kinetic energy

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Lot to Learn
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RE: kinetic energy

Postby Lot to Learn » Thu Jul 15, 2010 8:31 am

I am all for speaking in a language we all understand.  Where I differ, and I may be wrong, let's take my bow, Bowtech Airborne 101,  let's say my total arrow with 100 gr broadhead is 400g.   I don't know the speed for a fact but I would think this is close, let's say with that arrow it shoots 280fps.    That is about 69 ft lb of energy. Same bow with a 500gr arrow, it will be slower, how much slower?   If it slowed all the way to 250 fps then same kinetic energy, about 69ft lb.   Maybe it will not be much different in speed, I honestly don't know.

I understand where you are going with the equal but opposite reaction, but between release and hitting the deer,  I don't understand why the smaller arrow is losing so much more energy than the bigger arrow.  Maybe in my example the arrows are too close in size to make a difference, and maybe 30yds is too close also.

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RE: kinetic energy

Postby Bow Hunter » Thu Jul 15, 2010 9:00 am

KE and momentum are more important than speed. Almost always a heavier arrow shot out of the same bow will have more momentum and KE. Weight is more important. Think of it like this. What would you rather get hit with at 3 feet away, a ping pong ball or golf ball? The are the same size and about just as hard. Now throw both balls as hard as you can. The ping pong ball will leave your hand faster then the golf ball. But at 30 feet which will be going faster? The heavier ball will hold its speed longer (momentum). At 3 feet they both may sting the about the same but at 30 feet give me the ping pong ball.

It's the same with an arrow. A 350 grain arrow will loose it speed faster than a 500 grain arrow. At 20 yards the KE will be slightly more with the 500 grain but at at 40 yards the 500 will me much more. At 40 yards the 350 grain arrow will be much slower in comparision. It will lose a greater percentage of speed.

Guys have looked at this alot. You should be able to find lots of info on it. Search the mathews forum. It is a great resouce and no bashing like AT. Shoot through a cronograph at 2 and 50 yards with both a light and heavy arrows. The light will lose much more speed.

Here is a link to a KE calulator. Play with some numbers. 580 grains -250 fps and 350 grains -300 fps ... calculator

I shoot a 70# bow with a 584 grain arrow at about 250 fps. Thats on the heavy side (8.4 grains per pound) Most bow makers require 5 grains per pound. I think around 7 grains per pound is good.

Heavier arrows make your bow quieter, absorb more energy from the bow and hit harder. But yardage estimation is more critical. With todays rangefinders not a problem as I see it.

Light arrows are nice because they drop less. If you think its 25 yards and is 29 not that big of deal with a light faster arrow. With a heavy arrow it could mean 6 " low.

I hope this helps.


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RE: kinetic energy

Postby extroverted » Thu Jul 15, 2010 10:34 am

OK not to confuse you anymore, but speed is also a critcal factor when adding up your KE
Lets assume that you are comfortable shooting at 65# this would mean that you are not shaking when pulling the bow back, or pointing the bow up in the sky to bring it back.
just a single easy motion. Now lets again assume that your bow at 65# has an arrow speed of 250 using a 400 grain total set-up your KE will be 55.53 ft lbs of force hitting the object at lets say 25 yards.
Now lets use the same speed and yardage because this will remain constant.
using a 500 grain arrow set-up KE will be 69.41 ft lbs of force.
The change is in the arrow weight. or mass

Lot to Learn
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RE: kinetic energy

Postby Lot to Learn » Thu Jul 15, 2010 2:20 pm

"KE and momentum are more important than speed."  

kinetic energy is the sum of momentum from release to any given point, momentum(p=mv) and KE= (1/2) m v-squared.   so with no speed no momentum or kinetic energy. This is the type of statement, and I am not picking on anyone, this same statement has been made thousand of times and has led me to try to really understand what is going on. Again, academic, but I am going to figure it out. 

does anyone know the difference in weight between a 400 gr arrow and 500 gr arrow? 

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Woods Walker
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RE: kinetic energy

Postby Woods Walker » Thu Jul 15, 2010 3:00 pm

YIKES!!!! Math word problems! Thanks a lot for taking me back to one of the horrors of my childhood!

Here's what I know, and all I care about. It's probably the same result as the greek that was posted above........

When I shoot a 450 grain arrow at 15 yards into a block target with my 55# recurve, it'll go in about 1/3 of the way. Arrow flys nice and flat.

Then I'll take a 600 garin arrow and shoot it from the same distance with the same bow. The arrow flys straight, is noticeably more quiet, but with a little more arc. It also goes 2/3's of the way into the closed.
Hunt Hard,

Kill Swiftly,

Waste Nothing,

Offer No Apologies.....

