I think we can walks. Energy is by definition the capacity to perform work and work is defined as force applied over a distance. So KE for an arrow is a measure of the capacity of that arrow to apply a force over a distance. The more KE you have, the greater the potential force can be applied and/or for a greater distance. Clearly an arrow that carries 50 ft/lbs of ke can apply 50 pounds of force over a distance of 1 foot or 100 pounds for 1/2 foot. When penetrating some medium the force required to "push" the arrow through is a function of many complex factors so an attempt to calculate the force required to push through a deer would be daunting even if you knew the precise path the arrow would follow. In homogeneous mediums where the resistance to penetration is predictable however KE can be used to precisely predict the depth of penetration. In fact given the resistive force of the medium to the penetrating object, KE is the definition of how far it will penetrate. If for example the medium puts up 50 pounds of resistance then an arrow carrying 50 ft/lbs of ke at impact will penetrate precisely 1 foot. Even on a deer though we know without question the more KE you have means more potential penetration.

Just for the sake of expounding a bit. Penetration as a function of KE is not continous. There are at least 2 major disruptions in the function. When an object penetrates a medium at relatively low speed, the molecules of the material are stretched out of the way. The faster this takes place the greater the force of resistance. At some point the force becomes great enough that the molecules begin to break rather than stretch and there is a sudden change in the up to that point continuous funtion. When the molecules break off they are pushed ahead of the penetrating object and the force increases. This is basically the difference between the way bullets penetrate and arrows penetrate. Arrows penetrate because they are sharp and are very good at stretching or cutting the molecules at realatively slow speed leaving a hole that is clean and tends to seal back on itself. Bullets tend to "blast" there way through shearing material and leaving a round realatively ragged hole. If the force holding the molecules together can not be overcome by the force applied by the penetrating object then the penetrating object either breaks apart or bounces off or some of both. This is where momentum comes in to play. By keeping the energy high as a result of mass rather than speed, the point at which the penetrating object bounces off is increased relative to ke. The time it takes for the molecules to stretch out of the way is increased. This is why it is often stated that kinetic energy as the integral of force over distance is a beter predictor of the depth of penetration while momentum as the inegral of force over time is a better predictor of whether or not the object will penetrate at all. These concepts also explain why the arrow wins in the famaous arrow vs bullet shot into a sand bag contest.

This little explination is far from an in depth discussion of the mechanics of penetration but the concepts are simple enough to be usefull. They are not analogous to vortex shedding either. When penetrating a gas the rules change. They are also somewhat differnt when penetrating a liquid.

I'll still go back to my stated position regarding this matter though and that is that the arrows ability to penetrate is a function of its mass and velocity. If you want to multiply these 2 things together in differnt ways and say one way is more important than the other then fine, I'm not going to argue about it. It still comes down to if you want more penetration potential then put on a heavier arrow.

I believe it was the Razorback broadhead. You know the 5 blade head that rotated. The purpose of allowing the head to freely rotate on the shaft was that at impact the head would stay in a straingt path with the rotating arrow driving it. Theoreticall for more penetration. Doesn't seem that it would make that much difference to me but I really don't know.

I had thought for years that an arrow stopped rotating once it hit something, especially with a fixed blade head,, Not so! There must be plenty of rotational energy in a speeding arrow to do what it does in my solid foam back stop. Bullets blow up on impact sometimes due to the very high rotational speeds, if they are too lightly constructed for the speeds they are loaded to. This is a desirable trait with prairie dog bullets. I used to do a LOT of varmint hunting and reloading and until you do,, it's hard to beleive just what that little 25 or 50 grain pill will do

I wonder how many inch pounds of torque an arrow expends into a target??

I don't know what the rotaional rate is but I know that on slower bows you can see the fletching turn on its flight to the target.

Just thinking out loud...

It might be possible to get a rough idea from your foam target. If we knew the (1)velocity at impact (2) the depth the arrow penetrated) and (3) the number of turns when you pull the arrow out (assuming it follows the same path in I think we could get a pretty good estimation.

Another thought is that it can not exceed the total energy loss after the initial launch. As the rotational energy comes from the energy of the forward motion, then if you have 60 ft/lbs at launch and 55 when you hit the target then the rotaional energy must be less than 5 ft/lbs. If for example we then measured 4 ft/lbs rotational from the "walks with a gimp foam target experiment" we could conclude that 1 ft/lb of energy had been converted into other losses like friction and heat.

And another way at the answer might be to measure the ke of the helical fletched arrow at say 30 yards and then a straight fletched arrow of the same type at 30 yards. Assuming the straight fletch flys without rotating and the helical rotating then the differece measured in ke should be the answer. I think this would actually be more accurate than trying to measure the turns in your foam target. It seems to me that somebody somewhere must have done this experiment. Come to think of it anybody have access to a good arrow ballistic calculator? My bet is you could run these 2 conditions and anwer walks with a gimps question.

My personal opinion is that it is far more complex than that. For one thing, if an arrows direction changes, even slightly, it's penetration potention is sucked right out of the arrow because of friction on the side of the broadhead, shaft and fletch. This is why it is imperative that an arrow's path remain straight. It obviously won't travel very far if it starts moving sideways, regardless of how high the K.E. is. The loss in penetration when the path is changed, is far more drastic than the reduction in penetration from a bullet going sideways. So, when it hits the animal, the arrow's penetration potential will depend somewhat on it's K.E. but only if it's momemtum is high enough to insure a straight path. When is meets resistance (it always does) it will penetrate best when it's momentum is high enough to keep the path straight. This is why I don't worry at all about K.E. and only a little bit about momentum. Because of how an arrow is designed to work and because of the medium it's hitting (lubricated flesh, with some bone). Blade sharpness is still tops in my opinion when considering factors critical to penetration. I also consider a high F.O.C. somewhat important in overcoming side friction and keeping the path straight.

