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### 8 friction

1. 1. FRICTIONY ou might have seen a driver of a 12.1 Force of Friction car or a truck slowing down the vehicle at a traffic signal. You, too, Activity 12.1slow down your bicycle whenever Gently push a book on a table [Fig.needed by applying brakes. Have you 12.2(a)]. You observe that it stopsever thought why a vehicle slows after moving for some distance.down when brakes are applied? Not Repeat this activity pushing theonly vehicles, any object, moving over book from the opposite directionthe surface of another object slows [Fig. 12.2, (b)]. Does the book stopdown when no external force is this time, too? Can you think of anapplied on it. Finally it stops. Have explanation? Can we say that a forceyou not seen a moving ball on the must be acting on the bookground stopping after some time? opposing its motion? This force isWhy do we slip when we step on a called the force of friction.banana peel (Fig. 12.1)? Why is itdifficult to walk on a smooth and wetfloor? (a)Fig. 12.1 : A boy falls down when he steps on a (b) banana peel Fig. 12.2 (a), (b) : Friction opposes relative motion between the surfaces of the You will find the answers to such book and the tablequestions in this chapter.
2. 2. You saw that if you apply the force for this difference? Repeat thisalong the left, friction acts along the activity by wrapping a piece of juteright. If you apply the force along the bag around the brick. What do youright, the friction acts along the left observe?direction. In both cases the force opposesthe motion of the book. The force of Spring Balancefriction always opposes the applied force. In the above activity, the force of Spring balance is a device used forfriction acts between the surface of the measuring the force acting on anbook and the surface of the table. object. It consists of a coiled spring Is the friction the same for all the which gets stretched when a force is applied to it. Stretching of the springsur faces? Does it depend on the is measured by a pointer moving onsmoothness of the surfaces? Let us find a graduated scale. The reading on theout. scale gives the magnitude of the force.12.2 Factors affecting FrictionActivity 12.2 Tie a string around a brick. Pull the brick by a spring balance (Fig. 12.3). You need to apply some force. Note down the reading on the spring balance when the brick just begins to move. It gives you a measure of the of force of friction between the surface of the brick and the floor. Activity 12.3 Make an inclined plane on a smooth Fig. 12.3 : A brick is being pulled by spring floor, or on a table. You may use a balance. wooden board supported by bricks, Now wrap a piece of polythene or books. [Fig. 12.4 (a)]. Put a mark with a pen at any point A on the around the brick and repeat the inclined plane. Now let a pencil cell activity. Do you observe any move down from this point. How far difference in the readings of the does it move on the table before spring balance in the above two coming to rest? Note down the cases? What might be the reason distance. Now spread a piece of cloth148 SCIENCE
3. 3. Does the distance covered depend on over the table. Make sure that there the nature of the surface on which the are no wrinkles in the cloth. Try the cell moves? activity again [Fig. 12.4 (b)]. Could the smoothness of the surface of the cell also affect the distance travelled by it? Mark A I shall try the activity by wrapping a piece of sandpaper around the cell. Friction is caused by the irregularities on the two surfaces in (a) contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them (Fig. 12.5). Irregularities on the two surfaces lock into one another. When we attempt Mark A to move any surface, we have to apply a force to overcome interlocking. On rough surfaces, there are a larger number of irregularities. So the force of friction is greater if a rough surface is involved. (b) Fig. 12.4 : The pencil cell covers different distances on different surfaces Repeat this activity by spreading a thin layer of sand over the table. Maintain the same slope throughout the activity. In which case is the distance coveredthe minimum? Why is the distancecovered by the pencil cell different everytime. Try to reason why? Discuss theresult. Fig. 12.5 : Surface irregularitiesFRICTION 149
4. 4. We see that the friction is caused by friction and you find it somewhat easierthe interlocking of irregularities in the to move the box already in motion thantwo surfaces. It is obvious that the force to get it started.of friction will increase if the twosurfaces are pressed harder. You can 12.3 Friction : A Necessaryexperience it by dragging a mat when Evilnobody is sitting on it, and when a Recall now some of your experiences. Ispreson is sitting on it. it easier to hold a kulhar (earthen pot) Recall your experience when last time or a glass tumbler? Suppose the outeryou moved a heavy box from one place surface of the tumbler is greasy, or hasto another. If you have no such a film of cooking oil on it; would itexperience, get that experience now. become easier or more difficult to holdWhat is easier — to move the box from it? Just think : would it be possible torest, or to move it when it is already in hold the glass at all if there is nomotion? friction? The force required to overcome Recall also how difficult it is to movefriction at the instant an object starts on a wet muddy track, or wet marblemoving from rest is a measure of static floor. Can you imagine being able to walkfriction. On the other hand, the force at all if there were no friction?required to keep the object moving withthe same speed is a measure of slidingfriction.Fig. 12.6 : You have to push on the box to keep it Fig. 12.7 : A nail is fixed in the wall due to moving friction When the box starts sliding, the You could not write with pen orcontact points on its surface, do not get pencil if there were no friction. Whenenough time to lock into the contact your teacher is writing with chalk onpoints on the floor. So, the sliding the blackboard, its rough surface rubsfriction is slightly smaller than the static off some chalk particles which stick to150 SCIENCE
5. 5. the 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. 12.9 : Rubbing of your palms makes you feel warm for a few minutes. You can cite various other examples in which frictionFig. 12.8 : Soles of shoes wear out due to friction On 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 for Fig. 12.10 : Stricking a match stick produces firea few minutes (Fig. 12.9). How do you by frictionfeel? When you strike a matchstick produces heat. In fact, when a machineagainst the rough surface, it catches fire is operated, heat generated causes much(Fig. 12.10). wastage of energy. We shall discuss the You might have observed that the jar ways of minimising friction in theof a mixer becomes hot when it is run following section.FRICTION 151
