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kinetic friction
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Diagram showing four stages of frictional forces on a box atop a table: (A) only gravity and normal force act; (B) static friction opposes a small applied force; (C) static friction at its maximum resists further applied force; (D) kinetic friction opposes motion as the box moves.
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kinetic friction

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Diagram showing four stages of frictional forces on a box atop a table: (A) only gravity and normal force act; (B) static friction opposes a small applied force; (C) static friction at its maximum resists further applied force; (D) kinetic friction opposes motion as the box moves.
friction forces How friction forces, including kinetic friction and static friction, act upon objects at rest and in motion. In (A), a box sits on a table, and there are two forces: the force of gravity, Fg, which is the weight of the box, pointing downward, and the normal force, Fn, of the table on the box pointing upward. The forces are equal and opposite, and thus the box does not move. In (B), a force, Fapp, is applied to the object to push it to the right. However, static friction, Fs, acts to the left, with the same magnitude as Fapp, so the box remains motionless. In (C), the magnitude of Fapp increases until it reaches the maximum static friction, Fs max, which must be overcome for the box to move. In (D), Fapp has become greater still, and the box is moving, but with static friction replaced by kinetic friction, Fk.

Kinetic friction, also known as sliding friction, is the force that opposes the motion of an object sliding over another surface. This force is always directed opposite to the direction of motion.123

Key characteristics of kinetic friction include:
* It is largely independent of the contact area between the surfaces.2
* It is proportional to the normal force pressing the surfaces together.23

The ratio of kinetic friction to the normal force is called the coefficient of kinetic friction, symbolized by the Greek letter mu (\u03bc). This coefficient is dimensionless. For example, the coefficient of kinetic friction for a brick sliding on a wooden table is approximately 0.5. This means a force equal to half the object's weight is needed to keep it moving at a constant speed.23

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  3. coefficient of friction

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