Sliding weights

[Jens Nordlunde]

Sliding weights has been discussed more than once, but I don’t remember ever having seen with which effect the weight adds to the blow. Do we have one on the forum with an analytic mathematical brain, who can figure out which formula to use?

Let’s say that the sliding weight weighs 250 g, the sliding distance is 90 cm and that the impact speed without the weight is 40 km/h.

I know that 40 km/h probably is too slow, but if we have a formula it should be easy to change the speed. Besides I have chosen 40 km/h as the strength and enthusiasm shown in the start of a battle is likely to wear off during the day - like this, ‘fit for fight – fit – too tired to lift the sword’. Also the weight might not be heavy enough, but I don’t have any idea of how much the sliding weight on the Claymores weigh.

[fearn]

Hi Jens,

Just thinking about it, you might want to specify a few more things, namely the length of the blade, and the position of the slide relative to the blade (especially the center of impact).

Just out of curiosity, is a 90 cm slide reasonable? Most swords are less than 1 m long, and assuming a handle that is 20 cm long, you would need a 1-handed sword that has a 1.2 m blade simply to keep the weights inside, and to leave 10 cm at the tip for a solid hit. Then you have to assume that the weight can go from near the hilt (say 20 cm) in a vertically held blade, to 1.1 m on a horizontally held blade, as it hits, over about 0.1 second (I'm guessing, on the basis of 1 m/sec=3.6 km/hr). Does that sound reasonable? Ideally we'd want a perfectly frictionless slide, but I suspect that a bead in a slot (the weights I've seen), would also experience significant friction, especially if the sword was bloodied after a strike.

Neat problem though. I'll be interested to see what the physicists say about it.

Fearn

[Jens Nordlunde]

Hi Fearn

Thanks for the mail, and yes I did forsee that I most likely would miss some important pieces of information, so let me try again.

Weight 250 g.
Length of blade 90 cm, impact 60 cm down the blade.
Length of sliding 60 cm. Starting 15 cm from the hilt.
Speed of impact 40 km/h
We must assume, not to make it too complicated, that the blade is straight, that the weight is in one piece sliding in the middle of the blade (double edged), and that the friction at any time is close to nil, although you are right that while fighting blod would slow the slider down.
I will comment on what you write about the beads, steel balls later on this thread.

Jens

[Jeff D]

Hi Jens,

As always you ask a interesting question. The purpose of a sliding weight (if it is indeed functional not decorative) is to move your point of balance down the blade. Essentialy converting a rapier (light blade with a point of balance closer to the hilt) into an axe. As you know the cutting ability has a lot of variables such as edge geometry, draw cut etc. However the sliding weights function would be to add mass to the point of impact thus increasing the kinetic energy which then gets converted into cutting energy. KE=1/2mv2 (kinetic energy = one half mass times the velocity squarded). The amount of mass added will then depend on where your sliding weight is in relation to the point of impact which ideally will be on the point of impact.

As a disclaimer I am not a physicist
Jeff

[Jens Nordlunde]

Hi Jeff,

Thanks for the kind words.
Would the effect not be bigger when, like I have done, let the glider pass the point of impact with a few cm?
My idea with this thread is not to get an exact answer, but to get an idea of, how much extra power a gliding weight did/could add to the sword.
I will comment on you quertion, 'The purpose of a sliding weight (if it is indeed functional not decorative) is to move your point of balance down the blade.' later, as I find this very interesting.

Jens

[Jeff D]

Originally Posted by Jens Nordlunde

Would the effect not be bigger when, like I have done, let the glider pass the point of impact with a few cm?

Hi Jens

Yes, the effect should be increased as there is a rotational kinetic energy that should increase directly proportional to the distance past the point of impact squared (I=mr2).

Jeff

[Jens Nordlunde]

Hi Jeff,

I don't understand the formula, but I am relieved to say that I do understand what you write, and I find this interesting.
Now, if the blade was twice as long, and the glider could glide twice as long, but the point of impact would be the same - what then?

Jens

[fearn]

This is definitely a neat problem.

Just thinking about it, I'd break the contribution of the weights into two categories.

