Page 6 - Demo

152 SCIENCE12.4 Increasing and ReducingFrictionAs you have seen in the previous section,friction is desirable in some situations.Have you ever thought why the soleof your shoes is grooved [Fig. 12.11 (a)]?It is done to provide the shoes bettergrip on the floor, so that you can movesafely. Similarly, the treaded tyres ofcars,trucks and bulldozers provide bettergrip with the ground.In some situations, however, frictionis undesirable and we would want tominimize it.Why do you sprinkle fine powder onthe carrom board? You might havenoticed that when a few drops of oil arepoured on the hinges of a door, the doormoves smoothly. A bicycle and a motormechanic uses grease between themoving parts of these machines. In allthe above cases, we want to reduceFig. 12.12 : Powder is sprinkled on the carromboard to reduce frictionfriction in order to increase efficiency.When oil, grease or graphite is appliedbetween the moving part of a machine,a thin layer is formed there and movingsurfaces do not directly rub against eachother (Fig. 12.13). Interlocking ofirregularities is avoided to a great extent.Movement becomes smooth. Thesubstances which reduce friction arecalled lubricants. In some machines, itmay not be advisable to use oil aslubricant. An air cushion between themoving parts is used to reduce friction.We deliberately increase friction byusing brake pads in the brake systemof bicycles and automobiles. When youare riding a bicycle, the brake pads donot touch the wheels. But when youpress the brake lever, these pads arrestthe motion of the rim due to friction.The wheel stops moving. You might haveseen that kabaddi players rub theirhands with soil for a better grip of theiropponents. Gymnasts apply some coarsesubstance on their hands to increasefriction for better grip.(a) (b)Fig. 12.11 : Soles of shoes and tyres are treadedto increase frictionFig. 12.13 : Action of lubricantLubricant%u00a9 NCERTnot to be republished