Friction is a force which opposes relative motion. When ever you try to move a body,by applying a force,friction is generated and it is against the relative motion.At a basic level,here we are studying about friction between solid surfaces.It comes into picture between every two surfaces.It is because of irregularities that any solid surface has.With a normal eye, we may not be able to see them.When we keep a body under a powerful microscope,we can visualize them.When one body is placed over the other body, this irregular surfaces comes into contact and get interlocked with each other.When we try to move this body,this interlocking opposes the motion and it is called as friction.
The magnitude of friction does not depend on the surface area,as the point of contact of irregular surface are microscopic and it depends on the roughness of the surface and not the size of the surface.If both the bodies are at rest,there is no need to overcome the interlocking and there is no friction at all.When we try to move any body,opposition is developed and it is called friction.The friction between the two bodies that are in the state of rest is called static friction.If at least one body is moving,then the friction between the bodies is called kinetic or dynamic friction.If any one body is rolling,then the friction is called rolling friction.Static friction is highest as there is high interlocking.Once if the body starts moving,there is no that much of interlocking scope and time and hence,kinetic friction is less than that of static friction.
Frictional force is directly proportional to normal reaction.Normal reaction is the force applied by the upper surface of a body on the lower surface of a body in contact and it is perpendicular to surface of contact.It need not be always equal to weight of body and vary with respect to the given situation.
The proportionality can be eliminated with a constant called coefficient of friction.It is dependent of the nature of the rough surfaces but not on the applied force and normal reaction.
You can watch the following video lesson,to get the visual picture of what is explained above.
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