1) changing the centers of gravity and percussion. The best way to think of this is as a sliding weight that can be fixed in place. Fix it at the bottom of the slide, and the sword acts more as a back-weighted stabbing weapon. Fix the weight forward, and the sword acts better as a forward-weighted chopping weapon.

2) the sliding weight acts to increase the impact force by some amount. This gets weird. For instance, the weights could add a second impact to a stab. You use the blade to stab forward, and once the blade slams to a halt, the weights slide forward and add a second thump.

Conversely, when sliding, the weights move in an arc (potentially a straight line) out, from resting near the hilt to the outer end of the slide as the blade moves. Again, they add some force at the end of the slide, but it is at an angle to the cut. This might or might not be a good thing.

In both cases, there's a notable jerk or twitch when the sliding weight hits hits either end of the slide. This might or might not be a good thing.

The nice thing is that this would be relatively easy to dummy up. Put ball bearings (or whatever) in a closed pipe of the length of the slide, and attach this pipe to a practice blade (perhaps with duct tape? ). Some empirical experimentation would be informative, I think.

Fearn

[Jens Nordlunde]

Hi Fearn,

Your observations are very good. When I thought of the problem, before I started the thread, I wondered what the maximum weight of the glider might have been, if you did not want to loose your sword - torne from you hand by the force of the glider.
You write, 'In both cases, there's a notable jerk or twitch when the sliding weight hits hits either end of the slide. This might or might not be a good thing.' Yes there would have been, and if this jerk is too hard I don't think it is any good for the user, nor for the sword - therfor the slider must not be too heavy - but heavy enough to give the wanted effect. How heavy it that?

Jens

[Rick]

Wouldn't that be measured by a formula of the distance traveled by the weight , the mass of the weight and the speed of the swing ?
The important variable being the speed of the swing ?

[Jens Nordlunde]

Yes Rick, some can to day, but how could they do iot centuries ago?

Jens

[fearn]

Hi Rick,

With a perfectly frictionless slide and a perfectly rigid blade, you're right.

The problem is, it won't be frictionless, and since we're talking about steel, there's also the possibility of the weight rebounding off the end of the slide, plus some other weirdness with the sword flexing as it hits that we haven't even mentioned.

I'm wondering, at the moment, which is more important, the basic physics of a sliding weight on a frictionless surface and a rigid blade, or all the ugly complexities. Murphy's Law suggests that the ugly complexities might, be more important. But I'm a pessimist: either I'm right, or I'm pleasantly surprised.

Still, we need some physicists to speak up and point is to the right calculations.

Fearn

[fearn]

A side thought:

I keep thinking about an exotic Chinese weapon I've read about: the "seven stars" rod. Basically, it's a bamboo rod of seven segments. This is a "special species" of bamboo, which I take to mean that the bamboo has pretty thick walls.

The reason it's relevant here is that each of the seven segments is half-filled with mercury and sealed. Basically, it's got sliding weights all throughout the rod.

If someone knows how to make one of these suckers, please speak up. I'm too transfixed by the vision of the rod splintering and spraying mercury everywhere to want to make one of these things.

However, a simple version of it could be made with PVC and marbles (put small, closed sections of PVC pipe inside a bigger one, and secure them with glue or whatever.

Just a side thought, but it's a possible design for any one who wants to experiment with sliding weights.

Fearn

[Rick]

Originally Posted by Jens Nordlunde

Yes Rick, some can to day, but how could they do iot centuries ago?

Jens

Hi Jens , since you brought this subject up are we discussing a sword that actually exists ; or are we just brainstorming over the possibility of making a sliding weight cutter ?
In your initial post you refer to "sliding weight claymores" , does such a beast actually exist ?

If we are talking about a sword with metal balls i.e. 'tears of the afflicted' in my opinion that feature is entirely for show .

[Jim McDougall]

Wow! I had no idea that the clearly established knowledge of the members here included such command of physics!! I'm afraid I'm pretty much lost in this dynamic discussion, but its fascinating to see the potential and plausibility of such an interesting feature as the sliding weights on sword blades.

