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Chapter – 12 Friction
Friction is a force thatopposes the relative motion between twosurfaces of objects in
contact. Theforce of friction always acts ina direction opposite to that ofthe applied force.
Causes of Friction: Friction exists between two surfaces due to irregularities on the surfaces ofthe objects incontact, interlocking ofmicro-level irregularities ofthe two surfaces
and ploughing ofharder surfaces intosmoother surfaces.
Factors That Affect Friction Roughness of thesurface The extent to which thetwo surfaces presstogether Nature ofthe surface
Types of Friction:
(i) Static Friction: When a body is at rest, the force of friction is called the static friction and isalways equal andopposite to theapplied force. Theforce of friction which acts whenthe body is justat the verge of sliding onthe surface iscalled limiting friction. (ii) Sliding friction: The friction force which opposes the actual relative sliding motion between twocontact surfaces. Sliding friction is smaller than static friction. (iii) Rolling Friction: The frictional force that exists between two surfaces when a bodyrolls over theother. Rolling friction is smaller thansliding friction. Effects ofFriction Friction produces heat. Friction causes wear and tear. Friction opposes motion.
Advantages of Friction • Friction between pen and paperenables us towrite on thepaper. • Friction between our feet andthe ground allows our movements likestanding, walking and running. • Friction between the surface ofthe road andtyres of ourvehicles allow thevehicles to movewithout slipping.
Disadvantages ofFriction
Friction causes moving objects to stopor slow down.
Friction produces heat causing wastage of energy inmachines.
Friction causes wear and tearof moving partsof machinery, solesof shoes, etc.
Friction is a necessary Evil: As friction is advantageous to us it is considered as a friend but due to its disadvantages it is afoe. Depending onthe circumstance, friction can be ahelp or a hindrance. Thus itis a necessary evil.
Increasing Friction: By pressing the surfaces together more strongly. For e.g. when brakes areapplied on a bicycle orcar, the brakepads press against a moving partof the wheeland the force of friction increases. Friction can beincreased by increasing the roughness ofthe surfaces incontact. For example, treading of shoes andtyres is doneto increase friction.
Reducing Friction: Friction between the sliding surfaces of twoobjects can be reduced by making thesurfaces in contact smooth by polishing them. Sliding friction between the moving parts of vehicles andmachinery can be reduced byusing oil, grease,graphite or any other lubricant. Rolling friction is less than sliding friction. Hence, sliding friction is replaced by rolling friction byusing rollers, likeball bearings between the hub andthe axles in the moving parts of machines andvehicles. Friction is reduced by providing wheels, e.g. suitcase, school bags of kids, etc. Aeroplanes, boats, fishes and birds which move through fluids have bodies ofspecial shape, called streamlined shape, so asto reduce thefriction due tofluid and avoid energy loss.
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