Kinetic friction

Japanese: 運動摩擦 - うんどうまさつ（英語表記）kinetic friction

Also known as kinetic friction. Resistance that arises from a contact surface or line when an object moves along a surface or line, impeding that motion. There is sliding friction when an object slides along the contact surface, and rolling friction when it rolls. Generally, rolling friction is much smaller than sliding friction, and is used in rollers and ball bearings. Another type of kinetic friction is rotational friction, in which a torque acts on the tip of a rotating object in a direction that impedes the rotation, as in the rotation of a top. Torque is proportional to the normal force, and the proportionality coefficient is called the rotational friction coefficient. Rubies have rotational friction that is about one-third that of steel, so they are used as bearing stones in precision machinery such as watches.

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

Japanese:

動摩擦ともいう。物体が面または線に沿って運動しているとき，その運動を妨げるように接触面または線から生じる抵抗。物体が接触面に沿って滑るときの滑り摩擦，転がるときの転がり摩擦があり，一般に転がり摩擦は滑り摩擦より運動摩擦がずっと小さく，ころ，ボールベアリングなどに利用されている。運動摩擦にはこのほかに，こまの回転のように，回転物体の先端に，回転を妨げる向きのトルクが働く旋回摩擦がある。トルクは垂直抗力に比例し，比例係数を旋回摩擦係数という。ルビーは旋回摩擦が鋼の 1/3 程度に小さいので，時計などの精密機械に軸受の石として用いられる。

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