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Newton's Laws Of Motion & Friction

11. Friction and frictional force

11. Friction and frictional force
(a) If we slide or try to slide a body over a surface the motion is resisted by a bonding between the body and the surface. This resistance is represented by a single force and is called friction.
(b) The force of friction is parallel to the surface and opposite to the direction of intended motion.
(c) Force of friction is partically independent of microscopic area of surface in contact and relative velocity between them.
(if it is not high)
(d) However, it depends on the nature of material of the surface in contact (force of adhesion) and their roughness and smoothness.
(e) Normally with increase in smoothness friction decreases. But if the surface area are made too smooth by polishing and cleaning the bonding force of adhesion will increase and so the friction will increase resulting in 'Cold welding'
(f) Friction is a non conservative force, i.e. work done against friction is path dependent.
(g) In its presence mechanical energy is not conserved. Thus friction reduces efficiency of a machine.
(h) It is a general misconception that friction always opposes the motion. No doubt friction opposes the motion of a moving body but in many cases it is also the cause of motion.
(i) In moving a person or Vehicle pushes the ground backwards (action) and the rough surface of ground reacts and exerts a forward force due to friction which causes the motion if there had been no friction there will be slipping and no motion 
(j) There are three types of frictional force depending upon the nature of relative motion between the bodies.

IIT (Class X)