Archery Talk Forum banner

1 - 12 of 12 Posts

·
Registered
Joined
·
8,248 Posts
Discussion Starter #1
KE is not as important as what people think. I also believe people misunderstand what it actually is because of what they are told and not really learning exactly what it is. please don't take this the wrong way.,...here is some info for anyone who is interested in the real relationship of KE and momentum. lengthy read but may help clear up some info. http://tradgang.com/ashby/Momentum Kinetic Energy and Arrow Penetration.htm
 

·
Registered
Joined
·
418 Posts
You got a Cliff's notes version of that?
 

·
Registered
Joined
·
4,097 Posts
Very interesting reading.
 

·
Registered Elker
Joined
·
7,715 Posts
You got a Cliff's notes version of that?
Cliff notes:

While KE is important, the resistance of impact at higher speed causes penetration to suffer. In short, reduction in impact resistance is very important. Momentum is speed with direction. This alone can be increased without an increase in speed by ensuring 'straight' arrow flight. Minimizing fishtailing, etc. Striaghter flight reduces impact resistance.


Notable quotes from the article:

'Let us now assume an arrow weighing 700 grains for the slower bow (150 fps is easily achievable with that weight arrow and a ‘traditional’ bow) and a 390 grain arrow for the faster bow (the advertised velocity rating for one of the newest compound bows on the market, using that weight arrow). The slower arrow has 0.466 slug feet per second of disposable net force. The faster arrow has 0.519 slug feet per second.

Lets also assume these two arrows are of same materials, have equal physical external dimensions (easily achievable), and both have perfect flight characteristics. The tissue’s resistance increase is totally dependant upon the velocity of the arrow.

The lighter arrow has 10.22 percent more disposable net force (and 123.2 percent more kinetic energy) than the heavier arrow but, because of its higher velocity, it is met by four times the resistance to penetration. Which arrow will penetrate further in real tissues? Empirical evidence from the outcome studies provides an overwhelmingly definitive answer. Both the frequency and degree to which the heavier, slower, arrow out-penetrates the lighter one is of such a magnitude that it must be viewed as the norm.'

'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.'

"An arrow’s momentum is also a part of the arrow’s kinetic energy - the only part that relates to its ability to penetrate. Some of an arrow’s kinetic energy is dissipated as other forms of energy during flight and on impact. Even the ‘sound’ of a hit is derived from the arrow’s kinetic energy.

As shown above, the 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.'
 

·
Registered
Joined
·
4,097 Posts
'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.'

Heavy is better.
 

·
Registered
Joined
·
664 Posts
Yeah but when I say I have 118 ft lbs of KE it sounds better than all I have is .889 slug ft/sec of momentum.

Just messin as I for awhile now have said that KE is not the best formula for determining penetration but I do believe momentum is.

An analogy that I like to use is if ya have a smart car doing 100 mph and ya have a semi truck doing 50 mph and they both hit a concrete wall, which one is going to do the most damage to that wall.

BTW the numbers above are actual;)

OBH
 

·
Registered deer murderer
Joined
·
632 Posts
I would be cautious about what I believed as fact in that report. Although Ashby comes across as very "expertlike", that article was full of misleading analogies and misapplied science.

Kinetic energy is THE mathematical value that determines the ability to penetrate a solid.

Resistance increases as the square of velocity in a FLUID, not in a SOLID. Of course, an animal's body does contain fluid, but that is not the primary factor in stopping an arrow. It's the solid bones and muscles that limit penetration.

Resistance to penetration actually goes way down in some solids as velocity increases. Try slowly pushing a drinking straw into a potato, then try rapidly stabbing it in. Also try slowly pushing your fist through a pine board vs quickly punching it. It will become readily apparent that the faster you are moving, the less force you actually have to exert on the object.

The baseball analogy is completely absurd. It has far more kinetic energy AND momentum than an arrow. It doesn't penetrate a deer because it's round and blunt. Try putting a large round object on the end of your arrow and see how far you can stick it in a deer.

Momentum isn't meaningless, but it is not the primary factor in determining penetration. Don't believe everything you read on the interwebz (including my post). Study it yourself and make your own decisions.
 

·
Registered deer murderer
Joined
·
632 Posts
Yeah but when I say I have 118 ft lbs of KE it sounds better than all I have is .889 slug ft/sec of momentum.

Just messin as I for awhile now have said that KE is not the best formula for determining penetration but I do believe momentum is.

An analogy that I like to use is if ya have a smart car doing 100 mph and ya have a semi truck doing 50 mph and they both hit a concrete wall, which one is going to do the most damage to that wall.

BTW the numbers above are actual;)

OBH
Your average smart car only weighs about 1/10th what your average empty semi weighs. Assuming a weight of 30,000 pounds, the semi will have about 2,512,504 ft-lbs of KE at 50 MPH. Assuming a weight of 3,000 pounds, the smart car will have about 1,004,972 ft-lbs of KE at 100 MPH. The semi truck will obviously do more damage.
 

·
Registered
Joined
·
4,492 Posts
I'll take a heavier, slower, quieter, deeper penetrating arrow any day over a lightweight, loud one. Would you rather get hit with a baseball bat or a wiffle ball bat?
 

·
Registered deer murderer
Joined
·
632 Posts
I'll take a heavier, slower, quieter, deeper penetrating arrow any day over a lightweight, loud one. Would you rather get hit with a baseball bat or a wiffle ball bat?
Assuming that both were equally efficient at transferring their energy to the target, I would pick the bat with the most kinetic energy.
 
1 - 12 of 12 Posts
Top