Sliding weights

[Rivkin]

This list seems popular among the sword community - it's not the first time I've seen similar ideas expressed concerning the waves for example.

My problem is that for example it's hard for me to understand why the center of gravity is going to be a node for all waves (it should not be for at least for the waves with an odd number of halfwavelengths). Concerning the hilt, it seems more like a boundary condition to me, rather than a center of gravity. Concerning longer swords having higher frequencies and wider diaposon, it seems counter-intuitive to me - I would expect smaller swords to have larger frequencies and bigger separation in between of individual modes, but that's just my guess.

I'll be honest, I don't understand some of the ideas expressed above. Concerning the sliding mass question, again, what are the possible benefits of this construction vs. simply high momentum fixed mass weapon - nothing simple comes to mind.

[Ian]

Originally Posted by Rivkin

... My problem is that for example it's hard for me to understand why the center of gravity is going to be a node for all waves (it should not be for at least for the waves with an odd number of halfwavelengths).

The center of gravity is a balance point and the centers of inertia and percussion are located at different points along the blade. The respective centers are features of the weight distribution and mechanical properties of the sword (which mostly comprises the blade and the tang).

Waves do not originate from any of these centers. Waves are set up at the point of impact and spread out from that point. The further away the point of impact is from the vibrational node, then the more vibration will be transmitted along the blade and will be felt in the handle.

Originally Posted by Rivkin

... Concerning the hilt, it seems more like a boundary condition to me, rather than a center of gravity. Concerning longer swords having higher frequencies and wider diaposon, it seems counter-intuitive to me - I would expect smaller swords to have larger frequencies and bigger separation in between of individual modes, but that's just my guess.

The tang is continuous with the blade and would not be a boundary condition. Depending on the properties of the handle covering materials there may be some dampening of the vibration transmitted from the tang to the hand. My colleagues assure me that physics dictates that the longer the blade, the longer the sweet spot and the higher the frequency of vibrations.

Originally Posted by Rivkin

... Concerning the sliding mass question, again, what are the possible benefits of this construction vs. simply high momentum fixed mass weapon - nothing simple comes to mind.

I cannot see any real benefits to a sliding mass. That was the conclusion of my esteemed colleagues also -- they are skeptical that any greater force could be achieved, and the sliding mass would introduce unpredictable and inconsistent properties of the sword depending on how it was wielded.

Ian.

[fearn]

Hi Ian,

I think your overall conclusion is probably right--that a sliding mass is not beneficial, although it might be less troublesome in an executioner's sword.

I won't pretend to be a physics expert, but I do know a few things about swords and rods.

One thing that confused me was the difference between center of inertia and center of gravity. These are different because....?

So far as the vibrational nodes go, my limited observations are that straight swords are quite a bit like rods: the vibrational nodes are at the geometric center and the quarters. HOWEVER, the center of gravity doesn't have to be at any of these points. To give a crude example: imagine a rod two- thirds metal. It should be obvious to most people that the point of balance will be fairly close to the center of the metal part, because the wood is much lighter. In a sword with a heavy pommel and lighter blade, you can put the center of gravity and/or inertia pretty much where you want it.

So far as longer blades having bigger sweet spots due to higher vibrational frequency, I'll admit that I'm confused too. I agree that the longer blade should have a bigger sweet spot, but I'd bet a fair amount that it would have a lower frequency, just because it's longer. This is the same reason that cellos generally play lower than violins: the frequency is lower, not higher, in a longer string. I'm guessing that the word we're looking for is longer wavelength and bigger amplitude.

However, I'm still very glad that we had an engineer look at it. Now, if someone will get out there with the PVC tub and ball bearings, and find out what a sliding weight feels like when you swing it, we can all rest easily....

[Rivkin]

1. With given definitions the center of gravity and the center of inertia will be the same.

2. If its possible, I would really like to see the formula they use for sword's frequency as a function of length (do they consider it a string ? a thin and long prism ?).

