RE: Agree or disagree???
 

Actually, the subtle but major difference between throwing a pebble and a rock has to do with how efficiently you can transfer the energy from your arm to the object you are throwing. You can transfer more energy to the heavier rock because it takes time to convert the chemical energy stored in your muscles and as you throw the heavier rock slower there is more time for the transfer. You can't move your arm fast enough to transfer the same amount of chemical energy in to the pebble. It's similar with a bow. If 100% of the energy stored in the limbs were always transferred to the arrow, it would make no difference in the amount of ke regarding light or heavy arrows. Lighter arrows would be faster than heavier arrows but the energy level would be exactly the same. If you stored 60 ft/lbs of potential energy in the limbs you would wind up with 60 ft/lbs ke. Problem is bows aren't 100% effiicient and the lighter the arrow the less efficient they are. Virtual mass is the mathmatical value that tells us about how well a given bow transfers the energy. The lower the value the better. Zero virtual mass (though impossible) would mean 100% efficient.

RE: Agree or disagree???
 

You are correct in that I only picked the value for virtual mass out of the air but as the example is only for the pupose of showing relative change, actual measured values are unneccessary. Pick any value for virtual mass you want, the gist of what I'm saying is unaltered. Virtual mass like brace height or peak weight is associated with the bow not the arrow and in this example I am not comparing different bows just different arrows shot from the same bow. So what I am doing is absolutely valid. You can argue over a few ft/sec here and there but it makes no difference realtive to the point of the matter.

BTW, they're not MY formulas. They belong to everybody!

RE: Agree or disagree???
 

The only thing I'm confused about in this thread is.......how the hell did anyone get past mobowhuntr's sig pic?[&:]

I forgot what the question was.

RE: Agree or disagree???
 

Kinetic energy = 1/2 masstimes veolicty squared
momentum = masstimes velocity

As you can see jimmy is correct. They are simply different terms describing exactly the same phenomenon. That isa massin motion. Even the terms foot pounds and pound seconds come about merely because for ke you multiply mass by velocity squared and in momentum only by velocity. IMO in archery it truly is a non issue. I'll say is again. The facts are simple. Put on a heavier arrow and velocity goes down and both ke and momentum go up thereby increasing penetration. KE and momentum are just human inventionsderived from the only measureable quantities of the physical event which are mass and velocity. It makes little sense to me to worry about it. What matters is you put on a heavier arrow and you get more penetration potential. You also get a larger arch in trajectory. So there is the nuts of it. Youmust balance the postitive effect of increased penetration with the negative effect of greater trajectory. IMO the balance point is the fastest arrow you can shoot that still provides adequate ke/momentum for penetration of the game you are hunting and then maybe a little extra just in case... Unfortunately, there is no equation we can solve to give us the answer to this apparent paradox so the debate is destined tocontinue.

My advice is, if you're just interested in archery and hunting, I wouldn't worry about the difference between ke and momentum.Decide on a minimum level be it in ft/lbs or pound seconds or both if you like and then go as fast as you can while still maintaining that/those minimum(s). If you're just curious about the physics get a physics book or take a class. The subject is IMO too complicated to learn properly on an internet forum. But of course in the words of Dennis Miller, "that's just my opinion, I could be wrong".

RE: Agree or disagree???
 

Man, you got that right. I spent a little time pondering that subject myself! Ain't no equation going to solve that problem either![8D]

RE: Agree or disagree???
 

All the physics classes aside: What difference does it make? For the most part, in my experience, unless you're talking at the extreme end of weight and poundage, all modern arrows penetrate quite well. Most failures of penetration I have heard of are a result of poor shot placement, broadhead failure, or dull blades. I've yet to hear of one arrow bouncing off an animal because of lack of Momentum or KE or whatever unless we're shooting at 1000 yards. I guess if you're shooting a rhino or elephant, which may be the case concidering the magazine in question, does it really make a difference with today's bows? This reminds me of the theological debates of how many angels fit on a pinhead.

RE: Agree or disagree???
 

Well, think of it kind of like making love to your favorite sweetie. Would you want it fast and light or long and heavy. YUP....maybe somewhere in between.[8D] We did this all decades ago. That's where we come up with all these FOC etc etc etc. Heavy was our only real choice before the compound. Now with the blazing speeds we add another factor.....lower trajectory. ME, I kind of like the idea of being able to deliver a leathal blow if I'm off in range estimation a little. I'll give the ole girl a quickie and have a backstrap fried. We found to with this speed we have NEW problems such as finicky tuning and broadheads planing etc. Another vote for something middle of the road. I'm up to it and the ole gal can handle it.[8D][8D]

RE: Agree or disagree???
 

If someone could put into words we could all understand......it would be the man in the blue undies...

RE: Agree or disagree???
 

Well, I haven't been on here in a while , but I did want to say something on this thread.
I just wanted to remind everybody about bowhunting in real world situations. I'm glad you guys take an interest in KE , momentum, etc., I do too. However, I just wanted to offer a friendly reminder.

IMO , One of the most important things is stable arrow flight. Many bows, which are built for speed, do not provide a stable forgiving platform for the arrow to launch from. Not talking about the rest, just the bow in general. Many bows shoot so fast , it's hard to get stable arrow flight. Mechanical broadheads allow some guys to cheat on arrow stability. The thing is, An arrow flying with some lateral torque or "wobble" at the speed of sound many times will not penetrate as well as a slightly heavier, slower arrow will that's flying perfectly straight. Not only that, if an arrow hits a deer while in mid-wobble, it may plane through the deer at an undesirable angle. You can often see this on hunting shows and videos on the slo-mo impact scenes.
My dad was killing deer 40 years ago with a 50 lb recurve, fiberglass arrows and a big black diamod broadhead and slicing clean through almost every deer. these shots were always inside of 25 yds, mostly around 15, but still the equipment then was so primitive compared to today.

If you only focus on speed, or KE , you could be asking for trouble. I believe if you are using modern equipment, with your bow, broadheads/arrows,etc. perfectly tuned and flying like darts, and if you and your bow are solid platforms for the launching of the arrow , then you can almost throw the formulas out the window. At that point, the difference is how far it sticks in the ground on the other side.

I guess all i'm saying is although KE etc is important, don't overlook other more important factors in pursuit of KE alone. Chaotic arrow flight, or instability that is hard to see with the eye, can rob your shot of more KE than you gained by making hyperspeed adjustments.

I have never tried to calculate my KE. I ALWAYS have a tuned forgiving bow(ultratec) , stable arrows with fixed blade broadheads, tuned perfectly and spinning straight , and I would put mydead pass through deer up against anyones speed machine!

Good huntin' - cap