techinal kinetic energy question????

camp rut · 03-30-2003, 05:45 PM

I am trying to figure out what size carbon arrows to use with my new bow, I like the speed of a lighter arrow but also like the kinetic energy increase of a heavier arrow, I also realize you lose alot more speed then kinetic energy you gain going heavier.
I' d like to shoot as light a arrow as I can and still keep about 50-55#' s of kinetic energy and I want that 50-55#' s of kinetic energy at 30 yards.

I have a formula for figuring out kinetic energy given arrow weight & velocity.
KE=(total weight of arrow/450240)x(velocityxvelocity)

Finally to my question ........... at what distance using the above formula will I have this amount of Kinetic energy on impact of my target.
I am using this bow strictly for hunting and 30yards is about the max I' d shoot for whitetail and I want make sure I still have at least 50#' s ok kinetic energy at 30 yards.
So at what distance will formula I have reflect the kinetic energy? 10yard? 20 yards? 30 yards?
Greatly appreciated

Hunt2Kill · 03-30-2003, 06:35 PM

Ummm..... 27.39625 Yards is the Drop off from 50# to 49.9786372#

CBM SC · 03-30-2003, 06:43 PM

See if you can find Jackson' s archery page , there is a chart designed to figure out exactly what you want under ballistics !

dick_cress · 03-30-2003, 07:34 PM

Your KE formula is wrong:

KE=Arrow weight in grains (velocityXvelocity/540240)

You may also do a search on kinetic energy and you will find debates on both Kinetic Energy and Momentum.

littleal · 03-31-2003, 04:56 AM

[size=5]I thought it was 450240 as well![/size
AL

Arthur P · 03-31-2003, 06:58 AM

KE = (S^2 X W)/450240, where S = speed in feet per second and W = arrow weight in grains.

The 450240 figure is the grains to pounds conversion of 7000 multiplied by the gravitational constant (32.16 fps per second) to convert arrow weight (in pounds) to mass.

There is really no great KE benefit to shooting heavy arrows that will show up shooting thru a chony at 1 yard. It' s true that hefty arrows do absorb a couple more ft lbs from the bow and make the bow shoot quieter with less shock and vibration. But a few ft lbs is the only difference between the heaviest and lightest arrows your bow will shoot with some measure of efficiency. The real benefit to heavier arrows is their capability to carry that energy further downrange than light arrows can.

KE uses speed squared to calculate energy. Wind resistance uses speed of the projectile cubed in it' s calculation. Speed X speed X speed. So, wind resistance increases faster than energy does, the faster the arrow flies. Light arrows flying faster encounter more wind resistance and have to spend much of their energy just cutting thru the air to the target. Heavy arrows flying slower encounter much less wind resistance AND have greater momentum to help them cut the air, so they use less energy getting to the target. The less energy lost in flight, the more energy available to the arrow on impact.

Personally, I like big, momentum-hog arrows ranging between 9-11 grains per pound of draw weight for hunting. A lot of guys like getting down to 5 grains per pound for the speed. I don' t agree with that concept and think they' re pushing the limits beond reason, but I have to admit they can do it because today' s bows crank way more than enough KE into the arrow to get the job done. I guess a smart fella would split the difference and get some of both ends by shooting something around 7-8 grains per pound. Decent speed, moderate momentum, good energy transfer from the bow to the arrow and good retained energy downrange.

Playing around with different arrow weights and speeds on the ballistics program at Jackson' s site is something I' ve spent some time doing. Feed in the particulars about your arrow and it will spit out speed, energy, momentum and arrow drop in 10 yard increments. It' s real easy to compare downrange performance for arrows of various weights and speeds.

www.bowjackson.com

buckskin73 · 03-31-2003, 10:53 AM

Well, I was once a KE fan. I have since been converted.

I have done some research this past year and found some interesting information. I am an egineering student working on my master' s degree. Being able to utilize what I have learned has been excellent. Here is the low down.

Momentum is conserved. ALWAYS!!!!. To put this simply, momentum is always either transfered from one moving body with mass to another body with mass or it is " carried" by the moving body. Newton discovered this law and is one of the fundamental priniciples of the universe.

KE does not follow this law of conservation. KE can be transfered to another body in the form of potential energy, heat, or work.

There is another theory that relates to this topic and it is called the penetration theory. Very technical but in a nut shells describes the dependance of penetration on momentum.

Use bow jacksons calcualtor. Put in the numbers and see what happens. Play with a small light arrow at high speeds. Then try a heavy arrow at moderate speeds. You will see that the heavier arrow will maintain it' s momentum better than the lighter arrow.

I agree with Arthur P. Momentum is way under rated.

dick_cress · 03-31-2003, 08:53 PM

OOPS . . . I apologize, I transposed the numbers

450240 is the correct constant factor.

Bigpapascout · 03-31-2003, 10:07 PM

arthurP you are all over this like white on rice!
Good post and excelent explaination!

PABowhntr · 04-01-2003, 06:11 AM

Bow Jackson' s url is www.bowjackson.com . You can find more info there in addition to what has already been posted here.