3. Concerning tang not being a b.c., or even a separate body, I would prefer to hold a vastly different opinion.

4. Concerning waves propagating in swords and nodes - propagating waves usually do not have nodes. When people talk about nodes, they usually speak about standing waves, i.e. steady state solutions etc.
I suspect that the logic was that if sword can be considered a string, than a full wavelength standing wave will have a node in the middle, but it will basically be true only for even halfwavelengths mode... Plus I'm really too lazy to calculate the modes of a string with a variable mass, so I don't know how big percentage of the waves will have nodes at the center of mass.

5. Concerning the center of percussion - as far as I remember (and I remember it very poorly), the center of percussian is when you hit it, all the momentum is transfered into the rotation movement of the sword, without any daggling down or up.

[Jens Nordlunde]

Hi Ian,

Thank you for using your time on this topic, and thank you to your colleagues for their time. I had in the start expected the problem to be les complicated than it is, and I cant say that I can follow all the explanations, so I will have to read it one or two more times and see if it helps.

Fearn, I don’t have a sword with a sliding weight, only one with steel balls, and swinging that, the moving of the balls does not make much difference, it would not as the balls are not very heavy.

I think the conclusion is, like several has stated, that sliding weights on swords are non existent, and should such a sword be found, then it must have been made as an experiment – not for use.

[fearn]

Hi Rivkin,

You're right, of course: I'm thinking of standing waves, aka, the way the sword flexes when it hits something. The nodes are where it flexes the least, and those are where you want to hold it, unless you enjoy hand shock.

Hi Jens,

Yep, I think we've settled it. It's a good thing, too. Otherwise, we'd next have to deal with the mechanical advantage that the Chinese gain by putting those nine rings on the back of the nine-rings dao

Fearn

[Rivkin]

Originally Posted by fearn

You're right, of course: I'm thinking of standing waves, aka, the way the sword flexes when it hits something. The nodes are where it flexes the least, and those are where you want to hold it, unless you enjoy hand shock.

That's one of the things I don't understand - what is a node for one wave, will be a maximum for another wave.

[Ian]

Hi Kirrill:

You raise some interesting points and I will try to deal with them as best I can. My college physics is but a distant memory!

Both of my local contacts went on vacation on Friday, and will be out of the office for the next four weeks. Academics do very well with vacation time. I will do my best.

Ian.

Originally Posted by Rivkin

1. With given definitions the center of gravity and the center of inertia will be the same.

As described above, the respective moments vary with distance of the various components of mass from the handle, raised to the power of 0 (the zero moment, which is simply the total mass); with distance raised to the power 1 (the first moment); and with distance raised to the power of 2 (the second moment). The center of gravity relates to the first moment, the center of inertia relates to the second moment. If mass is distributed uniformly along the length of the rod, then I believe that the center of gravity and center of inertia will be the same. When mass is distributed unequally, then the two will be different. The difference will be demonstrated by the two tests I listed.

Originally Posted by Rivkin

2. If its possible, I would really like to see the formula they use for sword's frequency as a function of length (do they consider it a string ? a thin and long prism ?).

Need the experts for this one. I believe they modeled this as a solid rod.

The frequency we are talking about, then, is the resonant frequency of a solid rod, which (if I recall correctly) for a given diameter varies with the density of the material and its length. When we talk about a string, there is also a factor for the rigidity of the material or tension applied (a taught string resonates at a higher frequency than a slacker string). The resonant frequency is fixed for a rod of given dimensions and homogeneous construction. The amplitude of the vibration varies with the distance the rod is struck away from the resonant node.

An interesting example is the aluminum (aluminium) baseball bat, which has an outer aluminum shell and an inner core that is air-filled. Striking a ball with such a bat produces a brief, high-pitched "ching," and a lower-pitched "thunk." The higher pitched sound reflects the resonant sound of the metal shell, and the lower-pitched sound comes from resonance in the air-filled chamber.

These sounds are hard to distinguish with the human ear but apparently have been measured with sophisticated recording equipment. The low frequency sound is just a few hundred cycles per second, approaching the limits of detection for the human ear.

