Friction

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The component of the contact force from a surface that is parallel to the plane of the surface. Friction forces will arise when an object is sliding along the surface or when an object resting on the surface is subject to forces that would cause it to slide in the absence of friction. The force of friction will always resist the existing or intended sliding motion. Friction has two basic manifestations that are qualitatively different:

  • Static Friction: The specific manifestation of friction which attempts to resist efforts to move an object that is currently at rest with respect to a surface. If possible, static friction provides just enough force to keep the object stationary, and no more. When the net force attempting to create sliding motion exceeds a certain limiting value proportional to the normal force exerted by the surface on the object, static friction will be unable to prevent motion.
  • Kinetic Friction: The specific manifestation of friction that is directly opposed to an object's sliding motion along a surface. The force of kinetic friction has a size independent of the speed of the object, and proportional to the normal force exerted on the object by the surface.
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