Kinetic Energy...how exactly does it work?
11-19-2008, 07:53 PM
#1
Nontypical Buck
Thread Starter
Join Date: Feb 2007
Location: Allegany County Maryland
Posts: 1,977
Kinetic Energy...how exactly does it work?
I know it sounds dumb, but I was playing around with this speed/kinetic energy calculater thing and up until now I have always figured the heavier the arrow the more Kinetic Energy?? I guess I was wrong, I have a lot to learn!?!?
Total Arrow Weight:
391
Bow IBO Speed:
310
Adjustment for Draw Length:
-10
Adjustment for Draw Weight:
-12
Adjustment for Arrow Weight:
-14
Adjustment for Weight on String:
-20
Calculated Speed:
254 fps
Kinetic Energy:
56.03
Total Arrow Weight:
491
Bow IBO Speed:
310
Adjustment for Draw Length:
-10
Adjustment for Draw Weight:
-12
Adjustment for Arrow Weight:
-47
Adjustment for Weight on String:
-20
Calculated Speed:
221 fps
Kinetic Energy:
53.26
I was just thinking to myself what kind of energy could I gain with a 100grn weight tube in the arrow, and well apparently I would actually be hurting myself!?!?
Total Arrow Weight:
391
Bow IBO Speed:
310
Adjustment for Draw Length:
-10
Adjustment for Draw Weight:
-12
Adjustment for Arrow Weight:
-14
Adjustment for Weight on String:
-20
Calculated Speed:
254 fps
Kinetic Energy:
56.03
Total Arrow Weight:
491
Bow IBO Speed:
310
Adjustment for Draw Length:
-10
Adjustment for Draw Weight:
-12
Adjustment for Arrow Weight:
-47
Adjustment for Weight on String:
-20
Calculated Speed:
221 fps
Kinetic Energy:
53.26
I was just thinking to myself what kind of energy could I gain with a 100grn weight tube in the arrow, and well apparently I would actually be hurting myself!?!?
11-19-2008, 08:05 PM
#3
Nontypical Buck
Thread Starter
Join Date: Feb 2007
Location: Allegany County Maryland
Posts: 1,977
RE: Kinetic Energy...how exactly does it work?
ORIGINAL: MO_Bowhnter
Yes but your momentum would increase about 10%. This is much better than the loss in KE.
Yes but your momentum would increase about 10%. This is much better than the loss in KE.
I usually dont care about all of this stuff normally but am kinda thinking this is something that can EASILY be overlooked when equipping yourself beforeshooting!!
I guess what I am saying is, from what I am figuring I have the MOST kinetic energy from my setup right now with my arrow at 391grns!! I cant gain anymore KEfrom going up or down in arrow weight? I tried it a couple different ways...!!
But I can gain Momentum???? Ok the headaches coming on, im not about to try and figure this out..haha!! Please explain?[&:]
11-19-2008, 08:18 PM
#5
Join Date: Jan 2004
Location: Eureka, MO
Posts: 3,703
RE: Kinetic Energy...how exactly does it work?
The basic way to look at it is the more momentum an object has, the harder it is to stop. So when you are going through a deer, it's harder to stop its movement even though it has less energy. There is a point at which there won't be enough energy to help you through a deer but in your case, 491 gr. at 220 fps will still pass through with no problem.
11-19-2008, 08:26 PM
#6
Nontypical Buck
Join Date: Nov 2006
Location: Where the ducks don't come no more
Posts: 4,420
RE: Kinetic Energy...how exactly does it work?
You can argue this in either direction until you are blue in the face.
The issue in my mind is that depending on exactly what it is you are measuring from a physics standpoint, there are several different calculations for kinetic energy. The one we commonly use in archery is v^2m/450240..... that is twice velocity in fps times the weight in grains divided by the constant 450240.
I have read people both on here and on AT write dissertations about converting fps to mps or mph or mass to kilograms or some crap like that. Long and short of it is.... you can do whatever you want.... just so long as you do it the same for each bow/arrow you test... you'll arrive at basically the same result... plot it on a graph.... it'll look the same either way. Its just an even comparison.... kinda like IBO speeds are supposed to be.
All you can use kinetic energy for is an equalizing guideline.... it takes a produced speed with a given arrow and assigns it an arbitrary number that corralates with performance.... or potential performance. Based on that, you can make an appropriate choice of broadhead for your situation. Its like a stupid personality questionaire... I don't need a ten question survey in glamour magazine to tell me I'm sarcastic and cynical... BUT just how sarcastic and cynical am I??? Thats kinda how it works....
The issue in my mind is that depending on exactly what it is you are measuring from a physics standpoint, there are several different calculations for kinetic energy. The one we commonly use in archery is v^2m/450240..... that is twice velocity in fps times the weight in grains divided by the constant 450240.
