RE: Agree or disagree???
 

I've pretty much said what Iwanted to say regarding this matter and it has been my experience that this subjectquickly tends towind upin a not so friendlyargument.So I'll say good bye now and "kill em and grill em!"

RE: Agree or disagree???
 

Seemed to be pretty friendly so far.

RE: Agree or disagree???
 

I'm sorry, I think I left the wrong impression. I didn't mean that it wasn't friendly in fact just the opposite. I've just had some not so friendly experiences in the pastnot only here but on other forums when it comes to this kind of a subject so I just thought I'd just say my peace and then politely retreat. I thinkbecause answers to questions of physics and math are often either correct or incorrect, i.e. no room for opinion, as soon as there is a debate by definition, somebody is wrong. With most other types of subject it's easy to say to someone for example "IMO I think you are wrong" whereas in math if someone says 2 + 2 = 5there doesn't seem to be an easy way to say it without compromising what you know to be or at least are totally convinced to be true. People quite naturally, and of course I include myself in this even though I try not to, quickly become defensive. So anyway, sorry if I left the impression that anyone here wasn't friendly but again, I've said my peace and would only be repreating myself from here so like I said "kill em and grill em!"

RE: Agree or disagree???
 

I aim the bow, release the arrow, deer fall down... Practice with your setup and this will happen again and again. I am not an engineer but I get to eat a lot of backstrap!

RE: Agree or disagree???
 

Both increase and decrease at the same rate relitive to powerstroke , so the question is realy a nonissue when your bow creates more ke it will create more momentum , and visa/versa . Just deferant terms to define how much energy a flying arrow has .

RE: Agree or disagree???
 

Sylvan, you've both confused the hell outta me and inspired me both at the same time. You have made some sense, and I intend to investigate farther just to learn. Thank you for your good attitude with my ignorance. I appreciate you for that very much.

RE: Agree or disagree???
 

You're correct that when you increase energy you also increase momentum. But, when using the same bow and same draw weight, you can only slightly increase energy while really loading up on momentum. Doubling arrow weight will only gain a couple of foot pounds of energy (in theory)but it will double the arrow's momentum.

You're also correct that it's a non issue, but only when your bow is putting out way more energy than you need.A current model, 70 pound compound with hard cams is putting out more energy than the recurve Fred Bear used in the 60's to kill an ELEPHANT.So, yeah. It's probably a non issue for someone using such a bow for hunting puny little deer. (Although... Isn't it absurd that so many folks are shooting enough energy to kill an elephant and then coming on the forum, griping about not getting passthroughs on deer with light carbon arrows?)

If the bow is putting out barely enough to get the job done, it's a huge issue. Bear knew his arrow wouldn't be carrying a whole lot of energy in relation to the size of theanimalon that elephant hunt, so heused a 1200 grain arrow to get penetration into the pachyderm's vitals. He loaded up on momentum.

Sounds like you're saying the terms are interchangeable and they're not. Energy and momentum are interrelated, but not at all the same thing.

RE: Agree or disagree???
 

Arthur, we keep hearing that momentum and KE are related but not the same thing, I even said so in one of my earlier posts on this topic. I think we should define them both and explain the difference.

Kinetic Energy: the mechanical energy that a body has by virtue of its motion

Momentum: a property of a moving body that determines the length of time required to bring it to rest when under the action of a constant force

These are both definitions straight out of webster's dictionary. So, as you can see, KE is the energy it contains, and momentum is how long it will stay in motion by virtue of it's mass. I bring back the rock example. I can throw a pepple not very far at all, but I can throw it fast. But, because it is so light, it has no momentum (nothing behind it to keep it moving). I can throw a rock farther because of it's weight, but I can't throw a boulder at all.
Same is true with arrows. A light arrow may produce more initial KE because of its speed,but by 30 yards it has slowed down so much that the KE drops off quickly; momentum. A heavier arrow may or may not have the same initial KE because of it's slower speed, but it retains its speed longer downrange (momentum) thus retaining it's KE.
Let's say a 300 gr. arrow (I'm making these numbers up, but the concept is actual) is flying at 310 fps producing KE of 64#, but remember, that's at the point of release. By 30 yards let's say it's speed has decelerated to 260 fps, so the KE at 30 yards is 45#, a drastic decrease.
Now let's shoot a 450 gr. arrow. It's launch speed may only be 260 fps, giving a KE of 68#. Now, at 30 yards downrange it's speed is 240 fps (yes, the arrow has dropped more than the lighter one, but that's because of gravity, not speed, it's speed, by virtue of it's momentum has not dropped off so much) giving a KE of 58#.

So, yes, momentum and KE are both measured by velocity and mass, but they are not the same thing.



All numbers here, again, were fictional, (with the exception of the math) but I believe them to be fair and at least close.

RE: Agree or disagree???
 

So lets crunch some numbers. A bow that is throwing a 350 grain arrow 300 ft/sec calculates to be 70 ft/lbs KE and 0.47 pound seconds momentum. Now lets double the arrow weight to 700 grains and shoot it. Assuming 100 grains of virtual mass the new result is a 700 grain arrow flying at 225 ft/sec. That's 78.7 ft/lbs ke and 0.70 pound seconds momentum. So the heavier arrow has given us a 12.5% increase in KE and a 50% (not 100% like you said) in momentum. O.K. so what does that mean? Well if you agree that the heavier arrow will penetrate more it means that in this example a 12.5% increase in KE or a 50% increase in momentum (take your pick) resulted in precisely the same increase in penetration.

It's just like I said in my first post...

"If you want to attribute the increased penetration to the increase in KE that's fine. If you want to attribute it to the increase in momentum that's fine too. KE and momentum are merely the result of calculations based on the measureable quantities of arrow velocity and mass. From a given bow, the facts are quite simple. If you put on a heavier arrow, the veolocity goes down and both the KE and momentum increase thereby resulting in greater penetration. "

The relative percentage that each change is irrelevant.

Art, I'm getting a strange de ja vu feeling...

RE: Agree or disagree???
 

Sylvan...... You keep using virtual mass to calculate the heavy arrows FPS...... Not saying you are way off, but your calculations really mean nothing unless you have the exact FPS for the heavy arrow.... Someone needs to get a chrono and run us some numbers..... Then Sylvan's formulas would be alot more useful IMO.....