Saturday!

Avatar

Jaybird

Jaybird is Birdmojo on Xbox Live and Jaybirdmojo on Playstation's network. He's been playing consoles since the Atari 2600 and it was Zork that taught him how to touch-type. If you've got a song for Wednesday, a commercial for Saturday, a recommendation for Tuesday, an essay for Monday, or, heck, just a handful a questions, fire off an email to AskJaybird-at-gmail.com

Related Post Roulette

17 Responses

  1. Avatar Michael Cain
    Ignored
    says:

    Fencing, as usual. Opportunity to fence some different people this AM. Nice to be able to take someone aside after a bout and say, “I am not doing what you think I am doing. Here’s how I’m scoring against you…”Report

  2. Avatar Anne
    Ignored
    says:

    @ MIchael Cain Fencing?? SCA per chance?Report

    • Avatar Michael Cain in reply to Anne
      Ignored
      says:

      Contemporary sport fencing. Epee, for those who care about the differences in the three weapons. Heaviest and stiffest of the three weapons, enough to deliver bruises even through the protective clothing. Simplest rules, though, so you have to have a more complete “game” in terms of strategy and tactics.Report

  3. Avatar George Turner
    Ignored
    says:

    I once wrote a physics theory on why, under a point system that rewards first hits, all sword forms will shift from a rear blade, off-foot forward stance to a blade-foot forward, blade tip forward stance, and absent agreed constraints, blade weight, delivered energy and impulse will tend toward zero.Report

    • Avatar Brandon Berg in reply to George Turner
      Ignored
      says:

      I’d give you a wedgie for mixing nerdery and sports, but fencing was pretty nerdy to begin with.Report

    • Avatar George Turner in reply to George Turner
      Ignored
      says:

      Back in 2000 I got a nomination to the Martial Arts Hall of Fame for 200+ pages of math and physics on the dynamics of sword motions and impacts. Now that’s some geekdom! It’s also a pretty interesting story.

      At the time the theories being advanced were that a sword impact, in terms of where you needed to hit, was the result of fundamental, second, and third harmonics of vibration, with possibly strong effects from surface acoustic waves and electron spin resonance. Some argued that grip and foot placement played a huge factor during the impact. Some tried to calculate what fraction of an attacker’s body weight actually transfered into the blow. Some said the most important factor was the drawing action during the cut. Everyone was pretty much throwing s*** at a wall to see what stuck.

      I thought that all sounded a bit crazy and started doing the math. Turns out its just a club with a cutting edge, and all the physics had been done back in the 1600’s when explaining how a sword worked was the most famous problem in all of physics, probably suggested to Marin Mersenne by Galileo, but possibly going back much earlier, with one intriguing image of a piece of test equipment from Da Vinci. Fermat, Descartes, and Huygens all worked on an interesting aspect of the problem, which fundamentally was one of how bits of distributed mass determine momentum. Intriguingly enough, all major advances in physics in the hundred years prior to the Principia were the result of studies of pendular motion and impact. They were almost certainly trying to solve the sword problem, hunting for a generalized solution to how percussion points relate to pendular motion, and how force and momentum worked during an impact, especially one containing a rotational component.

      So the problem involves percussion points, mass, and moments of inertia. They had been able to find the percussion points for at least 20 plane figures with uniform mass distribution by about 1620, with a note implying they’d known how for at least a hundred years, and applied the results to impact diagrams of clubs, maces, bats, and tennis racquets.

      As an aside, centers of percussion are always conjugate pairs, and the physical meaning is that when you hit an object at one such point, it pivots around the other. As it turns out, if you suspend the object at one of these points and swing it like a pendulum, the other point is exactly at the length of a string pendulum that swings in time with the object, making it easy to experimentally find a pair of conjugate centers of percussion for anything.

      Handily, if you then measure the two lengths from the object’s balance point (center of mass) to each of the two conjugate center’s of percussion, and then use them to form two sides of a rectangle, you get an area. That area is constant for all conjugate center of percussion pairs in the same plane of rotation (such as edge-wise) for that object. So if you find one pair of percussion points for a sword experimentally, you can solve for all such centers just by dividing the constant area rectangle by the distance from the impact point to the balance point, and out pops the length of the other side of the rectangle, which is also the distance from the balance point to the center of percussion (hopefully somewhere in the grip). For many objects like rods and spears you can do it in your head.

      Unlike our modern approach of analyzing an impact around the center of mass, if you center the reference frame on one center of percussion, the result of an impact is a rotation around the other center of percussion (a pivot point). Amazingly, then you can treat the problem as an Archimedes class three lever, with the sword’s entire mass at the balance point, the fulcrum at the center of percussion in the grip, and the impact point as the effort. The mechanical advantage of the resulting lever is used to divide the sword’s mass down to an “apparent mass”, which you can call the object’s inertia, or resistance to acceleration, at the impact point.

