Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
The causes of the resistive force of friction are molecular adhesion, surface roughness, and the plowing effect. Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects against each other requires breaking these adhesive bonds
To stop a moving object, a force must act in the
opposite direction to the direction of motion. For
instance, if you push your book across your desk, the
book will move. The force of the push moves the book.
As the book slides across the desk, it slows down and
stops moving. The force that opposes the motion of an
object is called friction
The primary causes of friction are surface roughness,
the plowing effect and molecular adhesion. Surface
roughness is when serious abrasion occurs due to the
roughness of the materials in contact. The plowing
effect involves deformations of the objects that cause
resistance to movement when the materials are
relatively soft.