Introduction The simple fact of knowing how or why things work, doesn’t take away anything from their charm and at the same time allows for a correct, and ultimately more beautiful use for them. Does the fact that Newton discovered how and why apples fall from the tree negate our pleasure in eating them? Iaido doesn’t escape this rule and it is therefore possible to apply scientific knowledge to its techniques without worrying about unveiling secrets that will reduce its charm but that instead, if at all possible, will increase it even more! Did you ever ask yourself why a sword cuts? The edge, of course, but an edged sword might not be enough or, viceversa an unedged one can still easily kill: Furuichi Sensei (8th Dan Iaido, 7th Dan Kendo) often, during his Seminars, shows that a live katana (Shinken), a training unsharpened one (Iaito) and even a wooden practice reproduction pretty much cut in the same way if used properly. Physical Hypotesis and data Below we will apply some physics principles, with their connected math calculations, to the trajectories that the sword runs from its high starting position to its low arrival point. What we’d hope to demonstrate is that science agrees with a correct form and practice, and that sometimes sensations that lead you to believe that a movement technically not clean would be in practice more effective are actually wrong. We will therefore discover some significant values that scientifically agree with the perfect technical execution of a cut. First of all, let’s calculate how much distance the blade (or, to be more precise, its tip) covers from start to end in a proper cut, as opposed to a inaccurate one, when for example the practitioner moves by initially bending the elbows, then again extending them at the end, to create a sort of... “whipping” movement, which might seem more effective and it is not so – as Senseis teach us and science is going to prove! In these two images, A and B are always the starting point and the arrival point of a cut. RI is the initial radius of the cut. RL is the correct radius achieved by correctly swinging the sword in the widest possible arc. RC is the shorter radius achieved by bending the elbows in a whipping motion during the cut. In both cases, the sword will start from point A and reach point B, but in the correct technique the trajectory will be ABL, and in the wrong whiplike motion it will be the shorter ABC.
Now, given a sword of the standard 75 cm lenght and an added distance for the arms of the practitioner, we will arrive at a 150cm radius (RL) for the correct cut and at a 110cm radius (RC) for the whipping motion, not correct one, shorter to account for the bending of the elbows mid-way. Considering that to calculate an arc you need to multiply Pi*Radius*angle/180 we can approximate the distance covered by the two arcs ABL and ABC, which is respectively of 3.92m and 2.87m. Because speed is the measure of distance covered in a unit of time, and the time of execution of the cut is the exact same (0.35 secs): - Correct trajectory (ABL) is: 3.92/0.35 = 11.2 m/s, or 40.32 Km/h; - Wrong trajectory (ABC) is: 2.87/035 = 8.2 m/s, or 29.52 Km/h. Thus proving that the correct trajectory is sensibly faster (10 Km/h) than the wrong one, even if the practitioner sensation tells him sometimes something different. Incidentally, 10 Km/h is already the speed at which an average human bone will fracture by impacting against a solid surface. In the next installment of this article we will study and calculate the cutting power, pressure and results of a cut against a human body. Stay tuned! This article is based on the work and calculations of Davide De Vecchi, who is fully credited for the original material.
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