more real-world blade physics

DowntownDM said:
...tune into next week's episode of Kentucky Windage Gone Wild meets the Box o' Woodsman's Truth where the questions on everyone's mind will finally be answered:

your mom's Pie Server vs Grandma's Butter Knife - which one is best for chopping through frozen blocks of cream cheese for spreading on bagels while summering in the North Pole? does the extra length of the Pie Server make a difference? and, what sharpening angle is best for spreading cream cheese at high swing velocities?
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all I can say to that is: real men don't eat cream cheese...

but if you keep on with the insults, be forewarned: I'll be forced to challenge you to a duel: butter knives at twenty paces...
we'll fight til one of us is spread out on the hard, cold ground...

: )

but seriously: it seems clear that some of us enjoy these threads, while others don't... if you don't like them, then don't read them... not sure why you feel the need to mock those of us who do...?

for the record: I expect I've done as much real-world chopping and knife work as the next man... just because someone enjoys talking theory, doesn't mean they don't get out there and use their blades too...

don't make me use mine on you...
 
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just to make absolutely certain that no one misunderstands my last post:

I WAS ONLY KIDDING !



there's no way I'd bring a butter knife to a fight...

: )
 
Skytorn said:
Your supposition is correct and it can be proved in a variety of ways.

Impact-relevant, use Impulse for amount of force that changes hand. Since Impulse = delta momentum, a greater velocity given the same mass will produce a bigger impulse and therefore a better chop.

Using change in kinetic energy (0.5*m*v^2), mass coefficient is a linear multiplier while velocity is a quadratic multiplier (v^2). So velocity will produce a greater effect.

Using torque, as the swing will be in an arc and therefore rotational in nature, the centre of mass for a longer blade will be out further than a shorter blade, so torque, F x d(centre), would be higher as well.

*end geek-out*
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THIS!
You want the center of mass to be travelling faster, and in a rotational circumstance, the longer the radius, the faster the COM. This is why longer blades chop better and also why you want your weight to be forward.
 
no worries, killer, I was just goofing around anyway - and was poking fun at the trend of such threads more so than any given participant.

besides, I'll see your butter knife and raise you a tactical titanium Spork at 20 paces...

;)

LittleBill said:
all I can say to that is: real men don't eat cream cheese...

but if you keep on with the insults, be forewarned: I'll be forced to challenge you to a duel: butter knives at twenty paces...
we'll fight til one of us is spread out on the hard, cold ground...

: )

but seriously: it seems clear that some of us enjoy these threads, while others don't... if you don't like them, then don't read them... not sure why you feel the need to mock those of us who do...?

for the record: I expect I've done as much real-world chopping and knife work as the next man... just because someone enjoys talking theory, doesn't mean they don't get out there and use their blades too...

don't make me use mine on you...
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DowntownDM said:
no worries, killer, I was just goofing around anyway - and was poking fun at the trend of such threads more so than any given participant.

besides, I'll see your butter knife and raise you a tactical titanium Spork at 20 paces...

;)
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"No! NO!!! Anything but a Spork! I take it all back! Just please! don't Spork me!!!"
 
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but come to think of it: that might make an awesome thread:

'The real--life physics of the tactical titanium Spork'

or not...
 
Naga99 said:
Is Jerry gonna make us an INFI spork?
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what, you didn't get one yet?

mine is .32 thick, hand-shaped magnum G10 handles, and Satin. Sweetness.

sorry, no pics, it's one of those deals where it can't be shown just yet.
 
Well, as a hacker as well as knife knut, I know that it is vital that I continue accelerating my golf club THROUGH the ball. Stopping my force, or even maintaining constant momentum, during the swing will ensure I flub the shot. Deceleration begins after I've maxed out the arc. Prior to that, my swing is getting faster. Acceleration.

So, I'd say that force is the proper term. If you are properly applying a good swing to that knife, it IS accelerating. Unless you are swinging at something behind your knees where you lose the ability to keep the swing momentum (not the technical term) going.

And the tip of the 12" is definitely accelerating faster than the tip of the 8" knife.


That's the best I can come up with at quarter-o-midnight.
 
You are looking at the wrong measure of effectiveness. To cut things you have to break chemical/mechanical bonds. That is work, which takes "energy", not momentum. A non rotating object traveling in a straight line has "kinetic energy" equal to (1/2) x Mass x Velocity x Velocity. For an object that is primarily spinning you get "angular kinetic energy" = (1/2) x Moment-of-inertia x angular-velocity x angular-velocity. When you swing a blade you get a mixture of both types of kinetic energy. For equal mass blades with centers of mass moving at the same rate the kinetic energies will be equal. The center of mass is likely to move forward by about 1/2 of your increased knife length. The blade length also comes into play for the angular kinetic energy. Assuming a blade is similar to a rod the moment of inertia goes up as the square of the length.

If you simply spin a rod end-over-end (like a cheer leader's baton) the moment of inertia around the center is: (1/12) x Mass x Length x Length. If you swung it from one end like a club it is closer to (1/3) x Mass x Length x Length. Anyway you look at it for a fast rotating blade the angular kinetic energy goes up as the square of the length.

If you are cutting extremely thin flexible material you can have trouble transferring energy into cutting action. To slash through tall tough grass a long light machete is much more effective than a shorter one or a heavier one. You need speed to apply a high impulse to the contact point. You can't apply force to something that doesn't apply force back. There is very little mass in grass so you need to create a high acceleration to the stems to get any back force.
 
Jim malone said:
Yes the longer the blade the more momentum and potential energy is stored in the blade.
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I'd call that kinetic energy.. :P
LittleBill said:
two knives, of equal weight: one with an 8" blade and the other with a 12" blade:
...
so---everything else being equal---a longer blade is going to be a better chopper than a shorter blade...
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Well, a ''better chopper'' to me would be a blade that chops better while i get as tired as if i'd chop with the reference blade.

Now - if i would chop with two different blades that make the same effort to me - what would be equal? The kinetic energy i put in them? their (angular) momentum?
I guess it also depends on my arm and my way of chopping and another person would get totally different results..

Well, i got to go - writing an exam in "experimentalphysik 3"^^ (starts in 50 min..)

Have a good one! :)

s_f
 
i'm waiting for OldPhysics to chime in, not that i understand much of what's been said already and will be even more lost when he does....

Winston
 
Often I get tired pulling my blade out of what I am chopping more than from the down-swing effort. A flat machete is easy to swing into a tree, but then it tends to jamb. A thicker convex blade (almost an axe cross-section) is easier to back out of a deep cut.
 
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