Page 5 - Demo

FRICTION 151the black board. Could it happen if therewere no friction between the chalk andthe board?If an object started moving, it wouldnever stop if there were no friction. Hadthere been no friction between the tyresof the automobiles and the road, theycould not be started or stoped or turnedto change the direction of motion. Youcould not fix a nail in the wall (Fig. 12.7)or tie a knot. Without friction no buildingcould be constructed.Fig. 12.8 : Soles of shoes wear out due tofrictionFig. 12.10 : Stricking a match stick produces fireby frictionproduces heat. In fact, when a machineis operated, heat generated causes muchwastage of energy. We shall discuss theways of minimising friction in thefollowing section.Fig. 12.9 : Rubbing of your palms makes you feelwarmOn the other hand, friction is an evil,too. It wears out the materials whetherthey are screws, ball bearings or solesof shoes (Fig. 12.8). You must have seenworn-out steps of foot over-bridges atrailway stations.Friction can also produce heat.Vigorously rub your palms together fora few minutes (Fig. 12.9). How do youfeel? When you strike a matchstickagainst the rough surface, it catches fire(Fig. 12.10).You might have observed that the jarof a mixer becomes hot when it is runfor a few minutes. You can cite variousother examples in which friction%u00a9 NCERTnot to be republished