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Who said that physics wasn't fun?

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Who said that physics wasn't fun?

Old 03-29-2007, 01:37 PM
  #1  
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Default Who said that physics wasn't fun?

Kinetic energy is a wonderful thing. After all, it is one of the major contributors to job security for our beloved archery product engineers. As such, I read about it (KE) all the time. However, the physicist in me compels me to make an argument for another of our often-overlooked friends, momentum. Momentum is related to kinetic energy in that they are both functions of mass and velocity; KE = ½ mv2, where as momentum (denoted as p) p = mv. In the world of projectiles, it works kind of like this; KE gets the projectile to the target, momentum, on the other hand, drives the projectile into the target. In other words, momentum is the driving force behind the penetration of the projectile. Certainly there are a lot of variables that contribute to the penetration of any projectile, an arrow included (diameter, coefficient of friction between the shaft and the target, coefficient of friction between the broad head and the target, etc.). 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 (after all, velocity in the formula for kinetic energy is squared) when it reaches the target, but the more massive arrow will almost certainly have a greater momentum (due it its relatively large mass). As such the more massive arrow will penetrate the target farther than the light arrow. In a hunting situation the more massive arrow will have a greater probability of causing a fatal wound due to greater penetration!! Who said that physics wasn’t fun??!!
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Old 03-29-2007, 01:48 PM
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Default RE: Who said that physics wasn't fun?

Its true that a heavier arrow my carry more momentum. Contrary to popular believe you dont need a heavy arrow. Say a 365 grain arrow at around 300 fps. I will promise you there isnt a deer big enough to stop that arrow on a broadside shot threw the ribs. I dont understand why people are so caught up on the arrow weight thing, a 45lbs bow can get full penatration with medium weight arrows. My last set up was shooting a 360grain arrow at 310 fps, thats fairlya light arrow but it has enough KE or momentum to kill anything in north america, in most of the world for that matter.
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Old 03-29-2007, 02:29 PM
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Default RE: Who said that physics wasn't fun?

Because the heavier arrow retains a higher percentage of its available KE and momentum over greater distances. In other, words the lighter weight arrow bleeds off velocity at a much faster rate than the heavier arrow so the farther you are from the target the less penetration you will have in comparison to the heavier arrow.

Should you not make a perfect broadside shot through the ribs the heavier arrow will have a much better chance of getting deep into the vitals through heavy bonethan the lighter weight arrow will.

The bow is much much quieter with a heavier arrow.

There is much much less stress on the bow (limbs splintering) when shooting heavier arrrows.

There is much less hand shock and vibration with a heavier arrow.
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Old 03-29-2007, 04:40 PM
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Default RE: Who said that physics wasn't fun?

Dude, you missed the point. Your arrow's that always result in pass through do so not because of the KE of the projectile presented to the target, but because of the momentum of the projectile presented to the target. I hear KE referred to as doing all the work of the arrow. That is simply not true. It (KE) is groovy stuff to be sure. However, it is not the whole picture. Besides that, I not sure where you find people that are "all caught up in the mass thing", as I almost never hear about mass. I do, on the other hand, hear about KE/speed ALL THE TIME!
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Old 03-29-2007, 04:59 PM
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Default RE: Who said that physics wasn't fun?

Good post hunter44. Intresting stuff.
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Old 03-29-2007, 06:20 PM
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Default RE: Who said that physics wasn't fun?

I understand the need for momentum, but it is just like KE. You can get momentum from speed, you just dont need as much speed to get the equal amount of momentum from a hevier arrow. Bigbulls statement is true, a heavier arrow does retain more energy and speed at longer distances. Most people blow the heavy arrow vs. light arrow way out of perportion. The way people see it is like getting hit with a wiffle ball compared to a bowling ball. The whole thing is dumb, a 365 grain arrow can do all that a 500 grain arrow can do but with flatter trajectory. As for bibulls comment, it is true but the way he is talking it sounds like he thinks Im shooting a 200 grain arrow trying to shoot threw a moose length ways.
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Old 03-30-2007, 03:32 AM
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Default RE: Who said that physics wasn't fun?

I'm not sure why this is a mystery anymore. Dr. Ashby's studies have shown the importance of momentum when shooting an arrow into a live animal. We're dealing with animals that have bones that can deflect an arrow. Deflection kills energy. Momentum is what resists deflection. It's what gives adequate penetration on the not-so-good shots. When an animal is not only moving, but at an odd angle and the arrow hits a bone, kenetic energy is not the important factor.

Ashby has clearly demonstrated two important variables. One is momentun (heavy weight) and the other is FOC (where the weight is located). Make it heavy and put the weight on the tip and you get maximum penetration. The fact that you can kill an animal with less weight and lower FOC's does not make it the best way. When you're in the situation where there is a high wind and the arrow hits bone, at an odd angle, you might as well have the best chance at killing it.

Flat trajectories are great for those shooting at long ranges and who don't know the distances. Neither of those describes my hunting scenerios. As long as I know the distance, even long ranges are best shot with heavier weights. It seems like everytime I'm out, the wind is blowing, the cover is thick, with small twigs that can get in the way, and the animal presents less than the perfect angle for a shot. I'll take the advantages of high FOC and heavy arrows every time.


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Old 03-30-2007, 06:24 AM
  #8  
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Default RE: Who said that physics wasn't fun?

You can get momentum from speed, you just dont need as much speed to get the equal amount of momentum from a hevier arrow.
You are correct in that you don't need as much speed but that's not the point.
Model "X" bow set at "X" draw weight and"X" draw length will store a specific ammount of energy. The lighter the arrow the less time it spends attached to the string and therefore the bow is transferingless of that available energy stored in the bow.The arrow thenproduces less KE and less momentum. The heavier arrow stays attached to the string longer and more of that available stored energy is transfered to the arrow and this arrow has more KE as well as momentum and all other things being equal will penetrate deeper.


As for bibulls comment, it is true but the way he is talking it sounds like he thinks Im shooting a 200 grain arrow trying to shoot threw a moose length ways.
No, but if the hunter screws up and shootsthat moosesquare in the shoulder blade the 500+ grain arrow will penetrate quite a bit deeper than the 360 grain arrow will. Provided they are each properly spined and of the same diameter.
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Old 03-30-2007, 08:54 AM
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Default RE: Who said that physics wasn't fun?

Also forgotten is that a bow has a much more efficient transfer of energy to heavy arrow than a lighter arrow... We all know that the KE and momentum discussed both come from energy transfer! Without good energy transfer you might as well shoot a 15lb draw bow!
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Old 03-30-2007, 09:31 AM
  #10  
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Default RE: Who said that physics wasn't fun?

Bigbulls, you have it wrong.

KE is a function of the bow, not the arrow you are correct, but when the string is released, (excluding the losses due to bow efficency) the bow will transfer all of its potential energy to the arrow. The difference between a light and heavy arrow is that for a light arrow it will transfer that energy in a shorter time than for a heavier arrow. Time on the string a zero to do with the amount of KE transferred.

KE is essentially a measure of the area under the draw force curve for a particular bow. When the string is released, that potential energy is converted into kenetic energy. No where in that equation does the weight of the projectile come into play.
Now if we want to look at what happens to the projectile when this fixed quantity energy is transferred to it, we use the equation

KE = ½ mv^2

There is no variable for time in that equation. The amount of time it takes is irrelevant.

Based on a fixed quantity of KE, we can rearrange the equation to come up with:

sqrt((2*KE)/m) = v

With KE being fixed, the only thing in the equation that can effect the final outcome (velocity) is the mass.
As mass goes down, velocity will increase.
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