the physics of chopping

LittleBill

LittleBill

Joined
Jan 25, 2010
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86
leaving blade shape and grind out of it for the moment,
the momentum with which any object (whether a bullet or a blade) impacts its target is calculated by: mass X velocity squared

in other words, doubling the velocity has twice as much effect on impact than doubling the weight

thus it would seem that---all else being equal---a lighter blade moving faster is going to be a more effective chopper than a heavier blade moving slower...

but I'm not an engineer, and maybe I'm missing something... anybody out there who'd care to comment?
 
I don't think the difference in swing speed is large enough between say a BWM and a FFBM to overtake the size/mass advantage of the larger knife
 
blakem said:
I don't think the difference in swing speed is large enough between say a BWM and a FFBM to overtake the size/mass advantage of the larger knife
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^ This gets my vote. I don't have a BWM yet, but have had an FFBM. I think i could swing it just as fast as a CGFBM...so the difference in swing velocity is minimal. On the flip side, the effort it took to swing the extra mass on the FFBM was definitely noticeable.
 
Doubling the velocity has much more effect than doubling the energy.

ex.: if velocity is 10 units, v squared is 100;

If velocity is 20 units, v squared is 400.

So an increase in speed (if it can be maintained) will *usually* have a greater effect than an increase in mass.

That's why you can throw a .50 cal bullet at somebody, and it might sting, but they'll be OK.

Not so OK if a .22 cal. bullet is going 3,000 FPS.
 
If you look at the bullet theory light versus heavy. The light bullet starts out faster than a heavy bullet but in the end the heavy bullet keeps going where the light bullet falls off. Thats why heavys are used in africa. So I suppose the same holds true for heavy blades versus light blades. I would assume anyway.
 
Fubarski said:
Doubling the velocity has much more effect than doubling the energy.

ex.: if velocity is 10 units, v squared is 100;

If velocity is 20 units, v squared is 400.

So an increase in speed (if it can be maintained) will *usually* have a greater effect than an increase in mass.
Click to expand...

Thanks, brother, you're quite right; I stand corrected. You make my point much more eloquently than I did...

also: what complicates this debate in real life is that a heavier blade is usually also thicker, thus possessing a different edge geometry less conducive to biting into the wood... so the lighter (thinner) blade would have that advantage as well...
 
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Geez... good question.

All I can add is what I know with regard to bullet performance and ballistics, which seem to contradict that formula. Lighter bullets traveling very fast penetrate less than heavier bullets traveling at a lesser velocity, ceteris paribus.

Take a shotgun shell as an example. The smaller the shot, the faster it will move and less it will penetrate. 00 buckshot will penetrate much more than #8 birdshot, despite the greater velocity of the birdshot.

More to the point, as Ankerson recently showed, the FFBM clearly penetrates deeper due to the additional mass of the blade. But given the formula provided, one possible explanation is that Ankerson did not increase the velocity of the blade enough to improve the performance of the lighter blade. This seems to make sense to me, as I generally chop at about the same speed regardless of blade weight because I cannot accurately chop (aim) if I fully exert myself.

Still, interesting topic. I need a beer now... my thinking is done for the day. :D
 
d762nato said:
If you look at the bullet theory light versus heavy. The light bullet starts out faster than a heavy bullet but in the end the heavy bullet keeps going where the light bullet falls off. Thats why heavys are used in africa. So I suppose the same holds true for heavy blades versus light blades. I would assume anyway.
Click to expand...

true: as the distance from the muzzle increases, the heavier bullet retains momentum better, and ends up impacting the target with more energy...

but I'm not sure if that analogy can be carried over to blades though: as the distance of the swing is not enough that a serious drop in speed/momentum is going to take place...?
 
kdstrick said:
Geez... good question.

All I can add is what I know with regard to bullet performance and ballistics, which seem to contradict that formula. Lighter bullets traveling very fast penetrate less than heavier bullets traveling at a lesser velocity, ceteris paribus.

Take a shotgun shell as an example. The smaller the shot, the faster it will move and less it will penetrate. 00 buckshot will penetrate much more than #8 birdshot, despite the greater velocity of the birdshot.
Click to expand...

Yes; but aren't you comparing apples to oranges?

In a birdshot load, the total energy is divided among a lot of small pellets; so the energy per pellet will be less, and thus each pellet will penetrate less;

compared to the buckshot load, where the same total energy is carried by fewer pellets; so each pellet carries more energy and thus penetrates better...

But with two bullets of the same weight, traveling at the same speed---and thus carrying the same energy---the smaller-diameter bullet will also be longer, with greater sectional density, and thus tend to penetrate better...

