LBR-

Out of cuiosity run the numbers for the 630 grain arrow from the 85 pound bow, and see what comes up, that would be interesting.  My guess would be something that shoots faster than both, but with momentum between the two.

Man, 2 people posted in the time it took me to post once...

Completely understand about light/fast taking more energy to maintain its physical properties, O.L. never doubted that.  The amount of energy needed for velocity is exponentially related.  Just a tad more speed takes a lot mroe energy.

Jak, Agreed. Problem is for a given bow, you don't have more energy. You got what you got with out going to a different bow that is either more efficient or more draw weight. To shoot an arrow at 300fps requires 4 times more KE then it does to shoot it at 150 fps....O.L.

LOL!

The things we can get wound up in between hunting seasons!  I've never had an animal complain yet.

Good luck all, and I wish, I wish, I wish I didnt have to wait till Sept to hunt again----Them carps better look out in May!

Ryan

There was a good article in one bowhunting magazine this month about year round hunting.  Of course, most of us can't afford just to jaunt up and hunt black bear, mountain lion, or moose...

I should clarify, the 85@30 is what I have on order, so I had to make an educated guess at the fps on it, based on the bow I have now (same style and length).  By the same token, I can't shoot the 630 grain arrows through it to see how many fps they will get.  They wouldn't fly worth squat anyway........

I don't mean to stir the pot over what is right and what is not, I'm more looking to compare what has worked to what I plan on using.  Very interesting discussion nontheless.

Chad

I agree OL, but who has only one bow?  YOU of all people.

To make maters more interesting there is often an I-only-have-one-arm-thing going on here where people argue from the fixed point of their strength at any given time, while ilustrating many different outcomes possible from a universe of potential bows traditional or not.

The Momentum/KE thing goes way back past Holt, and eventualy past archery into discusions relative to various external ballistic in firearms.  I'm sure it was discussed in earlier archery times also, though I haven't ripped over it.  With firearms we had the mamoth black powder age, vs shooting Big Five witht he earliest smokeless military lightweights. Then we hade the Nitros, and after them the magnums, and the beat goes on.  It's a complex subject since impact energy is far moreso the killing force with guns than arrows, however at a certain point they become super charged field points dependant for killing power on penetration type drill wounds rather than nock down power.

These arguments got rolled over without reason into archery where the real truth needs to model the cutting power of the whole arrow, at least as much as any KE or Momentum. You can't go to ultimate KE arrows without loosing some column strength also.  

The facts are that there isn't any clean comparison between KE or momentum, neither fully encapsulates the event that will take place when whatever arrow impacts whatever structure.  It all starts with field testing, from which an engineer, not a Physicist will have to work backwards plunking in whatever values make retrospective sense in whatever situations.

So my math teacher was right,you do actually find a use for this stuff.    

I think the important thing to remember is that you match your equipment to the game.Ol is right,by design a particular bow will only produce so much energy.What we do with that is up to us.Bow efficency,how much energy is transferred to the arrow,goes up with arrow weight.If I'm heading out after mule deer or pronghorn out west I would opt for a middle of the road arrow that gives reasonable trajectory and still maintains enough energy down range to do the job.

If I'm like Chad and fixin to shoot a big tough critter,I want a heavy arrow to maximize penetration.I tend to think of momentum as the ability to continue forward when meeting resistance.A heavier arrow with more momentum will retain more energy and bleed that energy off slower than a lighter arrow with less momentum.I think there is a reason some relatively low KE set ups achieve good penetration in game.Heavy arrows with a lot of momentum and a low friction 2 balde broadhead.I think that was the consensus of Ashbys study.

I read the ozbow thread on momemtum.The idea that KE is the total amount of energy in all directions and momentum the linear or straightline energy is interesting.I'm not sure about that idea, it's beyond my depth of physics.

Of course there is one thing we can all agree on,if that arrow is'nt put in the right place it won't matter.    

