Friction

Kinetic Friction

Kinetic frictiοn is the frictiοn present when there is relative sliding between two objects, such as a crate sliding on a floor. This frictiοn force always acts opposite to the direction of motion.

The figure below illustrates this.


kinetic friction


Where:

μk is the coefficient of kinetic frictiοn between the two objects

N is the normal force perpendicular to the sliding surface

Fk is the kinetic frictiοn force


Static Frictiοn

Static frictiοn is the frictiοn present when a force is applied, but there is no relative sliding between the two objects. This frictiοn force always acts opposite to the direction of the applied force.

If the applied force (F) is less than or equal to the maximum static frictiοn force (Fs,max), the object won't slide. This means that the applied force equals the static frictiοn force (F = Fs in the figure below). If this "threshold" force is exceeded (F > Fs,max), the object begins to slide and kinetic frictiοn takes over.


static friction


Where:

μs is the coefficient of static frictiοn between the two objects

N is the normal force perpendicular to the sliding surface

F is the applied force

Fs is the static frictiοn force

Fs,max is the maximum static frictiοn force


Static frictiοn is usually greater than kinetic frictiοn because of molecular attraction between the two object surfaces. But once this bond is "broken" (and F > Fs,max), sliding begins, and due to the relative motion the molecular attraction is less, meaning that the kinetic frictiοn is less.





It is worth mentioning that the coefficient of friction between two objects is independent of contact area. It depends only on the normal force between the two objects, and not on the size of the contact area between the two objects.



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