The 6.5 and 7mm Mauser rounds are a case in point: carrying long, thin bullets, they provide awesome penetration despite not being all that powerful; which is why the old-time elephant hunters like Karamojo Bell relied on them: if you placed your shot correctly, they went right through that thick skull into the brain... whereas a fat bullet of the same weight and velocity would not...

the analogy to blades being: given blades of the same weight moving at the same speed, the thin blade will bite into the wood better than a thicker blade...
 
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kdstrick said:
More to the point, as Ankerson recently showed, the FFBM clearly penetrates deeper due to the additional mass of the blade. But given the formula provided, one possible explanation is that Ankerson did not increase the velocity of the blade enough to improve the performance of the lighter blade. This seems to make sense to me, as I generally chop at about the same speed regardless of blade weight because I cannot accurately chop (aim) if I fully exert myself.
Click to expand...

and that may explain why, in real life usage, the heavier blade seems to be the better chopper: in normal chopping, you don't move that lighter blade fast enough to make up the difference...
 
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Ill take a stab at it:

First mistake is that momentum is not m*v^2, it is simply m*v. Kinetic energy is .5m*v^2. This would mean that you would have to swing a knife half as light, twice as fast in order to have the same momentum.

Ok, its been a few years since I failed physics II the first time, but I did pass the second time around.

It gets a lot more complicated past this and I am not near drunk enough to BS my way through it. Impulse(change in momentum), friction, alignment of the planets and I am pretty sure even cup size of Playboy's centerfold come into play. I seem to remember studying all these before my physics final.
 
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the question brings up all sorts of questions. I would say that a normal person has a very limited swing speed. For example if you were able to swing a FFBM at 30 mph I doubt you will be able to double that speed in a blade 1/2 as light. Also you have the question of sectional density, or cross section and friction.

Too many issues to address by phone.
 
LittleBill said:
leaving blade shape and grind out of it for the moment,
the momentum with which any object (whether a bullet or a blade) impacts its target is calculated by: mass X velocity squared

in other words, doubling the velocity has twice as much effect on impact than doubling the weight

thus it would seem that---all else being equal---a lighter blade moving faster is going to be a more effective chopper than a heavier blade moving slower...

but I'm not an engineer, and maybe I'm missing something... anybody out there who'd care to comment?
Click to expand...

Doesn't work that way in real life. ;)

Thick heavy blades will chop better than thin light ones especially on harder wood. Not everything people chop are green 4" saplings, cutting soft green wood is easy.

Light blades will bounce off while the heaver blade have enough weight and mass to bite, speed can't compensate for lack of mass and weight.

Don't need any fancy math programs to figure that one out. LOL :D
 
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jdraizor said:
Ill take a stab at it:

First mistake is that momentum is not m*v^2, it is simply m*v. Kinetic energy is .5m*v^2. This would mean that you would have to swing a knife half as light, twice as fast in order to have the same momentum.

Ok, its been a few years since I failed physics II the first time, but I did pass the second time around.

It gets a lot more complicated past this and I am not near drunk enough to BS my way through it. Impulse(change in momentum), friction, alignment of the planets and I am pretty sure even cup size of Playboy's centerfold come into play. I seem to remember studying all these before my physics final.
Click to expand...


LOL! You just reminded me why I dropped physics and took geology instead. Formulas make my brain hurt. :D

Completely agree that cup size is critical however... I do remember that from college. ;) :p


Seriously though, maybe OP will visit this thread and set us all strait... :thumbup:
 
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Well, you see, Norm, it’s like this. A herd of buffalo can only move as fast as the slowest buffalo. And when the herd is hunted, it’s the slowest and weakest ones at the back that are killed first. This natural selection is good for the herd as a whole, because the general speed and health of the whole group keeps improving by the regular killing of the weakest members.

In much the same way, the human brain can only operate as fast as the slowest brain cells. Now, as we know, excessive intake of alcohol kills brain cells. But naturally, it attacks the slowest and weakest brain cells first. In this way, regular consumption of beer eliminates the weaker brain cells, making the brain a faster and more efficient machine.

And that, Norm, is why you always feel smarter after a few beers.”
 
Maybe we should all just stop guessing and drunkposting and wait for a real physicist ( Old Physics or Mustard Man ) to set us straight. :rolleyes:
 
I am typing from a cell phone, so have no intention to provide a physics lecture. Besides, the last time I taught physics it cost my students 35k a year for the priveledge :D
 
resinguy said:
Maybe we should all just stop guessing and drunkposting and wait for a real physicist ( Old Physics or Mustard Man ) to set us straight. :rolleyes:
Click to expand...

I would love to hear what both of those guys have to say.

Along with them sharing a bit of their knowledge it is good to have real life results also IMHO:)

As simple as I go outside with a machete next to a FBM an see which one chops better on different types of wood. That is probably going to be the most convincing. With that said, I am sure OP or MM could explain the physics behind my results in great detail:)

Anyhow, I agree and hope they stop by this thread and teach us a thing or two:thumbup:
 
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