arrow physics 2

[Rob DiStefano]

Originally posted by wingnut:
A long time ago in a place far far away, I was employeed to do some testing with a chronograph at 60 yds.

Yes it was a mechanical bow and at the time I was a pro shooter.

But that being said we did a study for the game dept on retained energy.

We shot arrows from 6 gpp to 12 gpp from a bow through a chrono that was 60 yds down range.

Not an easy task.

Anyway to make it short.  The difference in KE on the light arrow and the heavy arrow from the chrono at 3 ft and the chrono at 60 yds was 3%.

The heavier arrow retained 3% more energy then the light arrow over the distance.  Both arrows were flying perfectly.

Mike

real interesting, mike!

by 'mechanical bow', did you mean hand-held?  release aid?  what was the holding weight and let-off (if any)?

what was the 6pp and 12gpp arrow mass weights?

any foc numbers?

was any data collected at 20yds?

just curious.

[Richie Nell]

I apology if I quoted you out of context.  Gotta keep apples to apples here.

"I shoot about 11 grains per pound of pull, if anybody cares. 8-)"

I shoot around 12-14 (900-1050 grains).  I love the power but mainly for noise reduction.  
10-11 gr./lb would be plenty of arrow weight for my 71 lbs. but louder than I want.

[Richie Nell]

I wonder how the increase in the KE would equate to an increase in Momentum?

That would be interesting to know because Dr Ashby's research show that KE is insignificant as it pertains to penetration.

[Zradix]

wingnut... Sounds to me like...YES the heavy arrow in this setup will have more energy at the target.  YES the light arrow will have a flatter trajectory. The difference ,however may actually be very negligible when pertaining to the lethality on a whitetail. I would think that if you're gambling on a 3% chance to kill you're shooting entirely the wrong setup to begin with. It is surprising the trajectory difference in these two arrows when shot at 20yds. The faster arrow will get to the target a little more than .4 sec sooner. I say a little more because I don't know how to figure the extra distance the heavier arrow travels due to more arc. This extra distance would actually bring these two arrows even more similar to each other in energy at target. Mind boggle!!

[Jim now in Kentucky]

Just for fun, here is a post that looks at the relationship between momentum and kinetic energy:

 http://physics.ucsc.edu/~josh/6A/book/momentum/node3.html


The thing I hope can be gleaned from the above Web page is that kinetic energy and momentum are different ways of describing the same situation.

[oxnam]

Wingnut, that would have been a fun study.  3% loss isn't much.  Since you measured KE, I started doing some reading because others have talked about momentum being a more important number to consider.

I just found an interesting example on KE vs momentum.  
Summed up: In this extreme example, a meatball is way more dangerous than a truck at the given variables even though momentum would show they are equal.

Truck's momentum = mv = (1000 kg)(1 m/s) = 1000 kg m/s
Truck's kinetic energy = 0.5 mv2 = (0.5)(1000 kg)(1 m/s)2 = 500 Joules

Meatball's momentum = mv = (1 kg)(1000 m/s) = 1000 kg m/s
Meatball's kinetic energy = 0.5 mv2 = (0.5)(1 kg)(1000 m/s)2 = 500 000 Joules

Here's the source for some more good explanation:
 http://www.batesville.k12.in.us/Physics/PhyNet/Mechanics/Energy/KENOTMomentum.html

[Richie Nell]

Below are quotes from Dr. Ashby based on field research.

"ALL MOMENTUM IS NOT THE SAME

Given two arrows, identical in shaft and broadhead materials and profile, and having EQUAL momentum, but possessing UNEQUAL mass, the arrow deriving the greater portion of its momentum from its mass will penetrate better.  The Laws of Physics requires this to be true, and ALL of my field test data validates this to be the case."

"Laws of Physics dictates that momentum, and not kinetic energy, is the correct unit of measure to quantify the linear (straight line) "potential disposable net force" that is available to an arrow.  Momentum determines THE AMOUNT OF FORCE which an arrow has available to it for penetration."

"Kinetic energy is NOT the correct unit of measure for calculating ANY of the forces relevant to penetration.  It is applicable for calculating neither the force of a moving object; the disposable net force at impact; the net force at exit; net force consumed during penetration; the applied impulse; nor the resistance impulse force affecting penetration."

[Jim now in Kentucky]

"Laws of Physics dictates that momentum, and not kinetic energy, is the correct unit of measure to quantify the linear (straight line) "potential disposable net force" that is available to an arrow."

I don't think Isaac Newton knew about that one, he never mentioned it.

