Polar Moment in swords

[Phillip Rodley]

I usually lurk in the military sword forum.What Michael Stora calls centre of momentum is what military sword people call centre of percusion ,analogus to the sweet spot on a cricket bat and I presume a baseball bat.In the good old days(18th century) a P was stamped on the blade to mark this point.
Correction it is called the point of percussion not centre of percussion .

[Kris Hampel]

I've only read the first few posts of this thread so my apologies if this fact has already been established, but it seems you are using the terminology incorrectly to me, coming at this from an engineering background.

Polar moment is a measure of an objects ability to resist angular displacement under an applied torque. There seems to be confusion and an incorrect use of of polar moment when actually the moment of inertia (which can be used to measure angular acceleration) is what should be used.

EDIT: Actually I bothered to search it and wiki has saved me the time and probably expressed it more clearly:

Polar moment of inertia is a quantity used to predict an object's ability to resist torsion, in objects (or segments of objects) with an invariant circular cross-section and no significant warping or out-of-plane deformation.[1] It is used to calculate the angular displacement of an object subjected to a torque. It is analogous to the area moment of inertia, which characterizes an object's ability to resist bending and is required to calculate displacement.

The larger the polar moment of inertia, the less the beam will twist, when subjected to a given torque.

[I]Polar moment of inertia should not be confused with moment of inertia, which characterises an object's angular acceleration due to a torque. See moment (physics).[/I

Source: http://en.wikipedia.org/wiki/Polar_moment_of_inertia

I've seen a lot of swords being marketed as having a low polar moment but I think, as above, people have been confusing it with a low moment of inertia. These swords are being sold on the basis that they feel 'alive' or 'fast', that is to say that they are comparatively accelerative with light load. This is a characteristic of having a low moment of inertia and is not related to the polar moment. A low polar moment suggests that a sword would displace more easily under applied torque, it has nothing to do with 'how it handles', so to speak.

This confusion needs to be nipped in the bud and peoples marketing should be corrected as misuse of technical terms leads to major headache down the track to try and 're-educate' people who have come to misinterpret the terminology.

If people feel the terminology is being used correctly, then can someone point me in the direction of the marketing as I have not seen it. I hate to single out Angus Trim here but I just happened across it on his site will looking at some of his great products. I know he has a little disclaimer on his site stating the following:

Low Polar Moment as it pertains to swords means there is a lot of mass around the hilt, making for a very lively sword...

But why apply a technical term inappropriately, just for swords, doesn't 'moment of inertia' sound 'markety' enough or something?

[Arik Estus]

There are many words that sound the same but have different meanings in other contexts.
You cant apply apples to oranges and not get sour grapes.
Also check out: http://tinkerswords.com/forum
I do believe Tinker has had more to say in a lucid fashion about Polar movement in swords then most.

Just how many Swords with Low Polar Movement is a LOT? And where have you seen the phrase used? Ive only seen the phrase referenced in the past couple of years by people here or those affiliated here. Just wondering.

[Kris Hampel]

There are many words that sound the same but have different meanings in other contexts. You cant apply apples to oranges and not get sour grapes.

Thats largely my point, taking a technical term and changing its meaning for one application is a bad move, it just end's in confusion. Swords are not an exception to the rules used to develop the terminology, they are objects and what is actually being measured is angular acceleration therefore one should be describing them as having a low moment of inertia, suddenly switching to polar moment just for swords (in terms of technical analysis swords aren't far removed from steel beams either) is only to cause an increase in confusion.

Also check out: http://tinkerswords.com/forum
I do believe Tinker has had more to say in a lucid fashion about Polar movement in swords then most.

Will do, thanks for that link.

Just how many Swords with Low Polar Movement is a LOT? And where have you seen the phrase used? Ive only seen the phrase referenced in the past couple of years by people here or those affiliated here. Just wondering.

Well I suppose its largely on here that I've seen it and that's precisely the worrying trend, since these forums, particularly the one we are using now, are technical forums which are using misleading habbits. If anyone should be getting the terminology and understanding right I suppose its people who are interested in the technical side of things. I've been away from swords for a while due to uni and as you mentioned, this fashionable term seems to have sprung out of nowhere in the last few years and hence I'm either seeking an explanation as to why, or else trying to nip it in the bud so it does not go on to cause wider confusion. I suppose it seems like a lot to me as Tinker and Gus's blades are the ones I've always spent most of my time looking at (and learning from) so I'm most surprised to see them using the term out of context I guess.

If someone can give me a decent reason as to why polar moment is being used to describe the moment of inertia, when specifically applied to swords then fair enough, but I can't see why the exception needs to be made....

[Vincent Le Chevalier]

Hi Kris!

Thats largely my point, taking a technical term and changing its meaning for one application is a bad move, it just end's in confusion.

Well, welcome in the the world of sword physics Perhaps you might like to read this other thread:
COP = bogus?
Just to get a notion of how difficult it is to change the old habits in terminology Though you have some luck, as the words 'polar moment' are as far as I know only used by very few people (essentially Angus Trim and Michael Tinker Pearce), and even then not often. Others such as myself do use moment of inertia or other related quantities (if you are interested you can read about some work I've done here, not all that easy to grasp judging from the few reactions ).

There is a specific sub-forum over at tinkerswords dedicated to these discussions, but few people know about it:
Esoterica and Errata
Reading the threads there might bring you some more context.

The core problem at work here is that much of the 'theory' around swords has been made by makers, who most often are not all that proficient in general physics. Often the phenomenon is observed and some custom terminology is created to describe it, and yes marketing is possibly a factor here as well.

Which is precisely why we need more people like you, interested in swords and competent in physics Keep going!

[j.voss]

By all means, I welcome correction if I have a misconception here:

I like the term moment of inertia personally, since it gives a "gear head" something that is slightly understandable. Build race engines for a little while and you'll hear the term a lot in relation to lightweight parts and balanced everything. Why? Because it allows for more rapid acceleration of the moving parts and allows more power to be spun to the crank instead of being robbed by having to spin the extra weight and lack of balance.

Since power in a swing is created differently than in an engine, should the power of the swing actually come from the total mass, since I've yet to cut anything just by the movement of the wrist. While the rotational center *would* be in the hand for direction changes and deflection, the fulcrum of the swing would seem to be in the shoulder.

[Vincent Le Chevalier]

Hello !

Since power in a swing is created differently than in an engine, should the power of the swing actually come from the total mass, since I've yet to cut anything just by the movement of the wrist. While the rotational center *would* be in the hand for direction changes and deflection, the fulcrum of the swing would seem to be in the shoulder.

The matter is actually more complex because the center of rotation changes during cuts. In my experience, the center of rotation can be very far away at the beginning, and then reach the hand at the final stage of the cut, as it snaps into the target.

When you're trying to recover from a cut, ordinarily the hand is already stopped but you're trying to stop the sword's rotation around your hand, this is perhaps one of the moves where moment of inertia is most significant.

Even if the center of rotation was always at the shoulder and your arm extended, I don't think you could neglect moment of inertia: the sword is roughly as long as your arm so you can't pretend it's just a single point mass...

Regards,

[BlaiseCarlisle]

I own a bunch of very short swords/long daggers, that still feel "fast". I never really gave it much thought, but after reading this thread I realized it's because they're very bottom heavy. It seems obvious now, but I didn't really give it any thought before.

[Arik Estus]

Its nice when they work properly, isnt it?