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RE: Let's discuss.....
ORIGINAL: jmac_or THe two formulas M=m*V and KE=m*V^2, or KE=m*V*V. Both formulas have V or velocity/speed in them. An arrows highest speed is the second it leaves the bow. From that point forward, it is losing speed, just like a bullet. Your highest KE and M are at that first instant. Where things get dicey is the point of impact. If that arrow is flopping like a fish, the ability to transfer that KE or M in a straight line is reduced, as a function of the amount of deflection from straight. Basically you decrease efficiency at which that arrow can continue in a straight line. There is a finite amount of energy available to a projectile. If the projectile if flying straight, more of that available energy is spent continuing the path forward. The more side to side, the more energy you lose. It is the opposite of mechanical advantage. There is no more engergy being input into the projectile, so it is all losses. The fewer the losses, the more energy or momentum is available to continue in the straight line and overcome the resistance presented by penetration. JMAC KE changes very little with lighter arrows going faster If I remember correctly, the bow was not fully drawn when the guy was shot in the head. It may also be that the bow didn't go through the full power stroke because he was hit before the arrow left the string. I'm sure you're right but I wonder if there have been any tests done or if there is any statistical data to confirm that. Hell, I'm just a cook |
RE: Let's discuss.....
Actually KE will increase significantly with increases in speed, as the value is squared. In momentum, mass and speed are "weighted" the same. Bump the speed by 5 fps and you can add a factor of 25 to your KE, now it is not a straight 25, becasue there is a conversion to get back to your standard energy units, but the effect is the same. The squaring of speed is why it can be a decpetive measurement for penetration. It changes quite a lot, but is really only a small part of the total equation that is penetration. If you really think about it, friction is another main element to think about. How smoothly can that blade slide through flesh?
This is one reason why an arrow with a cut to tip broadhead like a Magnus can zip right through a deer, even if it is only flying aound 200 fps from a fast recurve. Of course those recurve shooters usually shoot big heavy lincoln logs for arrows too, as they realize the importance of momentum. If they can't speed it up, they can sure make it heavier. Ever shot a 3D course with a traditional guy shooting wood arrows. Those suckers really bury into the foam, usually just as far or farther than arrows out of a fast compound. Something to think about. JMAC |
RE: Let's discuss.....
Actually KE will increase significantly with increases in speed Any particular bow setup seems to have a certain KE that it produces - be it shooting a heavy slow arrow or a light fast arrow. **Edit** I reread your post:eek::eek: - If you are talking about increasing speed "without" decreasing weight of an arrow than I agree. |
RE: Let's discuss.....
Yes, Rack, that is what I was referring to. I was just crunching some numbers for an example, when I came back, I saw your reply.
Here is an example. Arrow mass 350 grains speed 300 ft/sec--KE = 69.96 M=.466 Increase same arrow by 5 ft/sec--KE=72.31 M=.474 Increase in KE is 3.35%, while the increase in M is only 1.72%. This seems small but it was only a small change. I was using conversion factors of 450240 for KE and 225120 for M, and I have not verified these yet, so there could be small errors. I think I am going to build a spreadsheet to show the different effects speed and mass have on energy, whether it be kinetic energy or momentum (which is not actually a measure of energy, but it is what we are talking about here). Might find some interesting results. Besides, pictures and graphs always look better than words. JMAC |
RE: Let's discuss.....
Which flattens out a hamburer faster? A light spatula, or a heavy one?
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RE: Let's discuss.....
Depends on how much force is applied and at what speed...
:) |
RE: Let's discuss.....
I was thinking about setting up a spreadsheet myself, but since you offered I won't. I just got to read this thread and as an engineer I'm impressed with you guys' thoughts on this....We really need to get archery season moved up a few months so you guys have less things to ponder!:D BTW, jmac, I agree with your calcs and if I had the time I would build up that spreadsheet for you....overtime has me too busy as it is and I should be hard at it right now!:);)
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RE: Let's discuss.....
Actually KE will increase significantly with increases in speed [link]http://www.huntersfriend.com/bowselection.htm#ketest[/link] |
RE: Let's discuss.....
Techy,
My statement was meant for the same arrow mass. For the rest, I just realized an error in my original formula, KE=.5M*V*V Someone should have caught that one.....:eek: I am going to try to work on the spreadsheet tonight. Just not sure how to post it when I finish. JMAC |
RE: Let's discuss.....
Neither do I, but If it's gonna have lot's of small print, I recommend posting it as a link so it can be viewed as large as possible.
I don't know how to do that either :) |
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