[Richie Nell]

It is a shame Sir Isaac died not having a clue why all the deer he shot looked like a pin cushion instead of swiss cheese.

[Richie Nell]

Below is from Dr. Ashby's research concerning KE and penetration.

"Using the TPI formula, a 60 pound longbow firing a 788 grain compressed cedar arrow, with a 190 grain Grizzly broadhead, at 148 fps has only 38.34 ft. lbs of K.E., .52 lb.-sec. of momentum, but has a TPI of 1.50.  That combination was used to repeatedly shoot through the scapula of a large zebra stallion and through the thorax to the off side, often breaking off-side ribs (never failing to penetrate the scapula and completely through the thorax).  This was compared to a compound firing a 555 grain aluminum shafted Black Diamond at 229+ fps.  This combination gives 65.21 ft. lbs. of K.E., a momentum of .57 lb.-sec., but a TPI of only 1.27.  That compound was, at best, able to penetrate only 5 to 8 inched beyond the scapula, and occasionally failed to penetrate the scapula at all on that same zebra carcass."

"The 60# longbow/788 grain arrow/Grizzly broadhead was also compared to a high energy compound firing a 450 grain carbon arrow tipped with a three blade head, with cut width of 1 1/8" and a cutting blade length of 2", at a velocity of 259+ fps.  This combination yields 76.56 ft. lbs. of K.E., .52 lb.-sec. of momentum, but a TPI of only 0.62.  It was unable to penetrate the zebra scapula."

"Penetration of this combination (compound, light weight carbon shaft and 3 blade head) on all the larger antelope species is marginal at best.  This bears out the reduced penetration capability of this set-up compared to the lower velocity longbow with its heavy arrow and single blade broadhead of high mechanical advantage.  This even though the momentum of both combinations is identical (.52 lb.-sec.), and the compound has 76% more kinetic energy (67.56 ft. lbs. vs 38.34 ft. lbs.)."

[oxnam]

Man I love reading Dr. Ashby's results, thanks Richie Nell.  His reports go way beyond just mathmatical equations and provides a wealth of real testing and experience.  Thanks Dr. Ashby once again.

[Rob DiStefano]

the doc has spent 25+ years compiling and learning from his exhaustive data collection.

i'll trust his real world bow hunting test results over that dead apple man any day.  

then again, maybe some of you are comparing apples to oranges ... ?  

all my personal testing shows that well flying, heavy, weight forward arrows penetrate 'stuff' better - maybe LOTS better - than lighter, faster, well flying low foc arras.  couple that with a truly sharp c-o-c broadhead and the hunting deed is well setup for killing as long as i do my part.

[James on laptop]

Well 135fps is getting on down to fish arrows speeds.It might penitrate best but it is below the threshold where I would hunt with so is too heavy for me.Sometimes there are more things to consider when picking arrow weights than what number crunching shows. Common sense needs to kick in sometimes.       :D

[Rob DiStefano]

imho, there is no question that higher bow holding weights, and heavier arrows, are always an advantage IF the bowhunter can *effectively* handle the higher bow holding weight under hunting conditions.

imo, hunt with the highest bow holding weight you can *master* - add in a heavy arrow and the prescription for success has been writ, it just needs proper execution.

[sj_lutz]

I love threads like this!

Sorry if it's already been mentioned and I missed it, but don't forget the bow noise aspect.  The heavier arrow will result in a quieter shot.

[ChuckC]

wow, still arguing this one heh ?

Why not throw in some more data.  Since deer are very prone to doing silly things,  like.. moving, and dead zebra are typically not quite as guilty of that crime, how much does a moving target change the formula (perfect arrow flight is no longer a perfect impact if the target is moving.. in fact with "bad arrow flight" there is a possibility, albeit slim, that the animals movement will correct for the bad arrow flight and allow for even more penetration)

And, as always... if a very light arrow  and a very heavy arrow both go thru the deer and into the ground beyond, which one is better ?
ChuckC

[Rob DiStefano]

Originally posted by ChuckC:
...
And, as always... if a very light arrow  and a very heavy arrow both go thru the deer and into the ground beyond, which one is better ?
ChuckC

however, if both arrows strike an inch or so off and hit bone or gristle mass rather than flesh mass, which arrow has the better chance to kill rather than wound?

[joevan125]

Great information Rob and couldnt agree with you more.

Im new to trad archery and all this is some great information for a newbie.

[Richie Nell]

And, as always... if a very light arrow and a very heavy arrow both go thru the deer and into the ground beyond, which one is better ?

The heavy arrow

[Don Stokes]

Jeepers. How long is it until deer season opens again?