Sylvan

Of course I understand the numbers but something tells me we are saying the same thing in a different way. Yes its easy to put on a heavier arrow and that is what I would recommend to anyone who thinks they have too little punch but when you do this you increase BOTH momentum and Ke not just one so for all practical purposes what difference does it make whether we talk about momentum or ke we do the exact same thing to increase our "punch" as far as the archer is concerned. Imagine the guy at the range who just asked us what to do to get more penetration. He watches us argue for an hour about momentum and kinetic energy and slugs and ft/lbs and pounds/sq inch and on and on and on. Suddenly we both turn to him and say "put on a heavier arrow". After he picks himself up off the floor laughing he does just that and low and behold it works!

Actually I believe this was all summed up in the 40 pound force required to push into the material. I do agree though that a deer is not homogeneous but of course the purpose of this little academic exercise was to demonstrate that KE predicts penetration depth in a homogeneous material while momentum only indirectly predicts it via time.

Exactly and that is what we are both doing and that is why I say the argument is silly.

Again, it comes down to the point that if you need more punch, put on a heavier arrow and/or head for the gym. Oh yea, i can't forget manboy, make sure your broad head is sharp too!

manboy

sylvan, on your reply that an arrow is a sharper than a .357 that is true.
lets say we load those bullets with a full metal jacket and load them backwards.[:-] we would see them then still not as sharp as the arrow, but shoot though the bag. why? because of the transfer of energy is not to explode, but a driving force though the bag.
heres is the reason i will never use any type of arrows but carbon they have a good drive on impact not the soft bend other types have. but that is just me

Sylvan

Because they would be effectively "sharper" than the other bullets. Softer bullets mushroom which makes them duller so to speak than a full metal jacket which holds its shape on impact effectively making it "sharper". Even backwards it would be sharper than a mushroomed softer bullet.

Arthur P

We got into the argument, Sylvan, because you said KE is THE indicator of how much penetration to expect. I don't believe it. It is AN indicator, even one of the prime indicators. But I think it's secondary to momentum. Your 330 grain arrow is probably generating more energy than the 75 pound recurve that Fred Bear used to kill an African elephant. He used a 1200 grain arrow for that job, so he was likely pumping out a good deal more momentum.

So, do you consider your rig an elephant bow? If not, then I believe my point is made.

ijimmy

More momentum , and ke , when you decrease one you decrease the other and visa versa . A light arrow will have less of both coming off of the same bow , and will have more of both coming off of the same bow with a heaver arrow

Sylvan

I guess we will have to agree to disagree. As I read and understand the physics of kinetic energy and momentum, KE dertermines the amount of penetration and momentum determines if penetration will or will not occur. The calculations I have shown you demonstrate that in a homgenious target requiring equal amounts of force to push the arrow through that 2 arrows of equal KE will penetrate an equal amount even though the cary different momentum.

Let me leave you with a couple of quotes from a popular web site regarding the issue. Your welcome to chek it out. http://homepage.ntlworld.com/joetapley/ here's another that might be more fun... http://www.batesville.k12.in.us/phys...TMomentum.html


On KE...
The arrow arrives at the target with a given amount of kinetic energy. This energy is lost when the arrow hits the target and the arrow comes to a stop. Most of the energy ends up as heat in the target and some is lost via the flexing of the arrow stuck in the target. If 'x' is the amount the arrow has penetrated into the target at any given moment then there will be a retarding force on the arrow from the pile/shaft behaviour described above at that moment F(x). The definition of Kinetic Energy is that it is the integral over distance of the force F(x). In other words it is the arrow Kinetic Energy that defines how far an arrow will penetrate into the (elastic string) target.

On momentum...
The arrow arrives at the target with a given amount of momentum which the arrow loses as it comes to a halt. The definition of momentum is the integral over time of the force F(x). In other words it is the momentum that defines how long it takes for the arrow to come to a stop. Because the momentum change relates to force x time it is also a measure of the strength of the 'impact' the arrow has on the elastic strings in the target. The arrow momentum is one of the factors which will determine whether the elastic strings will stretch or snap. As how many of the elastic strings are broken affects the value of the frictional force on the arrow F(x) the arrow momentum indirectly affects how far the arrow penetrates into the target. (An extreme example would be shooting at a suit of armour - not enough momentum and the arrow would bounce off i.e. zero penetration irrespective of how much kinetic energy the arrow had).

Sylvan

And that my friend imimmy is the entire story in a nutshell. All the rest is just bloviating!

Sylvan

Just found a better site to explain penetration. By Dr. Mark Timney...

http://www.bowsite.com/bowsite/featu...tion/index.cfm

Along with the physics, this guy provides some practical real world advice....

The title of his article sums up what I've been saying all along "Penetration, Much ado about Nothing"

Arthur P

Okay, I guess we'll agree to disagree then. Still, the answer to the original question, "what is the best way to get kinetic energy" for a given bow at a given draw weight and given draw length is to shoot a heavier arrow. In fact, it's the ONLY way. That's assuming the arrow doesn't get so heavy that it runs out the top end of the bow's efficiency range.

Did you read on down on Tapley's site about what is needed to penetrate a solid? He used shields in his example, but says it's momenentum that is needed to penetrate a solid and that an arrow that is too light will bounce off. Earlier, I mentioned using a heavier arrow for extra momentum just in case of a bad shot and needing the arrow to penetrate bone. Same concept.

I'm a strong believer in the philosophy of planning for the worst and hoping for the best. Better to have more momentum than you need than wind up needing it and not having it. To me, the idea of using light, low momentum arrows is just planning for the best and hoping nothing goes wrong. It might work 99 times out of a hundred but, sooner or later, that hundredth time happens when a thick bone IS hit and the animal gets away wounded.

Arthur P

This from Timney's article: "...consider how well traditional tackle does on game without the benefit of extra KE." We've agreed to disagree, but I just wanted to, once again, point out there are glaring contradictions in the argument that KE is top dog. To my mind, enough contradiction to disprove the theory. It was nice being able to use the words of a KE believer to do it.