I have read people both on here and on AT write dissertations about converting fps to mps or mph or mass to kilograms or some crap like that. Long and short of it is.... you can do whatever you want.... just so long as you do it the same for each bow/arrow you test... you'll arrive at basically the same result... plot it on a graph.... it'll look the same either way. Its just an even comparison.... kinda like IBO speeds are supposed to be.
All you can use kinetic energy for is an equalizing guideline.... it takes a produced speed with a given arrow and assigns it an arbitrary number that corralates with performance.... or potential performance. Based on that, you can make an appropriate choice of broadhead for your situation. Its like a stupid personality questionaire... I don't need a ten question survey in glamour magazine to tell me I'm sarcastic and cynical... BUT just how sarcastic and cynical am I??? Thats kinda how it works....
11-19-2008, 09:12 PM
#7
Join Date: Jun 2008
Location: USA
Posts: 393
RE: Kinetic Energy...how exactly does it work?
How much momentum would be (is) required to get a pass through on a "typical" whitetail standing broadside at 20 yards withgood shot placement?
Assumptions:
-arrow is flying true
-conservative fixed head (like Muzzy 75)
Assumptions:
-arrow is flying true
-conservative fixed head (like Muzzy 75)
11-19-2008, 09:49 PM
#8
Fork Horn
Join Date: Oct 2008
Location: Woodford County Illinois
Posts: 250
RE: Kinetic Energy...how exactly does it work?
http://www.youtube.com/watch?v=HNg-FAyeH90
hows that for kenetic energy.
Apparently I have 85.7lbs in a 407gr arrow total at 307fps arrow speed at 70lb draw but the bow scales at 75.
hows that for kenetic energy.
Apparently I have 85.7lbs in a 407gr arrow total at 307fps arrow speed at 70lb draw but the bow scales at 75.
11-20-2008, 01:47 PM
#10
Nontypical Buck
Join Date: Nov 2006
Location: Eldred, PA
Posts: 1,667
RE: Kinetic Energy...how exactly does it work?
What you are seeing is that when you increase your arrow weight to much, you are dropping to much speed. Theres actually a maximum arrow weight that will give you the maximum amount of KE. Use the KE calculator I posted yesterday to find this. Many times people think the heavier your arrow is, the more penetration you will get. WRONG. Lets look at this. If you shoot an arrow at exactly 350 grains, you might get 70 ft/lbs KE at 300 fps. But if you throw a HEAVY log of an arrow on there that weighs 650 grains, I'm going to see that my FPS has dropped to ABOUT 200. And my KE has dropped to about 58 ft/lbs. BUT WAIT, I thought the heavier the arrow, the more KE. That is a misconception.
And this is why. You have added so much weight to your arrow that you now have slowed it down so tremendously that the arrow energy has actually been decreased. (Check it in the KE formula if you need) Since the speed has dropped so much you have also lost KE. Its kind of like a teeter totter. When one side increases (arrow weight) the other side must decrease (arrow speed) and thus the KE is affected through these two things. If you think of the arrow weight as your variable you will notice that KE and your fps are its dependents. The KE and FPS are DEPENDENT on the arrows weight.
Lets think of this in a real world example. If I take a bowling ball and roll it into you at 5 mph and then I take a baseball and throw it at you at 50 mph which is going to hurt more?
For the guy who said that the KE may be lower, but the momentum will be higher is actually not exactly right. While there is some truth to his arguement, KE and momentum are so closely related and almost identical in formulas that their difference doesnt translate into real world use.
Bottom line is KE and momentum are almost identical scientific properties in a reality sense.
To sum all of this up in a snetence or two this is the bottom line. If you add to much weight to your arrow, the fps slows down so much that it negatively affects your KE. Likewise if you hunting arrow is to light you will gain massive speed, but will be too light to have high KE numbers.
And this is why. You have added so much weight to your arrow that you now have slowed it down so tremendously that the arrow energy has actually been decreased. (Check it in the KE formula if you need) Since the speed has dropped so much you have also lost KE. Its kind of like a teeter totter. When one side increases (arrow weight) the other side must decrease (arrow speed) and thus the KE is affected through these two things. If you think of the arrow weight as your variable you will notice that KE and your fps are its dependents. The KE and FPS are DEPENDENT on the arrows weight.
Lets think of this in a real world example. If I take a bowling ball and roll it into you at 5 mph and then I take a baseball and throw it at you at 50 mph which is going to hurt more?
For the guy who said that the KE may be lower, but the momentum will be higher is actually not exactly right. While there is some truth to his arguement, KE and momentum are so closely related and almost identical in formulas that their difference doesnt translate into real world use.
Bottom line is KE and momentum are almost identical scientific properties in a reality sense.
To sum all of this up in a snetence or two this is the bottom line. If you add to much weight to your arrow, the fps slows down so much that it negatively affects your KE. Likewise if you hunting arrow is to light you will gain massive speed, but will be too light to have high KE numbers.