CB

you got it....billy...marco #78 ...lets go shoot

I have noticed one thing that all of the KE vs Mommentum threads have in common, they give me a headache. I purposely try to avoid thinking about it at all costs, since it is just numbers and dosn't really tell you how well your arrow is going to penetrate. Yes, I know that in theory, two otherwise identical arrows,blah, blah blah. Did you ever try to make two identical arrows that only vary by weight, or spine? Human beings love to have things in black and white in front of them, so they can say, "ah, this is better", but animals are not impressed with numbers. Montys 1500 grain buff arrows, do really well on penetration even though they have a KE of only 43, but he uses long narrow heads and the shafts are very small diameter. The compound guys are very obsessed with the KE numbers and tend to use to light of arrows to get that number up. IMHO 900-1000 grains is a good compromise for most guys going after buffalo, if well tuned, it will give plenty of penetration and the tradjectory is not severe, as long as you get enough experience with it. You won't be taking long range shots at unwounded buffalo anyway. Rick

Kinetic Energy and Momentum are two distinct and separate units of measure.

KE does not apply to penetration.

It is illogical to use KE as an indicator of bow/arrow performance on an animal.

Bogsucker, OK, can I film you going after a big buff with a 40 pound bow?? From a distance of course!    ....O.L.

NO. The issue being Momentum vs Kinetic Energy, not 40lb vs 50 lb vs 70 lb bow.

Which would you rather shoot a big buff with, a heavy 70 ft-lb arrow or light 70 ft-lb arrow?
The heavy arrow having much more momentum than the lighter arrow.

KE just doesn't apply to penetration.

Bogsucker, I guess I've been shoting the wrong bows. I put ft/lbs into mine when I pull them back and get ft/lbs out when I turn it loose. That is all any bow does. What you do with those ft/lb is your choice. KE does matter, no KE, no momentum. More KE you have, the more momentum you can get....O.L.

No you've just been using the wrong units of measure. Momentum does not come from Kinetic Energy. You cannot store Kinetic Energy in a bow you may store Potential Energy but that is measured in different units (lbs). Kinetic Energy does not correlate with penetration, Momentum does.  This is a matter of physics not opinion.
The use of the term KE (Kinetic Energy) to indicate lethality on game is simply not logical.

70 lb bow 350gr arrow 300+fps 70 ft-lb KE .47 ft-lb-sec Momentum

60lb bow 540 gr arrow 230 fps 63 ft-lb KE .55 ft-lb-sec Momentum

This is very close to IBO and AMO ratings of several popular compound bows.

I haven't seen any IBO and AMO ratings for trad bows lately.

O.L.,

Actually I'd be honored to have you film me shooting a big buff with a 40lb bow. We'd just have to find someone to sponsor the event as I'm not financially able to hunt the Dark Continent.

Ya' know, every time I see one of these threads I think about the blind men looking at the elephant and all getting a different idea of what the animal is.  You need to see the whole picture.  Energy is energy, we give it different names based on where it is and how it is used.  We measure it in different ways for the same reasons.

When you draw back your bow the energy in the limbs is called potential energy.  When you release the bow, the energy is now used to to move things and it is then called kinetic energy.  The amount of kinetic energy is normally measured at the moment the arrow leaves the bow. The amount of kinetic energy remaining in the arrow at any point in time between the bow and the target gets smaller with every second of flight, which brings us to momentum, which I agree is a better number to determine what you can expect for penetration.  Momentum is a better way of expressing the remaining energy in the arrow, especially since the light arrow looses energy much faster than a heavy arrow and it also cannot caputre or store as much energy as a heavier arrow. That is why you get more noise with a lighter arrow off the same bow than you do with a heavier arrow.

So, the energy described as KE is still there in the arrow, it is the energy that fuels the Momentum of the arrow, if you will.

Again, Kinetic Energy (KE) and Momentum are two distinct and separate functions of weight and velocity(speed) and they are not linearly proportional to each other. The energy stored in a bow is not Kinetic Energy. Momentum does not come from Kinetic Energy it is a distinct and separate function. Momentum is a much closer measure of penetration than KE.

I've had enough. Sorry if I've upset anyone but KE don't mean squat.

No, I'm not done (sorry again). Question: Why should I use the unit of measure called Kinetic Energy to indicate performance of a projectile?

Bogsucker, No one ever said a bow stored KE, PE is measured in ftlb also. Get your momentum from where ever you want. I'll shoot my 12-13 grain/lb and get my MO from the KE......O.L.

Now your gettin close, however if you were to shoot a 7-8 gr/lb arrow out of that same bow you'd have  more kinetic energy but less momentum thereby disproving that momentum comes from the kinetic energy!
May we agree that generally a  "heavy" arrow is better for penetration than a "lighter" arrow out of the same bow?