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abe, , , , Friction, , en we push a box lying on the floor with a small amount of force, it does not, , (see Figure 1). [It means that the surface of floor, on which the box, ction opposite to the force of our push., which opposes its motion (due to, , move at all, is resting, exerts some force on the box which acts ina dire, , words, some force is acting on the bottom of the stationary box, , In other, This natural force between the, , which the box does not Move)., foor and the bottom of the box w hich opposes the motion of box, , onthe floor is friction (see Figure 1). Let us take another example., Aball moving on the ground slows down gradually and stops, We know that a force is required to, , after covering some distance., ans that a force is exerted by the, , stop a moving ball. This me, ground on the moving, ball whic h opposes its motion and brings, , ittoa stop. This force which opposes the motion of ball on the, | ground is also friction. We can now define friction as follows :, ‘the force which always opposes the motion of one object over, , | mother object in contact with it, is called friction) Friction, | occurs between the two surfaces which are in contact with each other. For example, when we push a heavy, , of friction occurs between the surface of floor and bottom of the box. And in, | the case of a ball rolling on ground, friction occurs between the surface of ground and the surface of ball., , Infact, friction acts on both the surfaces in contact with each other. Friction is a force which occurs when, h other and even when they are actually sliding (moving) over each, , , , Figure 1. When we push the box with a small, force, the friction between floor and bottom, of box prevents it from moving., , box kept on floor, the force, , the tw ., } two objects tend to slide over eac, | other,, , Dire :, ction of Force of Friction, , Th 5 se, , ie e force of friction always op, , will ace in a direction opposite to the direction in w, ‘come more clear from the following activity., , poses the motion of one object over another object. So, the force of, , frict, hich an object moves (or tends to move). This, , 213, , a

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214 \, , ACTIVITY, , Place a book on te table. Give a push to this book towards the right side las shown in Figure 2ay, will find that the book moves through some distance to the right side and then stops. Since the book Me!, towards right side stops on its own, this means that the force of friction is aang On it in the ome, direction (towards left) which is opposing its motion and making it stop. Thus, when the force is a Dlg, our push to move the book towards right side, then the force of friction acts towards lefi side (in Op, i, , UF ph Do, direction 10 motion of book) {see Figure 2(a)]. :, , , , Motion, , , , , , , , , , (4) When book moves towards right side, (b) When book moves ee left side,, friction acts towards left side friction acts towards right side, , Figure 2. The force of friction always acts in a direction opposite to the direction of motion of object,, , Lel_us now give a push to the book towards the left side [as shown in Figure 2(b)]. We will find that, book moves some distance towards the left side and then stops. Since the book moving towards left side, Stops on its own, this means that the force of friction is acting on it in the opposite direction (towards righ, which is opposing its motion and making it stop. Thus, when the force is applied by our push to Move the|, book towards fert side, then the force of friction acts towards the right side (in opposite direction to th, motion of book) [see Figure 2(b)). |, , , , , , In the above activity we observe that when the motion of the book is towards right side, then the force, of friction acts on it towards the left side. And when the motion of book is towards left side, the force of |, friction acts on it towards the right side. So, from this activity we conclude that the force of friction ags, in a direction opposite to the direction of motion of an object. This is why the force of friction always, Opposes the motion of an object. In both the cases described above, the force of friction Opposes the motion, of book on the surface of the table. This force of friction occurs between the surface of table and the surface, of book in touch with each other. |, , to move a book lying on a horizontal table top. The book can also be moved, , by the force of gravity of earth provided the table is tilted a little. So, if we, , tilt the table somewhat, then the book kept on it will start sliding down a, slowly. In this case, the force of gravity is acting in the downward direction oe, (see Figure 3). Since the book is mov ing in the downward direction, the force KH |, of friction must act on it in the opposite direction—upward direction (as, , shown in Figure 3) Figure 3., , | In the above examples, we have applied the force of push of our hand y, , , , Cause of Friction, , Every object has a rough surface, though the surface may appear to be smooth to the naked eye. Whet, we see through a microscope, it is found that the surfaces of all the objects have rough edges. Some of the |, particles on the surface of objects are in the form of tiny hills while others form grooves (see Figure 4) Tk, tiny hills and grooves on the surfaces of objects are called ‘irregularities of surfaces’. When we try to mo", one object over another object, the ‘irregularities’ present on their surfaces get entangled (or Jocked) wit*, one another (see Figure 4), The interlocking of irregularities of the two surfaces opposes the motion ae, object over the other and gives rise to force of friction. Thus, friction is caused by the interlocking ;, irregularities in the surfaces of the two objects which are in cOntact with each other) When we atten?, , . a

