Frictional force

Frictional force (f) is a component force between two surfaces that is parallel to the surfaces. It is divided into static frictional force (when the two surfaces do not move with respect to each other) and kinentic frictional force (when the two surfaces move, or slide over one another).

Static Fritional Force
Static frictional force does not depend on the area of contact between the objects, and the magnitude of the maximum static frictional force is proportional to the magnitude of the normal force, given by the formula:

The maximum static frictional force is just before the object begins to move. For example, an object is placed on the table. When you apply a force of 7N parallel to the table to pull the object, there will be a frictional force of 7N in the opposite direction to cancel the effect of the applied force. If more force is applied, then the object will start to move. The force that is just before the object starts the movement is equal to the maximum static frictional force. After that, the force you apply will overcome the static frictional force, and kinetic frictional force is involved. The µ_s is the coefficient of static friction and depends on the type and condition of surface and other factors such as temperature. It ranges from 0.01 to 1.5.

Kinetic Frictional Force
Kinetic frictional force does not depend on the area of contact and the speed of the object. Its magnitude is proportional to the magnitude of the normal force, given by the formula:

The value of kinetic frictional force is usually smaller than that of static frictional force.

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