Debunking My Own Video
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- Опубликовано: 31 май 2024
- Today we're doing an experiment to fact check a video I made last year. I build an air cannon and two fiberglass arrows to test whether penetration is driven by energy, momentum, or speed.
This is my first video after moving to Wisconsin to start my career as an engineer so it was a little bit of a challenge finding materials and locations for this video. This idea has been bouncing around in my head for a while so I'm glad I finally decided to test my own assumptions.
Outro Music: "Blast" from Bensound.com 
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I can't believe I've made it this far into my life without realizing kinetic energy and momentum aren't basically the same thing.
40 seconds in and I don't really care what happens after, this video has already been invaluable to me.
sound design is my passion
The Wisconsin mosquitos...
Great video. It’s unsurprising that the energy is the determining factor here. The arrow is doing work against the force required to break the target. The force doesn’t depend on the speed in any significant way in this speed range because it’s plastic deformation that’s doing most of the resistance. Thus the arrow stops when that stopping force does work equal to the arrow’s energy. More energy with the same resisting force means deeper penetration in solids.
Conversely in fluids, the resistance depends significantly on the velocity of the arrow and heavier arrow will be slower but will penetrate deeper into the fluid because the resistance will be significantly smaller for it. Maybe you can do a better explanation in a video someday.
This is more or less why I wanted to do this test. After the original video I realized that energy being force times distance may actually be the driver. The only counter argument I could think of was that force may not be constant with speed but looks like it is
@@ConHathy it has to be mostly constant. It’s basically the ultimate compressive strength of material times cross section of the hole. Maybe some friction as well but it doesn’t depend on the speed as well. The other way to look at it is to see where the energy goes initially. Most of it goes into breaking the bonds holding the solid. And the energy required to do it doesn’t depend on speed. Only tiny minority goes into accelerating the target or into pressure waves in it. Things may be different if the projectile is supersonic in the target. But it is not the case for these arrows.
Great video. It is interesting to watch your content evolve over the years. I've been watching since you old Arma/SE videos.
Keep up the great work!
It was nice to see you explain the different possible results before showing the testing! tho it took me a bit to understand why you were asserting that kinetic energy would be the same for different arrows fired at the same speed. My best guess is that since KE = (1/2)*m*v^2 and m is small, v^2 is large enough that KE1 is very similar to KE2. I'm still confused though, because KE1 will always be double KE2 because M1 = 2M2, which puts us back at "significantly different KE" territory.
The speed was actually different for the arrows! The light arrow averaged 22.95 m/s and the heavy arrow averaged 16.72 m/s. This gave them kinetic energies that were within 4% of each other. Despite this, the heavy arrow actually had about 44% more momentum because momentum doesn’t square the velocity. That’s why a single test can isolate momentum, kinetic energy, and raw velocity as potential driver’s for penetration depth, each would create a different result given the same setup!
Something I've seen from ballistic testing with bullets on other channels is that lightweight for caliber ammo has better penetration qualities, while heavy for caliber ammo stops faster. The light ammo may tend to zip through a target while a heavier one will dump its energy. Fast and light bullets are known for wrecking steel targets at close range, but losing energy with range more quickly from drag.
Someone even got their hands on exotic As Val 9x39mm subsonic AP ammo. It uses a stupidly heavy but slow bullet in order to prevent a sonic boom. While the bullet carried a ton of energy, it failed to penetrate a helmet even common pistols could with normal ammo. That said, the bullet left a massive, fatal dent in it.
Long winded way of saying it, but in bullets, the penetration seems to favor velocity rather than mass or even total energy. Makes some sense when you consider the equation for kinetic energy. Probably helps that the material may not be able to deflect quickly enough, and their inertia makes a better shearing force.
That last bit is largely guessing, but it's interesting to consider nonetheless.
I’ve heard the same thing which is why I mentioned it as one of the possible outcomes. I doubt this test translates directly to such high velocities, but I do wonder how much bullet shape/size changes things. Also, how well the heavy bullet you mentioned actually matched the energy of the lighter ones. Having the same powder load behind a heavier bullet would increase the pressure so the manufacturer may have reduced the load to avoid destroying barrels
knowledge is power 😀
Can you link a comment to the old video in the description of this video so people can see the original video after watching this video
Momentum has almost nothing to do with penetration.
At low velocities (knives, arrows) it is kinetic energy. Penetration of most softer targets is highly efficient.
At intermediate velocities (very slow bullets) it is a combination of kinetic energy and sectional density.
At high velocities (rifle bullets in soft materials) it is almost purely sectional density, and is logarithmic with speed of the projectile doesn't deform.
At extreme velocities (space debris), the object explodes on impact and it's the cube root of kinetic energy, as that is how explosions travel through matter.
And at even higher velocities (radiation) mass and energy doesn't affect penetration and the "object" (particle beam) penetrates based on velocity and the characteristics of the projectile's particles, rather than its bulk characteristics.