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Types of Friction

(i) Static Friction

Friction forces can also exist when the two surfaces are not sliding across each other. Such friction forces are referred to as static friction. Static friction results when the surfaces of two objects are at rest relative to one another and a force exists on one of the objects to set it into motion relative to the other object.


Suppose you were to push with 5-newton of force on a large box to move it across the floor. The box might remain in place. A static friction force exists between the surfaces of the floor and the box to prevent the box from being set into motion. The static friction force balances the force which you exert on the box such that the stationary box remains at rest. When exerting 5 newton of applied force on the box, the static friction force has a magnitude of 5 newton.




Suppose that you were to push with 25 newton of force on the large box and the box were to still remain in place. Static friction now has a magnitude of 25 newton. Then suppose that you were to increase the force to 26 newton and the box finally budged from its resting position and was set into motion across the floor. The box-floor surfaces were able to provide up to 25 newton of static friction force to match your applied force. Yet the two surfaces were not able to provide 26 newton of static friction force.

The amount of static friction resulting from the adhesion of any two surfaces has an upper limit. In this case, the static friction force spans the range from 0 newton (if there is no force upon the box) to 25 Newtons (if you push on the box with 25 newton of force). Precisely, Static friction is one where there is

  • No relative motion
  • The frictional force fs prevents sliding
  • (ii) Sliding or kinetic Friction
    Sliding or kinetic friction results when an object slides across a surface. As an example, consider pushing a box across a floor. The floor surface offers resistance to the movement of the box. We often say that the floor exerts a friction force upon the box. This is an example of a sliding friction force since it results from the sliding motion of the box.


    If a car slams on its brakes and skids to a stop (without antilock brakes), there is a sliding friction force exerted upon the car tires by the roadway surface.


    This friction force is also a sliding friction force because the car is sliding across the road surface.

    Precisely sliding friction is one where

  • The block is sliding
  • The sliding (kinetic) friction fk is opposite to v
  • Static friction is always greater than the sliding friction. Thus, it typically takes more force to budge an object into motion than it does to maintain the motion once it has been started.

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