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Who said that physics wasn't fun?

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Who said that physics wasn't fun?

Old 03-30-2007, 06:33 PM
  #21  
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Default RE: Who said that physics wasn't fun?

I didn't mean to imply one arrow was better than another. Actually, when I started this thread I had just been in a debate where KE was continually referenced doing momentum's job. Being a physics kind of guy, I felt compelled to defend momentum (in my experience with archery folks I have NEVER heard the term momentum used without bringing it up myself).

A story. I was with a friend of mine deer hunting in Utah. He shot a small buck poorly. The arrow entered the deer around mid rib cage and traveled back through everything until it hit the femur, which it broke slap in half! The shot was at 40 yards. The arrow was a big ol' lunker 2315 tipped with a 125 grain Bear Razor.A very impressive display of penetration!!
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Old 03-30-2007, 06:39 PM
  #22  
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Default RE: Who said that physics wasn't fun?

That is pretty good penetration Bow Hunter44. I do agree that momentum should deffinetly be refered to more than it is, all you ever hear about is KE. Dont get me wrong it takes both to do the job. I just think people are stuck back 20 years ago when you had to shoot a big old honkin lincoln log of an arrow to get penetration because the bows were so slow. The speed makes up for lack of mass to an extent, so why do people still act like shooting a lighter arrow is emoral or somthing.
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Old 03-30-2007, 06:41 PM
  #23  
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Default RE: Who said that physics wasn't fun?

I've tried staying out but I can't resist.

Big Bulls, time on the string has nothing to do with kinetic energy of the arrow. What you meant to say is "length of the powerstroke" has an effect on kinetic energy. Here's how it works.

When you draw your bow, you are putting Potential Energy into the bow. At full draw, you have maximum Potential Energy and Zero Kinetic Energy. The Potential Energy is equivalent to the area under the draw force curve. For those that remember calculus, it's the Integral of the draw force curve. You are multiplying the Force X the distance at each point along the draw force curve, and adding all those numbers together. For a typical hunting bow, that would give you something like 80 ft-lbs of Potential Energy, depending on the draw weight and draw length of the bow.

When you release the arrow, you convert the Potential Energy into Kinetic Energy. Kinetic Energy implies motion (hence the Velocity term in the equation.) The only two variables in the Kinetic Energy equation are mass and velocity. Time on the string has nothing to do with it. It only LOOKS that way because the heavier arrow is slower. The reason the hevier arrow is slower boils down to Newton: Force = Mass X Acceleration. If Mass goes up, Acceleration must go down for a given force.

The other variable in all this is the bows efficiency. Efficiency means what percentage of the Potential Energy at full draw gets converted into Kinetic Energy of the arrow. It's a fact that bows are more efficient with heavier arrows than lighter arrows. With lighter arrows, more energy is lost in the form of vibration and noise.

Also, it is NOT always true that the heavier arrow will always have more KE than a lighter arrow. If you plot Kinetic Energy on the Y axis and arrow weight on the X axis, you will see that there is an optimal arrow weight that provides MAXIMUM kinetic energy. If you have On Target, try different arrow weights from 300 to 600 grains out of your bow and notice how at first Kinetic Energy increases as the arrow gets heavier. But if you get TOO heavy, KE will start to decrease. But MOMENTUM will continue to increase as the arrow gets heavier (also up to a point).

And MOMENTUM, not Kinetic Energy, is what matters when an object hits another object. Momentum is Mass X Velocity. In this equation, velocity doesn't matter nearly as much as it does in the KE equation. Mass is much more relevant when talking about momentum versus kinetic energy.

Now what does this all mean to an average bowhunter? Not a whole lot since just about any arrow that can be safely shot from a 50-70 pound bow will pass through North American animals. But if I were going to Africa, I'd definitely be using 600 grains arrows over 300 grain arrows!
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Old 03-30-2007, 06:45 PM
  #24  
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Default RE: Who said that physics wasn't fun?

well put gzg38b
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Old 03-30-2007, 07:22 PM
  #25  
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Default RE: Who said that physics wasn't fun?

ORIGINAL: gzg38b

I've tried staying out but I can't resist.

Big Bulls, time on the string has nothing to do with kinetic energy of the arrow. What you meant to say is "length of the powerstroke" has an effect on kinetic energy. Here's how it works.

When you draw your bow, you are putting Potential Energy into the bow. At full draw, you have maximum Potential Energy and Zero Kinetic Energy. The Potential Energy is equivalent to the area under the draw force curve. For those that remember calculus, it's the Integral of the draw force curve. You are multiplying the Force X the distance at each point along the draw force curve, and adding all those numbers together. For a typical hunting bow, that would give you something like 80 ft-lbs of Potential Energy, depending on the draw weight and draw length of the bow.

When you release the arrow, you convert the Potential Energy into Kinetic Energy. Kinetic Energy implies motion (hence the Velocity term in the equation.) The only two variables in the Kinetic Energy equation are mass and velocity. Time on the string has nothing to do with it. It only LOOKS that way because the heavier arrow is slower. The reason the hevier arrow is slower boils down to Newton: Force = Mass X Acceleration. If Mass goes up, Acceleration must go down for a given force.