Actually this topic was discussed several years ago in a query about a much storied Scottish hero of the 17th century, whose claymore was said to have a 'ten pound weight on the back of the blade to add force to the cut'. While the hyperbole here is obvious, further research found no examples of such a feature on the back of sword blades. It seemed a bit useless to add any weight to the already heavy enough claymore let alone trying to control such a sword with moving weight changing the balance.
It seems that another tale similar describes a medieval falchion with a weight termed in literary fashion 'the apple' or 'pear' on a shaft at the back of the blade for the same express purpose of adding force to the cut.

Clearly the 'tears of the wounded' feature on Chinese and Persian edged weapons aesthetically correspond to this moving feature, but are generally considered to be ceremonial or parade weapons. There have been some accounts of blades containing mercury enclosed in a channel for the purpose of movement of force in the use of the blade, but again these tales appear to have literary origin rather than practical.

What I would like to know is if anyone has ever seen or heard of an actual weapon with a shaft and sliding weight, again without reference to the 'tears of the wounded' pearls or bearings.

Having asked that, I hope the discussion on the actual dynamics of this feature continues! Its fascinating to see how this feature would have been applied and helps in understanding possibilities. Excellent observations!!

Best regards,
Jim

[Ian]

It is some years since I addressed problems in physics, but the mechanics of this problem seem both simple and complex. We are dealing with a rigid bar traveling through an arc, which approximates the segment of a circle having the shoulder at the center of that circle. The radius of that circle is the length of the arm plus the length of the blade (or, more precisely, the distance to the point of impact along the blade).

The energy from the blow will depend on the angular momentum at the point of impact. So far, so good.

If this were a problem with a ball at the end of a weightless string it would be easy to solve (think of a yo-yo or a ball flail). But we have a bar with mass along its length, and we want to add a variable mass distribution to that situation. One way to address this variation in weight distribution is to consider several scenarios, with the two extreme cases being the variable weight at each end of its travel. I'm not sure how to address the question of mass distribution along a bar. Perhaps this requires an analysis of the moments around a fulcrum (which is the point of impact), although the "fulcrum" in this case is not an immovable object but yields with the blow.

I'm sure all of these issues have been worked out previously but it is a matter of finding a reference to the solution. Presently I'm traveling but will be back in the office next week and will talk with some engineering colleagues who have far more skills in mechanics than I do. The final solution may well include calculus, so be warned.

Ian.

[Jens Nordlunde]

Hi Rick,
It was actually Jim who got me started on this, as he mentioned a Claymore with a sliding weight, and this made me wonder how heavy such a weight would have to be, to be of any help. I did not know that the sword did not exist, and I have not seen such swords myself, although they may exist/have existed as experimental weapons.

Hi Fearn,
When it comes to friction, I think we should close an eye, or we will end up with too many ‘if’s’, but you are right, there are quite a few things which would/could have an influence on the weight, like blood as you mentioned, or maybe a hard blow on the blade, bending it just a little bit, would most likely stop the weight. Interesting what you write about the Seven Stars rod.

Hi Jim,
You are right, it is an interesting subject, and I hope someone will be able to show us a picture of such a sword. A sword with a sliding weight, weighing ten pounds would be impossible to handle, and more dangerous to the user than to his enemy. If the user of such a sword really held on to the hilt, he might end up with the hilt alone while the blade would be catapulted away. I don’t know, but I have a feeling that a weight of one pound probably would be too much.
We must not forget, that every time the user has used the sword, the weight is in its outer position, and he will have to get it back in order to strike again - this will make him vulnerable. The best would be if a spring could catapult the weight back into the start position, but this may be too far fetched.
I think ‘the tears of the wounded’ have a meaning, other than boasting, maybe to make the user remember the suffering of the wounded laying on the battlefield, but I agree with you that the balls, whichever material they are made of, are not sliding weights.

Hi Ian,
You are right, the problem seems more complex than I had expected from the start. It will be interesting to hear about the answers you will get next week.

Jens

[fearn]

Hi Jens,

While I like the topic of sliding weights, I can think of a couple of really good reasons why someone would "have a weight" on the back of a claymore. If the weight were _fixed in place_, it could be extremely useful either to tune the center of gravity and center of percussion, or alternately, perhaps to dampen shocks from the blade hitting a target.

