Angular momentum

Apologies for being so quiet lately. I thought that I’d share this little blog post examining sayabiki (the pulling back of the scabbard with the left hand to draw the Japanese katana with the right hand) from the physicist’s perspective of conservation of angular momentum:

Sayabiki and Angular Momentum

This is an interesting point, although I think that it is more relevant to, say, the dynamics of a throw in Aikidō than drawing the sword in Iaidō. While I don’t doubt that using sayabiki to exploit the conservation of angular momentum does allow a more powerful draw, I think that the primary reason for sayabiki is simple mechanics: if one does not retract the scabbard, then the sword will not come cleanly out of the scabbard mouth — if the sword comes out at all, then its cutting tip will damage the scabbard and indeed the Iaidōka’s hand!

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3 thoughts on “Angular momentum”

  1. (I think that comment was posted on fb, no?) Indeed, the first “pragmatic” function of sayabiki is certainly to free the blade. I was intrigued by other roles the movement can play.

    1. Yes, I posted the comment on FB, too, where I first saw the link to the article.

      Thinking about it some more, I think that the “angular momentum trick” in the Iaidō case must work mostly on the psychological rather than physical level: a katana is far more massive than its saya, and travels further from the axis of the rotation (nominally, the spine**), and so has a much greater moment of inertia. The sayabiki can’t possibly physically counterbalance the nukitsuke, but pretending that it does may help engage the right muscle groups and produce the correct movement.

      ** That said, nukitsuke is also a compound rotation, with one rotation about the spine and another about the wrist once the sword has fully exited the scabbard.

      1. It might be the case that it’s more psychological. Best to try it I’d say.

        While the saya has less weight and moment than the sword, the left arm and shoulder pull back vigorously as well, especially with a “square hips” posture. In my experience, I find it has a great impact on leg stability (compared to when I don’t do it), but we perform with “square hips”. Opening the hips would also add weight in the rotation.

        I feel it more when I perform from kneeling, as my left back leg can more easily rotate inward as I strike if I don’t pull back the sword correctly (it also has a lot to do with how I strike with the blade (trajectory and max speed point).

        Great point about compound rotation. It’s a complex mix of center, shoulder and wrist, all with different timings…

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