Interesting but short read on KE vs Momentum

[Will_m]

http://www.outdoorlife.com/blogs/ga...l&loc=whatsnew&lnk=physics-for-bowhunters-why

I think KE is starting to become less of a measurement standard for archery than it has in the past. Makes sense to me.

 

[Jacob Chapman]

Agreed. Momentum is much more important

 

[Will_m]

What really gets me is how many pro shops are still pushing the whole KE thing.

Just the other day I was looking at an elite impulse and the store owner was trying to push some ultra-light arrows on me to increase my speed. When I brought up the momentum aspect he immediately wanted to tell me the KE of his set up.

 

[jmez]

It is simply marketing. Speed sells, KE numbers change a lot when speed changes. It is also a big number whereas momentum is expressed as a decimal. .623 is not nearly as impressive as 90 foot lbs of KE. I don't look for it to change any time soon.

 

[elkguide]

Very simply stated. The reality of archery hunting.

 

[xcutter]

Makes sense.

 

[larryschwartz]

There is definitely an advantage to being around for several years...or decades. Back in the 80's (35 years ago) the compound manufacturers realized that the easiest way to differentiate their bow from others, and one that was measurable, was to focus on speed. They said that speed would give you a flatter trajectory and help minimize aiming errors due to range estimation. They said that the faster the bow the higher the KE would be. As mentioned above, it was really all marketing because those who actually think about the physics of the sport know two important things. First, what matters in the end is momentum because that it what gives you penetration, not KE. Second, the heavier the arrow (we are talking mass now) the more energy it will absorb and the slower it will loose that energy during its flight to its destination. So, a heavy arrow will absorb all or most of the energy stored in your bows limbs, which means there is less left over to become noise, vibration, and hand shock...all good things. It also means that the heavier arrow will loose less of that stored energy, unlike the light arrows which loose if much faster, giving you more energy on impact for penetration... again, a good thing.

 

[MattB]

Arrow weight is arguably more important than both.

 

[larryschwartz]

Arrow weight is arguably more important than both.

Well, for our purposes, weight and mass are equivalent.

 

[MattB]

Well, for our purposes, weight and mass are equivalent.

To KE and momentum?

 

[larryschwartz]

To KE and momentum?

No, mass and weight are equivalent to each other. The heavier arrow has more mass.

 

[tipsntails7]

Arrow weight is arguably more important than both.

p=m•v

For momentum, arrow weight is equally as important as velocity. Momentum is directly proportional to not only mass but velocity.

ke favors speed because it squares the velocity in its equation.

As pointed out ke is a scalar not a vector so it is not defined by direction.
So for a method of take that does not kill by imparting shock ke looks great on a magazine add but has very little use as gauging effectiveness for archery tackle. Other than if you have enough ke for an animal then it's most likely true that you also have enough p for that same animal in the archery world. Obviously outliers exist.

 

[MattB]

The KE calculation favors velocity as it is squared in the calculation.

Momentum is deemed by many to be more relevant, and the calculation uses both mass and velocity to the first power. This increases the emphasis on arrow mass.

But there are anecdotal studies that show that arrows with more mass but with lower momentum can out-penetrate lighter arrows with more momentum. I am not a proponent of all things Ashby, but I think he is onto something in this regard with his research in that arrow mass is a better predictor of arrow penetration than momentum or KE.

 

[jmez]

The other caveat with KE, momentum, speed and mass, you can easily change the mass of the arrow and appreciable amount. The speed of your bow is pretty much fixed. You can add 100 grains to an arrow pretty easily. You can only add speed by tweaking the tune and at best would gain less than 10fps.

As stated above, a heavier arrow is more efficient, retains energy longer downrange and sheds less of it on impact. That is why the heavier, slower arrow can out penetrate a faster lighter arrow.

 

[Dos Perros]

The one thing I never hear mentioned in this discussion (maybe it is assumed?) is that your bow is your bow. You don't really switch bows to get more KE or more momentum; well, switching to a faster bow would get you both, i.e. it's not like some bows are momentum bows and some bows are KE bows. They are what they are, striving to be as efficient as possible (and they are pretty dang efficient). So you have this bow, the next question is the weight of the arrows to put through it. The weight (mass) is what drives this KE vs momentum discussion.

I was playing around with The Archery Program last night. I kept the same bow, same arrow specs, and only modified one variable (arrow mass) and the program would output a calculated velocity. I varied arrow weight from 355 grains all the way up to 450 grains. Over that range, arrow weight increased 28.6%, speed decreased by 10.9%, KE increased by 0.7%, and momentum increased by 13.0%. So, because both KE and momentum increase with weight (even though KE by only a paltry amount) it makes sense to both KE and momentum believers to shoot the heaviest arrow you can stand to shoot, up to some limit I suppose (once you have enough you have enough). Really, since KE is almost relatively constant, the discussion should be SPEED vs momentum.

 

[Pac8541]

So let me ask this, of the below arrows which would you choose to hunt with and why?
A 445gr arrow @265fps
A 452gr arrow @260fps
A 477gr arrow @255fps
A 503gr arrow @248fps

Or, because they're fairly similar, does it matter...

 

[tipsntails7]

So let me ask this, of the below arrows which would you choose to hunt with and why?
A 445gr arrow @265fps
A 452gr arrow @260fps
A 477gr arrow @255fps
A 503gr arrow @248fps

Or, because they're fairly similar, does it matter...

Because they are so close, it would depend on other characteristics of the arrow for me. I wouldn't hesitate to use any of those arrows for anything in the states.

Now if you had an arrow like this

365g at 315 FPs

Or

477g at 255fps

I would always choose and recommend the latter.

 

[Pac8541]

...and that is where I'm standing at the moment: considering other characteristics of each one to make the decision. I'm currently stuck on the idea of FOC although I believe its the sum of the parts rather than focusing on a single attribute that makes a great arrow. But, as I've mentioned in private conversations with OR Archer, my accuracy and consistency benchmark is currently an overly heavy arrow for my rig (580gr) but with an FOC of over 17%. They're slow as hell out of my E35 @71lbs but I'll be damned if they don't literally stack on top of each other every single shot.

When I crunch numbers on 2 separate pieces of archery software, one of which calculates Ke and momentum, I see those values rise and fall as I change the overall mass of the arrow. I'm using those values along with FOC to make a decision, in addition to that fact that one is a FMJ and the other a Carbon Injexion; one allows a bit more flexibility by changing points; etc, etc. And as my decidedly unscientific penetration tests have shown, I think I'm honing in on the balance point between speed and mass.

 

[MattB]

"I'm currently stuck on the idea of FOC...."

Honestly, stop it. FOC is the bowhunting equivalent of the latest fad diet. Modern bows with almost any reasonable arrow/point combination will result in ridiculous penetration on any NA game. All the mental masturbation and $ expenditures to bang out that last 4% - or whatever it is - of penetration is frankly a silly exercise.

 

[MattB]

I'll add another comment - and probably against my better judgement. I'm fortunate to know a number of guys who are really accomplished bowhunters. Not one of them - not one - gives a cr@p about KE, momentum, or FOC. They put together an arrow of a reasonable weight with quality components, launch it from a bow of reasonable weight (50# or greater), and practice a lot. And then kill a bunch of animals.

I'd endeavor to be one of those guys rather than the guy who debates obscure variables on the internet when I should be at the range practicing or hitting the gym.