While I've never seen such a weight on a sword, I wouldn't be surprised if some enterprising soul didn't try it at some point. Fixing a lead slug (or similar) to a blade would certainly be simpler than remanufacturing the blade to have better performance characteristics.

Anyway, back onto the topic: one suggestion I would make is that a shorter slide would be more useful than a longer one, because the weight could move more quickly. Imagine, for instance, a weight that was on the hilt side of the center of gravity when the sword was upright, but which could be propelled across the center by some wrist action to make the sword tip heavy. Such a sword would be easy to accelerate and would hit hard, although it would be hard to recover after a blow.

Fearn

[Jens Nordlunde]

Hi Fearn,

We may, or we may not find swords with sliding weights, but I have a strange feeling, that I have seen one many years ago in a museum, maybe in Istanbul, or somewhere else – I am not sure.

What do you mean by writing that the weight should/could be ‘fixed in place’? How would it dampen he chock?

You write, ‘Fixing a lead slug (or similar) to a blade would certainly be simpler than remanufacturing the blade to have better performance characteristics.’ Yes you are right, that was what the headmen did, if you can believe, that the tree wholes at the blunt tip of their swords were for leaden weights, but they only had to hit once, so why would they need extra weight?

I agree with you, that a short glide would solve quite a lot of possible problems, although the shorter glide, the lesser power. I think I understand what you write next, but I am not quite sure – remember that I’m not brilliant in English. Is it possible for you to explain it in another way? Do I understand you right, when I think that you are saying, that the glider should travel from the back of the blade to the front? If yes, I think this would give the whole thing more force, but I can prove why I think so.

Jens

[Montino Bourbon]

actually the weight moving forward would make a longer pendulum, thus slowing it down, I think.

Besides, for the weight to have a real effect it would have to be big enough that it would add too much weight to make the sword useable.

[fearn]

Hi Montino,

You're right, but as I recall, the time of the pendulum works in part on the distance to the center of the pendulum's mass, not just its total length.

With a weight that slides for a long distance down the blade, you're right. The blade gets progressively more tip heavy, meaning that it will take more energy to move the tip and that it will hit harder (transferring more energy).

If the slide is short, around the blade's normal center of mass, what would happen is that the sword would go from back-weighted to front-weighted. Basically, this would mean that the sword accelerated a bit faster from rest (starting out back-weighted, as in a small sword), hit a bit harder (shifting to front-weighted), and was a bit harder to withdraw (still front-weighted). This type of design makes a lot of sense when you're only planning a single blow, as in an executioner's sword. In a battle, I'm not so sure that it's advantageous, but I don't know. We'll have to see what the engineers say.

One thing to remember is that a blade with a slot down the middle is probably more fragile than a solid blade of the same weight. Jens has proposed an interesting question, and it will be interesting to see some numbers around it.

Fearn

[Rivkin]

I truly believe that any sliding weight would be totally cool, fancy, and useless. I see no reasonable advantage over permanent placement of some weight at the sword's end, etc.

[Ian]

This topic is closely akin to the physics of hitting a baseball.

BTW, I notice that the on deck hitter often puts a ring weight around the bat barrel to take some practice swings, but does not try to emulate that weight in the bat itself. It slows down the bat speed and if the bat is overly heavy too much power would be lost -- kinetic energy is proportional to the square of velocity, but only linearly related to mass.

Ian

[ariel]

I remember years ago seeing a Chinese throwing knife that always hit the target with a point and got imbedded quite deeply. Apparantly the secret was a hole drilled within the entire blade along the whole length and filled with mercury. It moved freely in this channel and changed the center of gravity. Was very impressive.
I guess, the same principle as discussed here.

[Jens Nordlunde]

Montino, you may be right, we will hopefully know later, but I agree with you that a heavy weight getting stuck at the tip could be fatal.

Ariel, an interesting mail about the use of mercury in the blade, I can imagine that the result have been impressive.

