I've heard several memebers say that the momentum that an arrow has plays a bigger role in penetration of an arrow than KE does. So I would like to know what is a good momentum level considered. My bow produces over 80# of KE and depending on which arrow I'm shooting it has momentum of .536 to .555.
Is there a standard of what the acceptable level of momentum should be for a bow?
A person can become too scientific to the point that what is normally a simple given, based on historic standards, becomes very confusing.
My simplistic interpretation of KE vs. momentum is:
1. KE is velocity derived from energy transmitted by the bow.
2. Momentum is force derived from the weight of the shaft.
Properly combine the two and you have more than enough to do the job.
A 40# bow, stick or compound, shooting a shaft having recommended minimum of 425-450 grain weight will efficiently do the job on a Whitetail. Therefore, Lower energy (slower) bows are assisted by heavier shafts (momentum = force attained from weight) and cut-on-contact heads.
c903,
Actually, KE is the force derived from the weight of the arrow at a given impact velocity. Where in the equation below velocity is actually the impact velocity of the arrow.
KE = (1/2)*mass*velocity*velocity, in (ft*lbs)
Kinetic energy is the work done on an object over a given distance, so in physics terms:
work = Force * distance = KE
As expressed above, KE = (1/2)*mass*velocity*velocity.
In order to make the above equations come out in the units of ft*lbs, which is what we bow hunters like to know, you have to work out the above equations in SI notiation (kg for mass & m/s for velocity) this will give you an aswer in Joules which can then be converted to ft*lbs. Several archery manuals will have a unit conversion built into the equation which allows you to use a modified version of the KE equation above and you can use fps and grains instead of SI units.
Properly combine the two and you have more than enough to do the job.
I could not agree more. Relying on just one or the other does not give the archer an accurate picture of the entire function and characteristics of the arrow.
Quote:
KE is useless!
That I do not agree with. KE must also be considered when looking at the "entire package". What good is a heavy arrow that travels at a relatively slow speed if the archer is not very good at judging distance? A moderate level of both is usually the best option when applied to the game most commonly hunted with a bow in North America today.
No. But then the ballistics on a bullet and how it creates damage and death are somewhat different than that of an arrow fired from a modern compound bow.
Nobody really answered my question. I'm playing around with some different weight arrows and such and seeing how the speed vs weight debate always comes up, I would just like to know, what is considered an acceptable level of momentum.
As I said; rocket science is not required to determine the setup necessary to do the job. Of course, you can formula the hell out of physics. However, what great advantage is realized?
Mike from Texas:
Even if you shoot a lighter arrow than necessary, as long as the total shaft weight is not less than recommended for a 70# draw-weight, you will have all of the necessary momentum required.
Keep in mind: light arrows often means more bow shock, more noise, harder on bow, and can be more sensitive to post-shot recovery than a heavier arrow.
There is a somewhat accepted number but I cannot honestly remember it at this point. I remember a discussion on here awhile back when the Ashby report was reviewed. For some reason I remember the number .57 but that may have been the momentum figure that someone was asking about and not what you are looking for. If only ArthurP would catch wind of this one.
KE vs Momentum