Originally Posted by Rivkin

3. Concerning tang not being a b.c., or even a separate body, I would prefer to hold a vastly different opinion.

For full tang construction, there should be no boundary condition because the tang is essentially an extension of the blade. This is the same situation as a baseball bat, and the handle presents no boundary condition in that example.

As I mentioned above, there may be dampening of the vibrations by materials around the tang. For partial tang construction, I am unsure how much of a boundary condition there may be. It probably varies with the width and length of the tang, and again the wrapping materials will be important in how much dampening of the vibrations might occur for the user.

Originally Posted by Rivkin

4. Concerning waves propagating in swords and nodes - propagating waves usually do not have nodes. When people talk about nodes, they usually speak about standing waves, i.e. steady state solutions etc.
I suspect that the logic was that if sword can be considered a string, than a full wavelength standing wave will have a node in the middle, but it will basically be true only for even halfwavelengths mode... Plus I'm really too lazy to calculate the modes of a string with a variable mass, so I don't know how big percentage of the waves will have nodes at the center of mass.

This one requires the experts. The test described above speaks to a property of standing waves, I think, but the center so defined also identifies the "sweet spot" which relates to properties of propagated waves also -- at least that was how it was explained to me.

With respect to analogous models, I believe that a string as we usually think of it is probably not the correct one. A string can have variable tension. If we exert enormous tension on a string, and essentially make it highly inflexible or "rigid," then we may approach a more representative model. A metal rod has a high degree of rigidity, which is essentially constant for the purposes of this discussion.

Originally Posted by Rivkin

5. Concerning the center of percussion - as far as I remember (and I remember it very poorly), the center of percussian is when you hit it, all the momentum is transfered into the rotation movement of the sword, without any daggling down or up.

Could be, but we need the experts for this one too.

[Jeff D]

Hi Rivkin

I would like to address some of your questions and hope it can add to Ian's excellent answers.

Originally Posted by Rivkin

1. With given definitions the center of gravity and the center of inertia will be the same.

Inertia is the tendency for a body at rest to stay at rest or stay in uniform motion. It requires an external force to alter this state. With our sword example I think its relevance is more in the handling of the sword than in its ability to cut as per Jen's original question. So I would agree with you that for all intense purposes it would be OK to consider it the center of mass.

2. If its possible, I would really like to see the formula they use for sword's frequency as a function of length (do they consider it a string ? a thin and long prism ?).

I am not sure what model the engineers used, however I think it would be safe to use a bar with a clamped (fixed) end (similar to a hilt) as a model. Using this model the formula is;
F(1) = 0.162 {a/(L squared)} {the square root of(Y/d)} where a=thickness, L is the length of the bar, Y is Young's module (which is a variable of the elasticity of the material), and d is the density.

3. Concerning tang not being a b.c., or even a separate body, I would prefer to hold a vastly different opinion.

Using my model I would not consider the tang in the Length formula as the hilt and tang would be the fixed portion of the bar.

4. Concerning waves propagating in swords and nodes - propagating waves usually do not have nodes. When people talk about nodes, they usually speak about standing waves, i.e. steady state solutions etc.
I suspect that the logic was that if sword can be considered a string, than a full wavelength standing wave will have a node in the middle, but it will basically be true only for even halfwavelengths mode... Plus I'm really too lazy to calculate the modes of a string with a variable mass, so I don't know how big percentage of the waves will have nodes at the center of mass.

I think again we are confusing some of the issues. The vibrations only affect Jen's question in reguards to the energy transfer. The less vibration, the more energy transfered from the kinetic energy of the swing into cutting energy. If there is a lot of vibration then a lot of energy is wasted. The vibration will obviously affect the handling as well. I think a complex bar with varing thickness, fullers etc. would set up a series of standing waves that would cancel each other out so that there will be minimal palpable vibrations.

5. Concerning the center of percussion - as far as I remember (and I remember it very poorly), the center of percussian is when you hit it, all the momentum is transfered into the rotation movement of the sword, without any daggling down or up.

I am not sure of what your question here is but I think you are correct in that the center of percussion is where the blade strikes the sword, and should there for be where the resistance of the object balances.

Hope this adds a bit.
Jeff