      The impact can then be treated as a simple two-body problem, with a target mass and velocity and an impact point “inertia” and velocity, allowing you to easily solve for the resulting motion of the impact point and the target. Since the resulting motion must also produce a rotation around the center of percussion (the pivot point), that point’s prior linear motion is unchanged by the impact, and so you have the resultant motion of two points on a rigid body (the sword), and what looked like a dizzying complex problem can be solved rather trivially.

      In a general swing, the further you get away from the sword’s center of mass, the lower the inertia, but the higher the impact speed. So the best place to make an impact along the blade depends on the rotational and linear speed of the sword and the mass of the target. There is no single best solution, because it’s situationally dependent. I suspect that’s about where the problem was left, because at that point you can produce graphs that just show you should generally swing really hard at the guy’s head, and that there is no magic swing nor magic impact point.

      A second aspect of percussion points is that pushing on one moves the sword around the other, so the one place completely unaffected by your hand’s motion and mass during the impact (assuming you’ve struck near the percussion point relative to your hand), is the actual impact point on your target. During the impact the swordsman’s hand is pretty much completely irrelevant, as is, therefore, the rest of his body.

      One interesting aspect of this is that the center of percussion of a triangle (which was solved very long ago), relative to its base (pretty much where your hand will be), is right in the middle of the blade. The trick to move that percussion point on out towards the tip is to add a big pommel to the hilt, pivoting the blade at the cross guard and swinging it like a pendulum and adjusting the pommel so that the sword swings in time with a string pendulum that’s the same length as the blade, or perhaps a few inches shorter, so that you can strike with the sharp tip area (about a hand span on most medieval blades) and not get any significant hand shock.

      Taking things further, drawing the cut or using a curved blade probably doesn’t make a 1% difference in the impact, if that. The closing velocity is just too high for those effects to make a serious difference based on geometry. Those effects matter at low speeds, but won’t show up at high speeds.

      However, curving the blade does make a huge difference in the sword’s moment of inertia colinear with the blade (as when you balance it on its point and spin it like a top). That moment of inertia can be vanishingly small for a straight blade, so you have to put a big cross guard on it (either in front of the hand or behind) with little weights on the end. If you don’t, every time someone hits your blade it will spin in your hand right before the blade flexes, and if you’ve got gloves on, the blade angle as you grip the sword becomes almost random blow to blow.

      So to keep the sword usable as a cutting sword in combat, you either have to make the blade curve or you have to add a big cross-guard or a basket, or bend the hilt way over so the pommel doubles as a spin-rotation damper (some Indian blades are built like that). So physics speaks, and suddenly the big pommel and the cross guard with big balls on the ends makes perfect sense. Straight blades have to be built that way, or they suck.

      And then all that knowledge was forgotten, and various cavalry figures and British gentlemen pondered on the problem and produced a large set of completely wrong answers. Those probably found their ways into bodice ripping romance novels, and by 2000 the nonsense was so thick that people were actually trying to invoke electron spin resonance to explain a sword impact.

      Among the responses I got to my paper was the assertion that if a swordsman swings from a moving tank, he strikes with the entire mass of the tank. I pointed out that in that world of cartoon physics, I can plant my feet firmly and defend with the mass of the entire planet. A group of Asian martial arts aspects dismissed it as another academic attempt to explain the infinite mysteries of the blade, which perhaps at some point could be solved with a super computer, but nothing less. I explained that a sword has less than half as many moving parts as a pair of scissors, so how f***ing hard is it going to be?

      So, as they say, when you put a sword in a man’s hand his IQ drops by 20 points. It’s probably true.Report

      • Avatar Michael Cain in reply to George Turner
        Ignored
        says:

        Turns out its just a club with a cutting edge…

        In the military setting, absolutely. When the sword was the weapon of choice for heavy infantry, the preferred formation was a line of men, body armor to some degree, a bloody great shield, and a sword. The Romans adopted the idea and pretty much perfected it, chopping up anyone who made the mistake of facing them at short range on foot. Shift into a civilian setting sans armor and shield, and reach becomes critical so the weapon hand/foot has to come forward. Followed by figuring out that in that mode, a point weapon is much more deadly than a purely edged weapon, and it quickly quits being a club. At the peak of their usage, many rapiers weren’t even sharpened below the few inches closest to the point. The whole book on tactics became about speed, evasion, angles, control of distance, etc.Report

  4. Avatar dhex
    Ignored
    says:

    jb, have you played gone home yet?

    http://thefullbrightcompany.com/gonehome/

    it is amazing.Report

  5. Avatar Reformed Republican
    Ignored
    says:

    My gaming last weekend (and I did not have time for much) was mostly Diablo III and farming the limited time only Community Day heads and skins for Borderlands 2.Report

Leave a Reply

Your email address will not be published. Required fields are marked *