I think you are absolutely correct Straightarrow! This subject is very complex and simple explinations never cut it. There are gaping holes in all of them. An in depth discussion of penetration would fill volumes. My explination was of course a simple explination kind of with the old "all other things being equal" concept. Penetration is a fucntion of mass and velocity but of course its a function of a lot of other things too. The complexities you talk about are all valid and important. MORE important in my opinion. Like I said in my explination, there are "at least 2 major disruptions" in the function.

I guessed that it probably was. The only problem is that when building arrows of different weights, it's very difficult to keep thing equal. Althoug K.E. changes very little with different different weight arrows, the penetration potential can change drastically in some situations when increasing momentum, spine or F.O.C.. It would be my contention that very few people are limited by K.E. if they have adequate momentum, spine, F.O.C., blade design, blade sharpness, etc. Those that are truely limited by low K.E., would be better off paying close attention to almost everything else. In ohter words, take what working potential the arrow has, and put it to work in the most effective way.

WWAG, of course an arrow's energy is KE. This from Sylvan: Read that again, carefully. KE is the measure of CAPACITY to do work. It doesn't do the work itself. In our case, with arrows, it is momentum that does the work, using KE's capacity to do that work. Think on that definition. KE is capacity and that is all it is. A glass has a certain capacity to hold an amount of beverage. It doesn't drink the beverage itself though. That is similar to the relationship between KE and momentum.

Momentum also regulates how that KE is used in relation to time. Using that same arrow Sylvan used in his example, the one pushing a 50 pound object one foot. Momentum determines how long it will take to move that object. The lower the momentum, the faster it's moved. The higher the momentum, the slower it's moved. Which means, the lower the momentum, the faster the energy is expended. The higher the momentum, the slower energy is expended. With our arrows, We don't want our arrows spending up their energy fast. That's what has happened when you don't get a passthrough... The arrow has simply spent all it's energy too rapidly. In fact, if we want a complete passthrough, it would be ideal if they didn't use up any energy at all during penetration! Anyway, it does take an amount of time for the arrow to hit and go through the animal. So we want our energy conserved over a longer period of time. We want to arrow to spend it's energy slow enough to get through the animal before the energy runs out.

The greater the momentum the longer it takes to drain KE's capacity as the arrow is penetrating a target, animal, or just thin air in flight. So again, like I said many pages ago, if you use a light, low momentum arrow, you've got to be sure to give it enough KE to do it's job. If you cannot give the arrow a lot of KE, it's best to go with a heavier arrow to increase it's momentum, and make it more efficient with what KE you've got available. A heavy, high momentum arrow from a longbow with much less energy can achieve exactly the same amount of penetration as a high energy, low momentum arrow from a compound. A low energy arrow from a light draw weight compound can do the exact same thing, the exact same way.

Tradeoffs. Doesn't matter what aspect of the sport you want to look at, archery is all about tradeoffs. With the same bow, there's hardly any tradeoff with KE due to arrow weight. It's the tradeoff between trajectory and momentum that gets people in trouble. Those that insist on flat trajectory are preoccupied with KE. Since they have to use very light arrows to get their very flat trajectories, then they have traded off momentum to achieve their goal. So, they are absolutely right and correct to be preoccupied with KE. Without much momentum, they've got to have a lot of KE. Problem is, they've either forgotten or never knew WHY they need so much KE. Many of them are so obsessed with KE that they totally disregard momentum and hate to even hear the word mentioned. Sad state of affairs.

Arthur, I was unable to communicate with you on a far simpler subject. You have consistently either misunderstood or intentionally twisted what I have had to say either because of your obsession with momentum or simply a desire to argue. I'm not going to argue about this or explain how you have misunderstood what I wrote or try to explain how you are incorrect. It would be fruitless. My explination was a simple textbook explination. Though far from a complete discussion on the mechanics of penetration, what was said was accurate.

BTW, both KE and momentum exist only in the minds of men! The are both nothing more than mathmatical tools or "descriptions" or "measures" to aid in our understanding of the real events consisiting of and the result of mass in motion.

I agree again though I would like to point something out. I did some calculations...If we start with a bow that throws a 300 grain arrow 300 ft/sec and therefore 60 ft/lbs ke and 0.4 slug ft/sec momentum and we then shoot a 600 grain arrow from it... if the assumptions is 125 grains virtual mass then the 600 grain arrow will be moving at 229.7 ft/sec and carry 70.3 ft/lbs ke and 0.61 slug ft/sec momentum. This is a 17.2% increase in KE and 53.1% increase in momentum. Now some will argue that these numbers show that momentum is the more important player. Right? The only problem is that it is the same 600 grain arrow! All this really says is that in this case, a 17.2% increase in KE buys you exactly the same increase in penetrating potential as a 53.1% increase in momentum. In other words, don't be confused when someone points out that momentum numbers change differetly than KE numbers. Clearly the 2 aren't directly comparable.

Staightarrow, I don't mean to imply that was what you were saying. What you said just made me think of this.