Sliding weights

[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.