JR's Science Project
 

Hey fellas. Been away from archery for a few years. Swamp Jr. has a Sci project and decided to do it on arrow penetration upon an object. His hypothesis was that a heavier arrow would penetrate deeper into an object based on KE.

Well the test went as follows: 10 yard shot shooting into block target using a 85 grain tip, 100 grain tip, and 125 grain tip. The same arrow (Easton Acc 3-60's) and bow was used for all tests and the same spot on the target was hit on all shots.

The results:
85 gr 3 shot average: 28.75 cm of penetration of the target
100 gr 3 shot average: 32.5 cm of penetration of the target
125 gr 3 shot average: 30.5 cm of penetration of the target

Based on our theory, the 125 grain tip should have penetrated the target the most, but did not. I know the logical thing to do is to shoot thru a chronograph and get fps and weight of arrow to figure KE. But the local shop is closed today and the project is due tomorrow [:-].

Can ya'll give me some reasons why our theory did not hold true?

RE: JR's Science Project
 

The only thing you can really say is that because KE is composed of both velocity and mas (1/2* M*V^2) What you are seeing is a combined effect of changing both the mass and velocity of the arrow. As the mass increased the velocity decreased. I do not know how much each grain of weight difference would change the velocity but you can see by the equation that velocity is more important in terms of KE then mass. (After all it is squared which means it is basically taken into account twice!) Though I believe that mass is still important what I think you are seeing is that both increased mass and velocity increased penetration however because a increase in one decreases the other there should be a mass and corresponding velocity at which penetration is optimal. I think that is what you are seeing.

RE: JR's Science Project
 

"The Block" may not be the best material to shoot into.

If some of your shots are going more directly right between the layers of foam instead of hitting the edges as they enter- that can alter your findings. A fresh, solid foam block would be better for each shot- or better yet, ballistics gel. And just doing three shots to make up an average may not be enough either. How about 10?

If the spine of the arrow is reacting poorly to the heavier tip, that will degrade your penetration findings also.

RE: JR's Science Project
 

I would look at the spine of the arrow. Since you used the same arrow for all 3 heads. Also is the cut Dia the same. I hope you are using the same style broad heads too.
There is a huge dif between the 85 and 100 gr tips this does not sound right for 15 gr dif. I think your test might have a few bugs to work out.

RE: JR's Science Project
 

I think the spineis OK. .340 for 27.5" arrow. 29.75" draw length at 68 lbs. At 8.3 grains per inch, that is 228 grains for the shaft only. The same style field tip was used for all tests. I think I will go shoot ten shots per test though, to see if the results vary.

RE: JR's Science Project
 

Like mentioned, check spin. The 125 probably had more bend in the arrow, absorbing and releasing some of the bows energy in the arrows elastic potential.

RE: JR's Science Project
 

OK. The next big ? is the block.It might not be consistant enought for your test.

RE: JR's Science Project
 

Actually, you probably got a bit better penetration with the 100gr because it is ideal for the FOC of the arrow, whereas the 75 is obviously too light, and the 125 is too heavy.

RE: JR's Science Project
 

Thanks for the replies. I shot ten more shots with each field tip withvery similarresults.

I agree OHbowhntr. The FOC is ideal with the 100 gr tip. This is what I have been using for my hunting set up. Also, Ishot thru thechrono and determined the following.

Arrow with 85 grain tip = 383 grains.....................275 fps= 64 ft lbs KE
Arrow with 100 grain tip=398 grains.....................271 fps= 65.4 ft lbs KE
Arrow with 125 grain tip =423 grains.....................257 fps = 62 ft lbs KE

This makes sense that the 100 grain arrow penetrated the deepest with the most KE. I think Indianahunter83 hit the nail on the head. With velocity being squared, it plays a larger factor than the mass in determining KE. Am I missing anything??????

RE: JR's Science Project
 

Looking only at KE, what doesn't make sense now is that the 85gr produced more KE than the 125gr but penetrated less than it. Just an observation, I can't offer any explanation.