Calling All Shade Tree Physicists

[konrad]

Calling all Shade Tree Physicists


I was lying in bed one morning not too long ago and thinking about arrows. OK, all you folks about to make snide remarks about poking things and other suggestive comments, here is the place to get it out of your systems.

At any rate, I was thinking about arrows, the physics involved in impact drivers and energy transfer pulse/time intervals…OK, you can make a couple of more jokes here too. You won’t be the first (or last) to make fun of me…and I began to wonder about an experiment with the following conditions:

1.   With a “standard” arrow weight of 500 grains striking a target at a “standard” velocity of 250 fps obtain an energy transfer pulse length as a “base line”. In other words, obtain a measurement of the length of time the arrow actually delivers its energy to the target.
2.   Then, as a starting point, construct an arrow with a free floating, internal weight, occupying the rear one third of the inside of the arrow weighing 50 grains as a starting point.
3.   The Forward of Center (FOC) of the rear-weighted shaft would be 15% FOC.
4.   Then, the test arrow would be shot into the target at the same initial velocity of 250 fps and the energy transfer impulse time would be measured again.

My belief is that the weighted arrow’s transfer time would occur over a more prolonged period than that of the un-weighted shaft. If so, the effect would be similar to an impact driver, assisting in penetration due to the act of the internal weight transferring its energy to the tip of the arrow. In effect hitting a “double tap” upon impact.

My question is: Should I spend valuable sack time in more productive pursuits or might I be onto something?

Thanks,
K

[boneaddict]

Your arrow will probably hit fletching first, or do summersaults in mid air, but other than that....

[Todd_ID]

Since when do physics enter the witchery of archery? 

The impact of the arrow is such a violent situation that the arrow flexes as much or more than it does when fired from the bow.  Your weight would rattle around and maybe vibrate down the shaft, but I doubt it would be anything close to the double tap you are looking for.

With 50 grains in the rear, then you'd have to have a huge broadhead to get your 15% FOC.  A large weight and large broadhead only leaves you with a light shaft weight to get your 500 grain target, so spine would become an issue.  (However, putting the weight on the rear would help to negate this a little.)

The momentum of a 50 grain weight flying forward inside a 450 grain arrow would be equal to the momentum of a 500 grain arrow.

Even if you could get the double tap you are looking for, the result would be negligible increases in penetration ability due to needing to overcome static friction (which is much higher than dynamic, or sliding, friction).  You'd still be better off with just a 10% increase in arrow weight.  In this case 450+50 does not equal 500.

Overcoming friction (as the definition of arrow penetration) isn't dependent upon time.  It's a function of mass, surface area, velocity, and coefficient of friction.  The only ways to increase the penetration of an arrow are to increase the mass (heavy), decrease the surface area (skinny), increase the velocity (fast), or decrease the coefficient of friction (slick and flying straight).  Since friction is calculated with velocity squared, increasing the velocity provides incrementally smaller gains in penetration.  The aluminum arrows of 25 years ago penetrated as good as or better than what we see today with carbon because they were heavy, slow, straight and smooth.  Today's bows are light years beyond what we shot back then, but we still killed critters just fine.  Put an aluminum arrow in today's bows, and you're far beyond what we used to blow through critters with.

An impact gun doesn't work by extending the time of impulse; it works by delivering high torque blows in rapid enough succession that the user does not feel the full torque and thus can hang onto the tool.

Now, if your floating weight would happen to be a BB, it could be used to strike a primer inside that could set off a charge of powder, and then you'd have a more effective killing machine; I think the state would frown on this, however.

[KFhunter]

I'm digging the flying bangstick idea, I could really rambo it up 

[konrad]

Perhaps I wasn’t clear in the original post. Both of the completed arrows must be of the same weight.