The other variable in all this is the bows efficiency. Efficiency means what percentage of the Potential Energy at full draw gets converted into Kinetic Energy of the arrow. It's a fact that bows are more efficient with heavier arrows than lighter arrows. With lighter arrows, more energy is lost in the form of vibration and noise.

Also, it is NOT always true that the heavier arrow will always have more KE than a lighter arrow. If you plot Kinetic Energy on the Y axis and arrow weight on the X axis, you will see that there is an optimal arrow weight that provides MAXIMUM kinetic energy. If you have On Target, try different arrow weights from 300 to 600 grains out of your bow and notice how at first Kinetic Energy increases as the arrow gets heavier. But if you get TOO heavy, KE will start to decrease. But MOMENTUM will continue to increase as the arrow gets heavier (also up to a point).

And MOMENTUM, not Kinetic Energy, is what matters when an object hits another object. Momentum is Mass X Velocity. In this equation, velocity doesn't matter nearly as much as it does in the KE equation. Mass is much more relevant when talking about momentum versus kinetic energy.

Now what does this all mean to an average bowhunter? Not a whole lot since just about any arrow that can be safely shot from a 50-70 pound bow will pass through North American animals. But if I were going to Africa, I'd definitely be using 600 grains arrows over 300 grain arrows!

I will disagree with ONE thing,the time on the string IS power stroke.


High brace bows have less time on the string or maybe more correctly defined as length traveled on the string.And low brace high reflex bows have more time on the string or more distance on the string.



I definately agree with mass being the main part of penetration.ButI agree with the mass of the arrow meaning thickness.Mass thickness or stiffness will penetarte better than an equally weighted thin walled arrow.



Ke also plays a role but not as most will think.Ke helps keep an arrow straight.This with good foc and properly balanced and fletched arrows.


But don't overlook spine.It plays a HUGE role.


Speed even plays a role in the cutting department but sharp heads will take care of most of this,but it doesn't really make that big of a difference on most animals.Remeber that speed fights friction.I know this will recieve flack but it is like when you slide a heavy block across a table.It is hard to get moving but once it is moving,it becomes easier as the speed increases.It takes less force to do the same job because of speed.This is a law of dry friction.


Another words,don't worry so much about it.Most HUNTING arrows are designed to do the intended job.Make sure the arrow is properly spined and built with the proper foc and all will be fine.If heavy boned animals are on the horizon,then make sure you utilize a heavier massed and properly spined arrow and you will be fine.


Oh yea,all of the above means squat if the bow isn't properly tuned.[8D]
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Old 03-30-2007, 07:36 PM
  #26  
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Default RE: Who said that physics wasn't fun?

and all of that means nothing if you dont put the arrow where it needs to go.
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Old 03-30-2007, 07:48 PM
  #27  
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Default RE: Who said that physics wasn't fun?

ORIGINAL: passthru79

and all of that means nothing if you dont put the arrow where it needs to go.
I disagree,you can see how well you penetrate the ground or trees.


Just kidding,that is absolutely correct.
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Old 03-30-2007, 08:04 PM
  #28  
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Default RE: Who said that physics wasn't fun?

Ha Ha TFOX, thats pretty good. I dont think pope and young has a catogory for trees yet, maybe they will come out with one next year.
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Old 03-31-2007, 03:42 AM
  #29  
 
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Default RE: Who said that physics wasn't fun?

ORIGINAL: gzg38b

ow what does this all mean to an average bowhunter? Not a whole lot since just about any arrow that can be safely shot from a 50-70 pound bow will pass through North American animals. But if I were going to Africa, I'd definitely be using 600 grains arrows over 300 grain arrows!
I guess this is the part I have a problem with. In my opinion it means everything, especialy to the average bowhunter. The guaranteed passthough just isn't true. What if you hit the shoulder blade of a large north american animal? What if you hit the front legbone of a smaller north americn animal? Does anyone think the 300 grain arrow will do as good a job as a 600 grain one? Heck, the average bowhunter is the one most likely to not make a perfect shot everytime. He is much better off with the heavier arrow that will do a better job at penetrating when he messes up. You can never have too much penetration.

________________

I definately agree with mass being the main part of penetration.ButI agree with the mass of the arrow meaning thickness.Mass thickness or stiffness will penetarte better than an equally weighted thin walled arrows.

But don't overlook spine.It plays a HUGE role.
TFox, this is why high FOCs are so important. Put the weight up front, and those factors won't be as critical.
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Old 03-31-2007, 04:11 AM
  #30  
 
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Default RE: Who said that physics wasn't fun?

In response to the original statement
"However, all things being equal, two projectiles, one with a greater mass strike a target with the same velocity. The lighter projectile will almost certainly have a greater KE ...."
By formula this is not correct. m=mass=w/g which is the only variable in this statement since the velocities are equal and acceleration do to gravity "g" is constant (location of the test is the same location on earth). If you work out the solution (by the equation for KE) for different arrow masses but same velocity you will see that the heaver (more massive) arrow has more KE. Byassuming same velocity for both arrows and the same instance in time it is implied that there are no losses (drag, heat loss, etc.)


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