I have been following this thread, and I am really enjoying the theories and thoughts, they are definitely making me think!

But what I have noticed is that there is a trend in everyone's thinking that Momentum is the better penetrating factor in comparing it to KE. (MxV vs. (½)MV2) I definitely agree with this theory, as I use momentum as my deciding factor in arrow design.

There is only one scenario being played out in everyone's thought process and which is setting KE equal between two non-equally weighted arrows AT THE TARGET.

But there is another scenario as well, setting momentum equal AT THE TARGET between two non-equally weighted arrows.

I do not have the answer, and this is the reason, b/c this conundrum is an inverse resulting problem. Bear with me here.

SETTING KE EQUAL AT THE TARGET:
-   500 gr arrow @ 146 ft/s
-       KE = 533 x 10 to the 4th
-       Mom = 73,000
Vs.
-   600 gr arrow @ 133.33 ft/s
-       KE = 533 x 10 to the 4th
-       Mom = 80,000

SETTING MOMENTUM EQUAL AT THE TARGET:

-   500 gr arrow @ 160 ft/s
-       Mom = 80,000
-       KE = 640 x 10 to the 4th
Vs.
-   600 gr arrow @ 133.33 ft/s
-       Mom = 80,000
-       KE = 533 x 10 to the 4th

So ignoring the units of measure, b/c that becomes a cumbersome task to do the unit conversions into standard KE and Momentum units the recap is as follows:

SETTING KE EQUAL AT THE TARGET:
-   The heavier arrow has more momentum at the target than the lighter arrow. I would conclude that the heavier arrow would penetrate better.

SETTING MOMENTUM EQUAL AT THE TARGET:
-   The lighter arrow has more kinetic energy than the heavier arrow. I would conclude that since momentum is the same, then the lighter arrow has the better ability to penetrate.

So the conclusion or the secondary question one must ask himself, and I do not have the answer to this question, would be as follows:

Shooting THE SAME BOW, and adjusting your arrow weight, which of the two variables (momentum or KE) would be the same or closer to each other than the other variable. Is it more realistic that momentum will have closer values AT THE TARGET, or will KE have closer values AT THE TARGET when comparing two non-equally weighted arrows. Whichever the two scenarios proves out with your bow and your arrow weights in comparison, would be the one to go with.

But to the original question, I do not believe that two arrows of the same KE but different weights will penetrate the same... No way.

Your thoughts?

Good answer cpnhgnlngct to a simple physics question. Since a bow will give about the same KE to different weight arrows (actually a slight bit more as arrow weight increases) The answer to your secondary question is that the heavier arrow will always have greater momentum than a lighter arrow with the same KE and all else being equal will have greater penetration. Simple numbers.

cpnhgnlngct,
This is a good question, but from the same bow you will never get , in the 8 to at least 20 gpp range, the same KE neither momentum, because any time you add weight to your arrow, you will increase both KE and momentum when you measure it in front of the bow.
Then,from the same bow, a heavier arrow (identically built than a lighter), will always out penetrate the lighter one because both KE and momentum are higher.
AT THE TARGET this heavier arrow will out penetrate even more than really close:
At 25 yds, air drag on the light arrow (if same length, fletch, and diameter...) will be higher than on the heavier arrow. So drop both in KE and in momentum AT THE TARGET from the same bow will be bigger with the lighter arrow, increasing the difference you had already in front of the bow and lead the heavier to out penetrate more.
In fact, the only advantage of a fast arrow, is that it will hit the target  a little sooner than the heavier at hunting range.

oops: on the lastsentence you should read
" In fact, the only advantage of a light arrow, is that it will hit the target a little sooner than the heavier at hunting range with quite flatter trajectory"

The one most tuned to the bow....

And shot by the most accurate shooter with the smoothest release.

After all the discussion, THAT is your answer.      

i was just wondering if someone could prove this persons claim. seems no one can.

It is difficult to "calculate" it but easy to offer an easy experience to the hunters we all are supposed to be.
Let's build exactly the same looking arrow, one 850 gr and 2nd 465gr.
If we shoot the first one from a 70+# top of the line recurve @ 28" we could get around 165 fps what is really close to the set up you used succesfully  for your water buff hunts....

KE with that set up (165 fps- 850 gr arrow) is very close to 51.4 ft.lbs.

To get similar KE=51.4 with an arrow weighting 465 gr, we need something like 223 fps. With a trad bow it will be difficult unless you don't mind about life expectancy of the bow BUT with a Wheely bow rated 315-320 fps IBO, that is something you  would get for...55 LBS @28".

Who will be brave enough to give a try for buff hunt with that 55 lbs wheely and 465 gr arrow??


 

QuoteOriginally posted by Terry Green:
The one most tuned to the bow....

And shot by the most accurate shooter with the smoothest release.

After all the discussion, THAT is your answer.          

In any reasonable weight range with any reasonable weight bow, this is your answer, folks.

It's easy to get so wrapped up in which is "better" that we forget to ask which is enough.

Maybe turkey season will offer some distraction from the winter doldrums...

QuoteOriginally posted by Don Stokes:
Maybe turkey season will offer some distraction from the winter doldrums...

I couldn't agree more