Kinetic Energy question


Swamphunter

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Well here goes.. over the winter I received a martial arts related injury to my right shoulder with a partial separation. I need surgery but am putting it off until this winter. I went out to shoot my bow today (70lbs), and man did it hurt! :jaw:

There is no doubt I now have to lessen my draw weight from 70 down to around 62 until next year. I have scheduled my Ted Nugent safaris Russian boar hunt for this November 5th and 6th. Is 62 lbs enough to kill a wild boar?

Dang I am messed up right now.... :bummed:

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Most 45 pound compounds today shoot with more energy than our old recurves. Kinetic energy is not a good way to figure an arrow's efficiency anyways. It gives too much value to the velocity which is squared. Momentum (and impulse) is a better indication of how it will kill. Velocity is not squared in this formula and given the same consideration as weight. All it means is that as heavy an arrow as you can shoot and still have a decent enough trajectory to hit your target is the one to use. We are not trying to impart energy to our target but want the arrow to keep it's velocity as long as possible. I would turn your bow down even further for awhile. A 400 grain arrow from a 50 pound compound will probably do what you need to get done with a good cut on contact broadhead.

Mark

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Most 45 pound compounds today shoot with more energy than our old recurves. Kinetic energy is not a good way to figure an arrow's efficiency anyways. It gives too much value to the velocity which is squared. Momentum (and impulse) is a better indication of how it will kill. Velocity is not squared in this formula and given the same consideration as weight. All it means is that as heavy an arrow as you can shoot and still have a decent enough trajectory to hit your target is the one to use. We are not trying to impart energy to our target but want the arrow to keep it's velocity as long as possible. I would turn your bow down even further for awhile. A 400 grain arrow from a 50 pound compound will probably do what you need to get done with a good cut on contact broadhead.

Mark

So kenetic energy then would be better for a bullet right?..But for an arrow shaft we want penetration..at least thats the way I read your post.

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So kenetic energy then would be better for a bullet right?..But for an arrow shaft we want penetration..at least thats the way I read your post.

If you want to blow up prarie dogs KE is great. Light fast explosive bullet/massive immediate energy dump. If you want to penetrate an elk and mess up the vitals you need some momentum of a heavier bullet to do that. A balance of the two. An arrow doesn't have much energy to kill with and needs to penetrate and cut as it does.

mark

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Kinetic energy is the force you are hitting something with force and the sam egoes for bullets or arrows. Whether it is either of these you will not get the results that you want with out speed. Speed does play a big role in achieving KE. I agree a heavier bullet or arrow will retain its energy at a slower velocity but if you want to shoot lighter poundage and maintain effective KE of 40# or higher. You will need to keep your speed up bullets and expandibles are more critical of speed than a fixed blade since there is expansion involved.

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Kinetic energy is the force you are hitting something with force and the sam egoes for bullets or arrows. Whether it is either of these you will not get the results that you want with out speed. Speed does play a big role in achieving KE. I agree a heavier bullet or arrow will retain its energy at a slower velocity but if you want to shoot lighter poundage and maintain effective KE of 40# or higher. You will need to keep your speed up bullets and expandibles are more critical of speed than a fixed blade since there is expansion involved.

If you agree that a heavier bullet or arrow will retain it's energy (KE) at a lower velocity then why is it important to have speed since the energy to open it is the same regardless of speed? Energy is energy.

Mark

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Nevermind the kinetic energy thing. I want to know why you scheduled a pig hunt during our north country peak rut? :D

But it is Uncle Ted so I understand. I would miss my own wedding to meet Uncle Ted. :D

Thats it Ranger.... I have a martial arts test near the end of October... don't want it to end up the same weekend, but knew I was safe in November...

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If you agree that a heavier bullet or arrow will retain it's energy (KE) at a lower velocity then why is it important to have speed since the energy to open it is the same regardless of speed? Energy is energy.

Mark

Because you can't get more energy without more speed. Even though a lighter arow might lose energy faster than a heavier arrow. The lighter faster arrow will still have more energy down range.

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You can get more Kinetic Energy by increasing either speed or weight. The speed is squared so it will more noticeably increase KE over weight.

The weight will be the same no matter how far the arrow travels and the speed varies (decreases) so KE at further distances will drop more dramatically than momentum (the formula doesn't square the velocity as in KE).

Mark

Edited by m gardner
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If you increase the weight, you need to increase the speed to keep up with the weight increased. If not, your KE decreases because it doesn't have the momentum to maintain that weight increase therefore reducing your KE. I know all about this, I've done chrono studies out to 40 yards with different arrows, different weights, and all the above. Besides there are calculation charts that will reassure you of anything you might not be to sure on. Just be sure to change the speeds when you change the weights and you will see, I'm done.:pop:

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