Axis of rotation

Japanese: 回転軸 - かいてんじく（英語表記）axis of rotation

When an object rotates uniformly around a fixed point, the points on a specific line that passes through that point do not move at all. This fixed line is called the axis of rotation. Any rotation can be considered as a continuous addition of infinitesimal rotations, and the axis of each infinitesimal rotation is called the instantaneous axis of rotation. When the axis of rotation coincides with one of the three principal axes of inertia of an object, no couple acts on the axis that tries to change the direction of the axis during the rotation, so the principal axis of inertia is also called the free axis. If the axial direction of an object rotating around one free axis is slightly changed, two of the three axes will act as couples that return the axis to its original direction, and the remaining axis will act as a couple that changes the axis's direction even more. The former two axes are called stable axes, and the latter as unstable axes. Wheels, pulleys, gears, cams, and other rotating parts of machines have stable axes as their axes of rotation.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

物体が1つの定点のまわりに一様な回転運動をするとき，その点を通る特定の直線上の点はまったく動かない。この定直線を回転軸という。任意の回転運動は瞬間的な微小回転の継続的な加え合せとみなすことができ，個々の微小回転の軸を瞬間回転軸という。回転軸が物体の3個の慣性主軸のどれかに一致するときは，回転運動中に軸の方向を変えようとする偶力が軸に働かないので，慣性主軸は自由軸とも呼ばれる。1つの自由軸のまわりを回転している物体について軸方向を少し変えてやると，3つのうちの2つの軸では軸の方向をもとに戻す偶力が働き，他の1つの軸では軸の方向をますます変えるような偶力が働く。前者の2軸を安定軸，後者を不安定軸という。機械の回転部分をなす動輪，プーリー，歯車，カムなどでは安定軸を回転軸にとる。

出典　ブリタニカ国際大百科事典小項目事典ブリタニカ国際大百科事典小項目事典について　情報

<<: Rotating seesaw - Rotating seesaw

>>: Rotary Bowling - Kaitenshiki Boringu