The internal weight I am going to use in testing is shot shell buffering media.
It is non-hygroscopic, light spherical plastic, free-flowing and does not stick to anything. I currently have some on order and as yet do not have a good idea of its density in grains per cc. As it happens, I use alloy shafts in a modern bow and the internal finish is mirror bright and smooth. While it (the buffer media) will easily be moved to the rear of the shaft by tilting the arrow up, as soon as deceleration due to target contact is encountered, it will begin moving toward the front in one extended (hopefully) “pulse”.

My primary issue (as I have already decided to proceed with experimentation assuming I can get appropriate weights before running out of internal space) has been in the manner in which quantitative data is to be collected. I have imagined a crude method would be the use of ballistic gelatin or perhaps clay. First shooting a “solid” arrow into the medium and then shooting into a fresh block with the movable weight arrow, both tipped with the Saunders Combo-point. One sees the use of ballistic gelatin in firearms testing as a standard media for projectile/load experimentation. It would seem to me that the same media should give excellent results at least giving an “apples to apples” comparison.

Look at it this way, I could be drinking whiskey and picking fights with other drunks or still asleep!

[chrisb]

Doesn't this "prolonged" energy transfer only have real impact if the arrow doesn't go all the way through and exit?

[jaymark6655]

Isn't that the stuff that sticks to everything due to static and is really light.  On testing shoot each arrow 20 or more times then measure depth of penatration, then see if there is any difference in the averages.  What about using some really small lead shot or powder like that used in dead blow hammers?

[KFhunter]

I think you'd loose about 90% of your pulse effect from the buffer media into the walls of your arrow.  The first little bit would slam to the front of the arrow then as the rest slammed into the leading media the forces would be diverted to the walls of your arrow shaft.

You need a solid slug to slide back and forth.

[JPhelps]

Energy equation from physics would state that no more energy can be created once the arrow was in flight.  Only deduction of energy (wind resistance, gravity, hitting flesh or bone, etc...).  You would also add in friction loss from the sliding action which would contribute to worse penetration.

You want an arrow as heavy as possible flying as fast as possible that is flying straight.

[yajsab]

I think right when the arrow leaves the string, the arrow will grab the free floating weight making it flying backward.  Hopefully the knock will not explode or come loose.  If everything is intact, I think the motion of the free weight will reduce the arrow speed to some degrees.  My .02.

[Todd_ID]

Better glue your nocks in well for a downhill shot. 

Sounds like a fun project, but I don't think you'll beat the laws of physics.  Jason is right: energy cannot be created after the arrow leaves the bow.  Time has no part in figuring penetration (friction).  Watch some super slow motion videos on youtube of arrows when they hit a target to see the horizontal forces you have to overcome to get your weight to slide forward.  I predict the arrow will hum on the shot, penetrate just shy of equally with the solid 500 grain arrow, and hum on the impact.  The hum is energy lost.

[JPhelps]

The arrow will be spinning in flight.  The RAM would also spin with the shaft.  So in order for it to slide freely it couldn't be to tight.

I am imagining a wobbly arrow because of the loose ram?

[BULLBLASTER]

I was under the impression that a deadblow hammer is to eliminate the hammer bouncing back up off the hammered item or surface. Arrows usually won't bounce off a soft tissue. I also agree with Jason in that all arrows of the same weight shot the same speed will have the same energy when leaving the bow. Arrow broadhead and fletching differences will change drag and downrange energy. I did an experiment a number of years ago regarding arrow speed. I filled a fat shaft arrow completely full of 125 grain screw in tips and shot from my 70 lb x force. On impack with the target the tips inside bulged against the shaft walls and created cracks all over the shaft.
On another note have you ever seen anyone knocked their inserts out using a drill bit inside the shaft and swinging the arrow? I think your weight inside the shaft would just knock the insert out inside the target. 

[coachcw]

Now this is a good waste of your brain power . a 420 grain arrow at 300 fps will blow right through a elk at 50 yards what else do you need . try curing cancer with that nogan instead.

[TommyH]

I seen somthing similar on a hunting channel just a couple days ago?