Jens

[Mare Rosu]

Jens
You are at it again, from magnetic blades to now sliding weights.
Are you sure you moved to Switzerland and was not run off, to hide/escape?
Anyway the idea of sliding weights is a good one, while I have little to add to this subject I do think a distance related subject might be of interest to you and others and that is; Atlatls and banner stones used with them. I have found these banner stones in years past and this is just two web sites on their use. http://donsmaps.com/atlatl.html and http://www.primitive.org/atlstealth.htm.
To me a very interesting subject you have started Mr. Jens

[Jens Nordlunde]

Interesting subject you raise Gene. You are right, this is about giving the weapons more dynamic power, and the links you give, show that this has been very important for thousands of years. This shows, that people in ancient times did know a lot about dynamics, although they did not know the formulas. When throwing a javelin or a light spear, it is important that you ‘arm’ is long, and here the Atlatl helped a lot, as the longer the ‘arm’ the more power you could put behind the throw. Interesting also to see that they adapted the length of the Atlatl to the surroundings, but what I find really intriguing is the use of Banner Stones, as I did not know such stones were used. I wonder how they figured out that the use of Banner Stones would make the use of the Atlatl more silent.

[fearn]

Hi Jens,

This is getting a bit off subject, but I was partially thinking of atlatls when I was talking about weights absorbing shock in a sword.

The other source of inspiration was a dha-style sword I made out of a machete. I very quickly learned that there were only two good handholds on the long handle: at the end, and about 1/4 of the length from pommel to tip. This was because the sword flexed on impact, and holding it at other points was a recipe for getting a very sore hand (the best handholds were at the nodes: end, 1/4, 1/2). Personally, I think that there are a lot of hidden shock-minimizers in well-designed dhas and other long-handled blades. Otherwise, they wouldn't be worth swinging.

There's no reason that a moving weight on a blade couldn't serve a similar shock-absorbing function, but that's another design question entirely.

Personally, I'm surprised that the atlatl people weren't talking about tuning their dart and thrower systems to make the things accurate. After all, if the dart flexes the wrong way, it's not going to hit whatever the person was aiming at.

Fearn

[Jim McDougall]

Outstanding discussion everybody!!!! It is extremely fascinating to read the well explained and supported observations here concerning the feasability of these weapons features. While edged weapons were in themselves primarily simple in thier dynamics, it is amazing how very complex their actual use and construction became. While the sliding weight feature discussed here remains purely conjecture found in literature thus far, many other interesting features are well established in developed examples, such as the thumb rings brought up on a concurrent thread and both hilt and blade shapes and construction.

It would appear that the sliding weight feature remains a figment of literary imagination as no existing examples have been seen, and such features are not mentioned in contemporary narratives or material describing edged weapons.

The mercury filled channel blades, while early examples seem to remain somewhat in question, the concept seems to have been applied in some latter examples of weapons, with this likely to be in response to the earlier tales of such weapons. An illustration of this occurs in at least one 'Bowie' knife produced in the 1860's with a hollow channelled blade containing mercury. Whether it was ever used is not described, however by its appearance (published in "The Bowie Knife", Norm Flayderman) it seems an extremely clumsy weapon.

As this excellent discussion continues, I remain hopeful that in some hidden corner of some archives or museum we will find evidence of the sliding weight on some long forgotten sword. I'm confident that if such a sword can be found it will be by the members and readers here!!

Meanwhile, the comprehension of physics and dynamics has always eluded me, but you guys make it understandable!!! Thank you so much!!

All the best,
Jim

[Jens Nordlunde]

Hi Fearn,

I don’t think you are off subject, or if you are, it is only a fraction, as the whole subject is about dynamic power/shock-minimizing and stuff like that, so the way I see it, you are on track. I find, what you write about your dha-style sword most interesting. I have always been wondering about the long hilt, but I was wondering about it – you did something about it and tried it out, so now you have a knowledge, which I fail to have. Thank you for sharing.
In the start I did not think about the shock-absorption made by the weights, but I guess you are right, this could be an extra plus to such a construction. The hand would not have to take the whole of the blow – it would be divided – but with how much to the weight and how much to the hand?
I am sure, that those using an atlatl centuries ago, had everything tuned in, so that they were sure to hit the target – they could not afford otherwise.


Hi Jim,

You are right; the discussion gets more and more interesting, as we get more and more implements into the discussion. What I think we can agree on is, that sword smiths in the old times must have had a very good idea about dynamics and shock-absorption – more than we normally think of, when we buy a sword for our collection.


Jens