others that insist that the correct units for torque are pound-feet. So if it's confusing to you, you are far from alone.
After all this, I hope that satisfactorily answers your question.
I think you deserve a short synopsis that answers your question a little more briefly than this thread has done:
Pounds (English) and Newtons (SI or metric) are measures of force. Something that weighs a pound has a downward force of one pound acting upon it. Similarly, in the SI system you might say that an object weighs a Newton. As has been pointed out, weight is a poor standard for measurement. There is roughly a 1% variation in what an object will weigh at different points on Earth, let alone off the Earth. That's half a grain in a 50 grain powder charge. That's why scientists (and European reloaders) normally use mass, which is the same everywhere. Weight = mass x acceleration of gravity, about 9.8 m/s/s or 32 f/s/s. Mass is measured in grams and Kilograms. Balance scales measure mass. Digital scales measure weight.
Energy can be measured in foot-pounds. If you slowly lift a 1 pound object 1 foot, you have done 1 foot pound of work, and the object has 1 foot-pound of potential energy. If you then drop the object, it will land with 1 foot-pound of kinetic energy (conservation of energy). The SI unit of energy is the Joule, which is 1 Newton-meter, which is also 1 watt-second (one watt of power for one second).
Torque happens to come out in very similar units, but it's not at all the same thing as energy. Torque is defined as lever arm length x force applied at right angles to the lever. A 1 foot lever, with a 1 pound force, does obviously come out as pounds x feet. And a lot of people call it foot-pounds. There are those that simply put up with this confusion, and others that insist that the correct units for torque are pound-feet. So if it's confusing to you, you are far from alone.
After all this, I hope that satisfactorily answers your question.
harrygunner
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They both reduce to the same units.
I won't try to correct all the stuff in this tread. 'denton's post #47 above does a good job of answering the original question.
MachIVshooter
Member
I know you said as much, but just to clarfiy, digital or mechanical weight scales reading out in grams or kilograms will have the same small error at different locations as those reading ounces or pounds. A reloader's balance scale reading in grains measures mass same as it would with metric units.
There's more distinction with Imperial, pounds being used as weight (I've never heard anyone who understood the difference refer to the pound as mass) and the slug for mass. Metric should be using Newtons for weight and grams for mass, but how many people have ever expressed their weight in Newtons?
Since a pound of strawberries will have denoted on the container "net weight", usually abbreviated to "net wt", technically the gram mass measurement below it is incorrect.
The tiny error that exists around the world just isn't enough to justify measuring mass with expensive, cumbersome balance scales. Can you imagine truck stops and weigh stations moving around giant counterweights with cranes?
Exactly, only momentum is conserved in collisions. The confusion about kinetic energy and momentum within the shooting community is a legacy of mass marketing by inprint gunwriters. For more than a century, Gunwriters have attempted and continue to attempt to make lethality a precise science, Sort of like that dismal science, Economics. If you examine economic literature, the majority of what you read are mathematical models based on neo classic economics.
If anyone did not notice, neo classicist mathematical models did not predict the Great Recession of 2008 and they are not going to predict the next recession or depression. Economists and gunwriters both have the same problem, they want to predict the future, given some simple inputs, and both camps have failed totally.
I remember when the Hatcher Index was dogma in the IPSC community. Hatcher created a simple model, based on momentum, added a shape factor for the bullet. The 45 ACP came out of to 20 on the revised Hatcher models, and is the baseline for determining what is a “major” round, lesser recoiling rounds are dismissed as "minor". Hatcher assumed that momentum was correlated to lethality, but never tested his assumptions in any scientific manner to determine if his intuition was correct. (It was not) You can find Hatcher’s stopping power table on page 432 of his Textbook of Pistols and Revolvers The 22 lr has a “relative stopping power” of 3.0, the 45 Colt, a 79. As an example of the pseudo science in his book, on page 416 there is a picture of the holes different calibers make in lead. All very interesting and all, but, are animal, or human tissue, like lead? Is any divot in lead an accurate predictor of lethality in a mammal?
The Kinetic energy school has been dominant among gunwriters primarily because it favors velocity. This is typical:
Weatherby promoted high velocity bullets, and according to Weatherby, you just have to hit the animal somewhere, anywhere, and that “wallop” does the rest. Kinetic energy is easy to calculate, Gunwriters have created chapters of tables, ranked by kinetic energy, and animal type. It takes this much kinetic energy to kill this size of animal, more for this size. Of course, it is all nonsense and drivel, but it evolved from the problems gunwriters have with selling new rounds which are essentially identical to existing rounds. It is a lot easier to increase velocity, and hence KE than it is to increase momentum. Momentum is mass times velocity, it takes dramatic increases in velocity and/or mass to increase momentum significant ally. Conveniently, kinetic energy is mass time velocity squared. A little velocity increase hugely increases kinetic energy, because the velocity number increases the product by the square. A small increase in velocity, the KE number increases massively, which makes the difference between essentially identical cartridges, appear significant. The end result: more profit for the firearms industry!! And it is within that contrived difference that gunwriters have convinced us that newer, faster, cartridges are so much better than those old and slow cartridges already out on the marketplace. And it works like a charm, every time.
In terms of lethality, this is worth looking at:
Wound Ballistics Everything you ever wanted to know and didn't know what to ask
https://www.thehighroad.org/index.p...ed-to-know-and-didnt-know-what-to-ask.833872/
Hummer70:
Martin Fackler https://en.wikipedia.org/wiki/Martin_Fackler and other lethality testers came to the conclusion that KE was not a measure of lethality. A simplified Fackler statement would be, if it lives and breathes, if you make it bleed enough, it will stop breathing. Fackler's papers are very positive about big through holes. You can read Fackler's papers, he nicely dismisses the knock down theories of in print gunwriters, saying in one article, they are nothing but advertising. If you research this topic in enough vintage popular press articles, the further you go back in time, the more incoherent they become, not merely from the current physical world, but from each other. Mass marketing is after all, a type of propaganda, another form of deception. What is presented does not have to be true, does not have to be consistent, it only has to generate profit now.
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