Kinetic Energy VS Momentum

[deeraddict]

Having some good back and forth about KE and momentum in another thread. I was talked into staying with a larger diameter, long arrow a few years back by a guy at a pro shop, and the reason was he said I wanted to carry KE down range especially if I ever wanted to go after large game like Elk. i have stuck to that way of thinking and have had great success, even getting a complete pass through on a big doe in an open field at 52 yards. Is it the KE energy and the momentum together that help achieve this? I have pasted an exert from the other thread below. Good stuff, I like learning new things..... What rule do some others follow and why? What knowledge do some of you have on this? I am just going with what I was told by a source I trusted.

Originally Posted by huntyak
Thats the great thing about IW. 99% of the time you get great input with respectful discussions and this thread is no exception. Good points by SS and JJ.

Deer addict, that is an excellent analogy and the difference is that KE essentially dissipates onces it hits an object, and momentum carries on. Throw a whiffle ball and a baseball with equal KE but notice that the whiffle ball rolls a few feet, a baseball rolls a few yards AFTER it hits the object (ground).

Its interesting to discuss these things but with whiteatails I think we are spliting hairs. Shooting a moderate weight arrow with a good head that you can shoot accurately, and you are in great shape on whitetails. :drink2:"


Thanks huntyak. Yep, just trying to learn a bit. I understand that KE disapates, but would this not also add to the momentum? For example Send a baseball out of a pitching machine at 50 mph at panes of glass. It is going to get through a few panes. Now, shoot about 200 finishing nails to that same ball and send it again. (we do this to balls to weight them for pitching strength.) Is it not going to go through more panes as the extra weight adds to the KE which will help to increase the momentum? Not trying to be argumentative just trying to understand completely. I have had great success with the rages myself but think everyone should shoot what they are comfortable with. I have the same experiance as SureShot with the Monotechs, I was really pleases with the way the shot, and am going to be using them this year as well.

 

[203ntyp]

I have always leaned more toward kenetic energy versus vilosity. I do believe there's a perfect balance of the two depending on what your set up is. I shot a shafts that were 12.3 gpi for years with great penetration out of a bow set at 70 lbs. I wanted to go with a smaller diameter so I did slim down to 10.3 gpi which did increase velosity without losing too much KE. I know my buddies complain when we're shooting 3-D when they go to pull my shafts versus their lighter ones since they do seem to penetrate more than the fast set ups they're shooting. Going too light also is hard on the bow limbs so finding the perfect balance for your set up is something you may have to experiment a with. Just my experience.......

 

[deeraddict]

I know my buddies complain when we're shooting 3-D when they go to pull my shafts versus their lighter ones since they do seem to penetrate more than the fast set ups they're shooting. QUOTE]

Funny you mentioned that. Same here. I think my arrows are 10 gpi and my arrows always seem to be buried in the targets when we shoot compared to my friends as well.

 

[oldgene]

I knew there was a reason I should have taken physic.

 

[Gladiator]

Not to ruffle feathers but I think there is some misinformation above. KE is found in the following formula: KE = 1/2 mass x velocity x velocity or (one-half times mass times velocity squared). You can't compare velocity to KE...it is part of the formula. As you increase the velocity, you increase the KE. BUT if you increase the velocity, you actually square it...and that increases the KE by a bunch. That is the whole thought process behind increasing velocity of bows...

Momentum's formula for comparison is p=mv (momentum equals mass times velocity). Note, there is no squared factor for the velocity...

Another thing with pulling arrows out, I shoot a bigger diameter field point on my arrows than most - only because it makes arrows pull out a LOT easier....that doesn't mean I'm not getting penetration. Many people shoot way less poundage and KE and have arrows that are tough to pull out simply because their heads are smaller than the diameter of their arrows.

In reference to huntyak above, Iad make a valid point with a baseball and wiffle ball, but if they shot out at the same KE, the wiffle ball would have much, much more velocity and would have traveled much farther in a very short time than the baseball ever did....

All of that being said, everyone is right in that once that broadhead hits the deer, you definitely want some momentum to carry it through the animal....i.e., you don't want a straw or piece of hay being shot at a deer extremely fast.

There is a balance at some point........where, I'm not sure. Until then, I'll shoot 291 fps, with 100gr head pulling 70lbs. Seems to work OK for the time being....

Again, not trying to ruffle feathers....just extending my duties of "science teacher" to the deer world instead of my class. :way:

 

[deeraddict]

Good input Gladiator.
I am still fuzzy on all of this.
If I am getting it though more velocity means more KE which means more Momentum? KE will dissapate quickly but the momentum stays? So would it be correct to say the KE helps hit the deer hard and start the penetration, but then the momentum carries it through for a pass through?

 

[Crimson Arrows]

Good input Gladiator.
I am still fuzzy on all of this.
If I am getting it though more velocity means more KE which means more Momentum? KE will dissapate quickly but the momentum stays? So would it be correct to say the KE helps hit the deer hard and start the penetration, but then the momentum carries it through for a pass through?

KE is more dependent on velocity as you square this component while momentum is mass x velocity. The lighter the object, the faster it will stop. Its energy dissipates rather quickly compared to an object that's heavier at a slower speed...to a point as Gladiator explained with the whiffle ball example. It will slow down at a faster rate than a heavier object.

Mass and velocity carry equal importance with momentum, velocity carries more weight with KE.there is a happy medium but where you transfer that energy is where it matters in archery, so you wouldn't want a very high KE attained through speed as its squared, and a super light arrow as that energy will not carry over once it hits the object in question. The arrow should be heavy enough to transfer some of that energy via the momentum equation.