Coeffiecient of friction:

The coefficient of friction is ratio of the force of friction acting in between 2 surfaces in contact, and the force required to slide one over the other.

Coefficient of friction, frictional coefficient, or friction coefficient as it is also known, can't be determined by calculations, but by the practical experiment. The values of it depend on the materials in contact with each other, and it varies from near zero to above one. If the coefficient of friction is zero, it means there is no friction force between the surfaces. But, this is a hypothetical value only, since no surfaces in contact have been found to be frictionless. The higher the value of coefficient of friction, the greater the frictional force acting among the surfaces, and this certainly means that the larger force will be required to slid one of them over the other. Be needed to slide 1 of them over the other. This goes with the fact that the static friction is higher than kinetic friction; but, there are certain surfaces whose coefficient of static and kinetic friction is the same Force of Friction.

When surface of 1 object slides across the surface of other object and some other force is pressing the surfaces together, a drag force is created between the 2 surfaces which are parallel to the surfaces. The coefficient of static friction id depends upon roughness of the surfaces. The coefficient of static friction doesn't depend upon the contact surface area, provided the 2 surfaces are made of different material so that any cohesive force between the 2 surfaces is small.

The force of static friction (such as normal force) is a reaction force in that the value of it depends upon the magnitude of some external force trying to push the object along surface. The force of friction is always in opposite direction of the other forces imposed. The force of static friction increases up to a maximum value, after which the object breaks loose and begins moving.