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passin through
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RE: kinetic energy

Postby passin through » Fri Jul 16, 2010 5:45 am

ORIGINAL: Lot to Learn
does anyone know the difference in weight between a 400 gr arrow and 500 gr arrow? 

About 100 grains[:D][:D]LOL

Sorry ... too easy!

Speed is equally important to the equation its just one that to signifigantly change it upwards usually it taks new cams or new bow IMO. 

I'll give you an example of what i'm working with to help illustrate

I shoot a 375 grain total weight arrow/head combo 261fps...that equates to 56.7 #s of KE.  (helps to have a calculator that has a lot of decimal places I just discovered)
This fall I am hunting with a new set up that will come in around the 455 mark finished.  Say that costs me 10 feet per second (I doubt it but just say it for grins)  so a 455 grain arrow at 251 fps will register in at 69.3 pounds KE.  12 pounds difference just by adding 80 grains to my shaft.  To do the same thing with speed and leave my current target arrow alone would require me adding 17 fps to my bow.

Thats the way I understand it accurately figure your KE you have to know your FPS for sure as well as finished arrow weight.

The formula I use is (Mass divided by 450240) multipled by velocity squared.

There,s a couple other formulas but all will get you close.
It matters not the weapon nor its caliber, rather the caliber of the one who wields it.

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RE: kinetic energy

Postby metrx » Fri Jul 16, 2010 6:27 am

The ultimate goal for a bow hunter is a quick, clean kill.  With a razor tipped arrow this is done by hemorrhaging.  More hemorrhaging is done by more penetration, which is directly connected to kinetic energy, not speed. 
Woodsy, you have written many detailed, informative posts regarding this topic.
Think about it, what would feasibly cause more damage, being hit by a baseball at 100 mph or a bowling ball at 50 mph?
Modern bows allow you to shoot a variation of arrow weights.  You want to shoot the heaviest arrow at the fastest speed possible for the greatest amount of kinetic energy to be produced.  Check arrow charts...Go to a pro shop (if possible) pick two or three different arrow weights and shoot each from your bow and see which penetrates deeper and at different yardages. 

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RE: kinetic energy

Postby berudd » Sat Jul 17, 2010 5:50 am

when someone says that energy and momentum are more important that speed they are not ignoring that speed affets both of these. What they are saying is do not focus on the absolute maximum speed and ignore everything else. If you achieve max speed by using minimum arrow weight the result will not be the most lethal combination. It is my belief (and not I have not tested this) that for bow hunting, momentum is more important than KE. Since KE is based on the square of the velocity you could increase KE and decrease momentum by using a lighter arrow. However, if you sacraficed some speed by going to a heavier arrow you could increase momentum. Momentum is important to us becuase it represents the tendancy for a object to continue moving. High momentum means that the arrow will continue to move (penetrate) even after it strikes an object (a deer). High momentum results in deep arrow penetration. But, if you make the same weight arrow fly faster it will make it more effective assuming it you can shoot it as well and you don't generate excessive noise.

Speed is a factor but it is not THE factor; that is what these statements mean. These statements probably arrise from the preception that we have become too focused on speed. It is the number that all manufacturers present to you as the reason you need to by their bow instead of the other guy. Its a great marketting tool but that does not mean it as effective for determining how effective your weapon is. I think it also became a focus point because a faster arrow flys flatter and gives you more room for error on judging distance.

Keep in mind as well that a heavier broadhead on the same shaft shifts the weight of the arrow forward. This contributes to improved accuracy as well as deeper penetration.

I might just pick up some 85, 100 and 125 gr field points and measure how deep each of them penetrates my target and then use that weight when I hunt assuming it does not travel an arc like a rainbow.

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RE: kinetic energy

Postby jzhorton » Mon Jul 19, 2010 8:20 am

[size="5"][font="courier new"]this explanation made the most sense to me "if you were standing 30 yards away would you rather be hit by a baseball or a wiffle ball thrown at the same speed"


Lot to Learn
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RE: kinetic energy

Postby Lot to Learn » Tue Jul 20, 2010 11:48 am

guys, appreciate all the comments, I was hoping someone could point me toward articles showing science behind.  I get more is better,  I did not ever think as speed being a constant, but for a given bow that does make sense to me.   Like I said, I am bored, season is over 2 months away so I thought I would dig into it. 

As to the last analogy, and others,  would you rather get hit by a #11 softball or a #12 softball, that is probably much closer to what we are talking about. 


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