Page 7 - Demo

FRICTION 153Activity 12.4Take a few pencils which arecylindrical in shape. Place themparallel to each other on a table.Place a thick book over it (Fig.12.15). Now push the book. Youobserve the pencils rolling as thebook moves. Do you feel it easier tomove the book in this way than toslide it? Do you think thatresistance to the motion of the bookhas been reduced? Have you seenheavy machinary being moved byplacing logs under it?12.5 Wheels Reduce FrictionYou must have seen attaches and otherpieces of luggage fitted with rollers. Evena child can pull such pieces of luggage(Fig. 12.14). Why is it so? Let us findout.Can we reduce friction tozero by polishing surfacesor using large amount oflubricants?Friction can never beentirely eleminated. Nosurface is perfectly smooth.Some irregularities arealways there.Fig. 12.15 : Motion of the book on rollersWhen one body rolls over the surfaceof another body, the resistance to itsmotion is called the rolling friction.Rolling reduces friction. It is alwayseasier to roll than to slide a body overanother. That is the reason it isconvenient to pull the luggages fittedwith rollers. Can you now understandwhy wheel is said to be one of thegreatest inventions of mankind?Since the rolling friction is smallerthan the sliding friction, sliding isreplaced in most machines by rollingby the use of ball bearings. Commonexamples are the use of ball bearingsbetween hubs and the axles of ceilingFig. 12.14 : Rolling reduces friction fans and bicycles (Fig. 12.16).%u00a9 NCERTnot to be republished