K.E. VS Momentum

[SPIKER]

Wow, I almost got the same result from the formula. I just didn't have the string add on factor.

[smj]

If you know the mass and speed, the math is very straight forward and there are many calculators out there to help you out. With what I suggest, you'd change the drop height to allow the speed to increase or decrease as required to match the KE at the point of impact. Drop one arrow, measure the speed, calculate the KE. Then work backwards: For the given KE of the first arrow, and knowing the mass of the second arrow, calcuate the speed required to give you that energy with the second arrow. Now, you could calculate the drop height, but it would be just as easy and no doubt quicker to just adjust the drop height until you get the right speed for that second arrow! Using a chrono would help, too!!! KE = 1/2 (mass*velocity squared)

Here's a couple links:

en.wikipedia.org/wiki/Kinetic_energy
www.bowhuntinginfo.com/KineticEnergyNew.htm


Oh, yeah, the acceleration due to the force of gravity on Earth equals 9.8 m/s2 in the metric system and 32 ft/s2 in the English system. This is the acting force on both arrows, a constant, hence the change in drop height requirement.

[smj]

Again, I have found over time that most bows will have some arrow mass that will result in the most energy. The lighter arrow is not always the one that gives the most bang for the buck. Then again, a really heavy arrow might not be the right choice either, one has to get a couple of arrows, shoot them and clock the speed, see what you get out of the bow. The exception may well be longbows and recurves, where the bow simply likes a heavyer arrow - but I've not clocked this yet. I am only going on what I have been told by another. I'll have to test that in the very near future! For my Bowtech Pro 38 - it seems to like around 450 grains. That seems the best trade-off for speed, KE, and flatness of shot. I think you have to check each bow to really know.

[davidhelmly]

This is a very interesting thread. I can't wait to see the test results or better yet a video clip!

[BT]

Thanks for bringing this back up!

I had completely forgotten about it and it has to wait till summer to be done.

I need a rolo dex!

[royden]

I think for most of us we have enough k.E. that really any more just bury's our arrow farther in the dirt on the way out. However the question I have is for women and kids... does a heavy arrow flying at 200 fps out penetrate a light arrow flying at 240? Those numbers may not even be attainable for many of the women/kids.

[Greg Krause]

If you are on the cusp of KE required for a paticular animal, it's a given that your shots should be limited in range. say 15-25yds or so. A heavy arrow will generate a little more KE. with that being said you would want the most KE you can squeeze out and flat trajectory is not an issue, because of the limited range. I would lean towards the heavy arrow.

[smj]

Somewhere along the line I am going to set up a test to get some comparison data. I think I have everything I need except a target to drop the arrows on to for checking penetration. Any suggestions material wise? I suppose I could get a dozen boneless buffalo steaks and have a BBQ once done... Kabobs anyone?

[michihunter]

Wet yellow pages is a good cheap medium.

[BT]

smj said:

Somewhere along the line I am going to set up a test to get some comparison data. I think I have everything I need except a target to drop the arrows on to for checking penetration.

That test wont work due to the fact that gravity is relative to everything within it.

If you drop two arrows the heavier one will always win due to it having more mass and traveling at the same rate of speed.

As to what head design will penetrate better....this would be a good test if the arrows remained the same.