Page 4 - Demo

150 SCIENCEWe see that the friction is caused bythe interlocking of irregularities in thetwo surfaces. It is obvious that the forceof friction will increase if the twosurfaces are pressed harder. You canexperience it by dragging a mat whennobody is sitting on it, and when apreson is sitting on it.Recall your experience when last timeyou moved a heavy box from one placeto another. If you have no suchexperience, get that experience now.What is easier %u2014 to move the box fromrest, or to move it when it is already inmotion?The force required to overcomefriction at the instant an object startsmoving from rest is a measure of staticfriction. On the other hand, the forcerequired to keep the object moving withthe same speed is a measure of slidingfriction.friction and you find it somewhat easierto move the box already in motion thanto get it started.12.3 Friction : A NecessaryEvilRecall now some of your experiences. Isit easier to hold a kulhar (earthen pot)or a glass tumbler? Suppose the outersurface of the tumbler is greasy, or hasa film of cooking oil on it; would itbecome easier or more difficult to holdit? Just think : would it be possible tohold the glass at all if there is nofriction?Recall also how difficult it is to moveon a wet muddy track, or wet marblefloor. Can you imagine being able to walkat all if there were no friction?When the box starts sliding, thecontact points on its surface, do not getenough time to lock into the contactpoints on the floor. So, the slidingfriction is slightly smaller than the staticFig. 12.7 : A nail is fixed in the wall due tofrictionYou could not write with pen orpencil if there were no friction. Whenyour teacher is writing with chalk onthe blackboard, its rough surface rubsoff some chalk particles which stick toFig. 12.6 : You have to push on the box to keep itmoving%u00a9 NCERTnot to be republished