6. 6. 12.4 Increasing and Reducing In some situations, however, friction Friction is undesirable and we would want to minimize it.As you have seen in the previous section, Why do you sprinkle fine powder onfriction is desirable in some situations. the carrom board? You might have Have you ever thought why the sole noticed that when a few drops of oil areof your shoes is grooved [Fig. 12.11 (a)]? poured on the hinges of a door, the doorIt is done to provide the shoes better moves smoothly. A bicycle and a motorgrip on the floor, so that you can move mechanic uses grease between thesafely. Similarly, the treaded tyres of moving parts of these machines. In allcars,trucks and bulldozers provide better the above cases, we want to reducegrip with the ground. Fig. 12.12 : Powder is sprinkled on the carrom board to reduce friction friction in order to increase efficiency. When oil, grease or graphite is applied between the moving part of a machine, (a) (b) a thin layer is formed there and moving surfaces do not directly rub against eachFig. 12.11 : Soles of shoes and tyres are treaded other (Fig. 12.13). Interlocking of to increase friction irregularities is avoided to a great extent. We deliberately increase friction by Movement becomes smooth. Theusing brake pads in the brake system substances which reduce friction areof bicycles and automobiles. When you called lubricants. In some machines, itare riding a bicycle, the brake pads do may not be advisable to use oil asnot touch the wheels. But when you lubricant. An air cushion between thepress the brake lever, these pads arrest moving parts is used to reduce friction.the motion of the rim due to friction. LubricantThe 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. Fig. 12.13 : Action of lubricant152 SCIENCE
7. 7. Activity 12.4 Take a few pencils which are Can we reduce friction to cylindrical in shape. Place them zero by polishing surfaces parallel to each other on a table. or using large amount of Place a thick book over it (Fig. lubricants? 12.15). Now push the book. You observe the pencils rolling as the book moves. Do you feel it easier to move the book in this way than to Friction can never be slide it? Do you think that entirely eleminated. No resistance to the motion of the book surface is perfectly smooth. has been reduced? Have you seen Some irregularities are heavy machinary being moved by always there. placing 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. Fig. 12.15 : Motion of the book on rollers When one body rolls over the surface of another body, the resistance to its motion is called the rolling friction. Rolling reduces friction. It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull the luggages fitted with rollers. Can you now understand why wheel is said to be one of the greatest inventions of mankind? Since the rolling friction is smaller than the sliding friction, sliding is replaced in most machines by rolling by the use of ball bearings. Common examples are the use of ball bearings between hubs and the axles of ceiling Fig. 12.14 : Rolling reduces friction fans and bicycles (Fig. 12.16).FRICTION 153
8. 8. is fluids. So we can say that fluids exert force of friction on objects in motion through them. The frictional force exerted by fluids is also called drag. The frictional force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid. It is obvious that when objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimize friction. So, objects are given special shapes. Where do you think the scientists get hints for these special shapes? From nature, of course. Birds and fishes have to move about in fluids all the time. Their bodies must Fig. 12.16 : Ball bearings reduce friction have evolved to shapes which would12.6 Fluid Friction make them not to lose much energy in overcoming friction. You read aboutYou know that air is very light and thin. these shapes in Class VI. Look carefullyYet it exerts frictional force on objects at the shape of an aeroplanes (Fig.moving through it. Similarly, water and 12.17). Do you find any similarity in itsother liquids exert force of friction when shape and that of a bird? In fact, allobjects move through them. In science, vehicles are designed to have shapesthe common name of gases and liquids which reduce fluid friction. Fig. 12.17 : Similarity in shapes of an aeroplane and a bird154 SCIENCE
9. 9. KEYWORDS WHAT YOU HAVE LEARNT BALL BEARING Friction opposes the relative motion between two surfaces in contact. It acts on both the DRAG surfaces. FLUID FRICTION Friction depends on the nature of surfaces in contact. FRICTION For a given pair of surfaces friction depends upon the state of smoothness of those INTERLOCKING surfaces. LUBRICANTS Friction depends on how hard the two surfaces press together. ROLLING FRICTION Static friction comes into play when we try to move an object at rest. SLIDING FRICTION Sliding friction comes with play when an STATIC FRICTION object is sliding over another. Sliding friction is smaller than static friction. Friction is important for many of our activities. Friction can be increased by making a surface rough. The sole of the shoes and the tyres of the vehicle are treaded to increase friction. The friction is sometimes undesirable. Friction can be reduced by using lubricants. When one body rolls over another body, rolling friction comes into play. Rolling friction is smaller than the sliding friction. In many machines, friction is reduced by using ball bearings. Fluid friction can be minimised by giving suitable shapes to bodies moving in fluids. Exercises 1. Fill in the blanks: (a) Friction opposes the _____________ between the surfaces in contact with each other. (b) Friction depends on the _____________ of surfaces.FRICTION 155
10. 10. (c) Friction produces __________. EXERCISES (d) Sprinkling of powder on the carrom board ________ friction. (e) Sliding friction is ___________ than the static friction. 2. Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement. (a) Rolling, static, sliding (b) Rolling, sliding, static (c) Static, sliding, rolling (d) Sliding, static, rolling 3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be : (a) Wet marble floor, dry marble floor, newspaper and towel. (b) Newspaper, towel, dry marble floor, wet marble floor. (c) Towel, newspaper, dry marble floor, wet marble floor. (d) Wet marble floor, dry marble floor, towel, newspaper 4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it. 5. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why? 6. Explain why sportsmen use shoes with spikes. 7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why? 8. Explain why the sliding friction is less than the static friction. 9. Give examples to show that friction is both a friend and a foe. 10. Explain why objects moving in fluids must have special shapes.156 SCIENCE