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The friction force is a contact force that opposes the relative movement between two surfaces in contact. It depends on the nature (rough or smooth) of the two surfaces and the normal action or reaction force N of one body on the surface of another.

Friction force = Ff = μ N (= μ * weight of the body )

Coefficient of friction = ratio between the normal force and the force of friction. Ff is just equal to the force required to move the body on a surface.

Coefficient of friction μ
             = Force needed to just move an object Ff / Normal reaction force R

It is always between 0 (smooth surface) and 1 (totally rough surface).