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. 215, , , , , , , , , , bject over the other, we h,, sone 0 , ave to apply, ve. i i a ply a force to Wooden, ‘ i ne interlocking of the irregularities in their surfaces ne, “ere 1 roughness of a surface, larger is the nub S., , ore s on its surface and hence greater mber of, , , , , , , , : ‘ r, ) te will be the fricti Irregularities _ Pu, lat cabin i : e the friction. =, iO pe foree of friction is greater if very tough surfaces ave in surfaces >,, 157 ., 1M ocd 4 h h 7 Wooden 8, tt soure + We have shown a ghly maenifi 4 block, i agnified >, n BE ¢ two wooden blo: 5 diagram of gularities, , es cks A and B Fioure 4, Friction arises due to Te:, faces 9 nd B kept on igure 4. ene of objects:, ine suf jease note the rough surfaces of these retin Bi a (or roughness nthe surfaces of “, , thet irregularities. When ny BIOCKS 7, aang tot of Wreah we pull the upper wooden block A over the lower wooden block B by, , wn a force, then the irregularities of their rough surfaces get entangled (or locked) with one another., , 20F “ qterocking of surfaces gives rise to force of friction which opposes the motion of upper block A over, ower block B. We are able to move block A over block B because we apply sufficient muscular force, , ig ulling to undo the interlocking of surfaces and overcome the opposing frictional force. The block, , i noveS over block B only when the pulling force applied by us becomes greater, , ijan the force of “ee them together. Friction is small for smooth surfaces Cy, , (ike glass and ice). Friction is much greater for rough surfaces (like sand paper and Sf, , -onctete)- f, , * Before We 8° further, we: should know what a spring balance is because it will be ~), , sed t0 perform some activites based on friction. The spring balance is a device, , which 1S used for meee uring force acting on an object (see Figure 5). The spring | , palance contains a coiled spring which gets stretched when a force is applied to its | ., , jeeend (having a hook). The extent by which the spring gets stretched is a measure “, , ofthe force applied. Larger the stretching of spring, greater will be the magnitude of , force applied. The stretching of spring or magnitude of force is indicated by a pointer, , attached to the spring which moves ona graduated scale. The reading on the scale of Ls, , spring balance (as indicated by the position of pointer) gives us the magnitude of, , force. When the spring balance is held vertically (as shown in Figure 5), it is said to J, , measure the weight of an object hung from its hook (because weight of an object is a ms, , also a force). And when a spring balance is held horizontally (attached to an object nee Taee, , and pulled), it can be used to measure the force being applied to pull the object on a —e—, , horizontal surface., , , , FACTORS AFFECTING FRICTION, Ithas been found by experiments that the friction between two surfaces depends on two factors :, (i) the nature of the two surfaces (smoothness or roughness of the two surfaces)., (ii) the force with which two surfaces are pressed together., The force of friction, however, does not depend on the ‘amount of surface area’ of the two objects, which is in contact with each other. We will now study how the friction depends on the nature of two, surfaces as well as on the force with which the two surfaces are pressed together., , Dependence e Nature of Two Surfaces, , Friction is not the same for all the surfaces. Friction depends on the smoothness or roughness of the, two surfaces which are in contact with each other. When the two surfaces in contact are smooth, then the, friction between them will be small] (because the interlocking of smooth surtaces is less), As the degree ot, roughness of the two surfaces in contact increases, the friction also increases. And when the two surtaces, m contact are very rough, then the friction between them will be very large (because the interlocking of, ery rough surface is too much). We can study the dependence of friction on the nature of surfaces by, Performing some activities as follows., , ha

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Y, , 216, , era , hread) around the brick and connect it to the hoop., , spring e f, spring balance. Apply a pulling force to the brick by pulling the other a pit De hand Mil “, brick just begins to slide (move slowly) on the floor (see Figure 6). Note on aes Se fee oe, when the brick begins to slide. This reading of spring balance will ive us the | : 8! ; ras Ofce of frig, between the surface of floor and the surface of brick (which are in contact with eac other). OH, , , , Place a brick on the floor. Tie a string (strong, , iis Spring, Ns String balance, pee ———, , [OP peta os, PE —————, , Floor, , , , , Figure 6. Activity to study the dependence of friction on the nature of surfaces., (7) Let us now wrap a piece of polythene around the brick and repeat the above activity. We NOt th, | spring balance reading when the polythene wrapped brick just begins to slide on the floor. We will find thay, this reading of spring balance is smaller than the first reading of spring balance (when there was no Polytheng, around the brick) indicating that the force of friction has decreased. From this observation we Conclude thay, wrapping of polythene sheet makes the surface of brick smooth due to which the friction with floor, decreases., , (7) Remove the polythene sheet from the brick. We now wrap a jute cloth around the brick and Tepeat, the above activity once again. We note the spring balance reading when the jute wrapped brick just begins, to slide on the floor. In this case we find that the reading of spring balance is greater than the first reading of, spring balance (when nothing was wrapped around the brick) indicating that the force of friction has, increased. From this observation we conclude that wrapping of jute cloth makes the surface of brick more, rough due to which the friction with floor increases., , , , , , From the above activity we learn that when the surface of brick is made more smooth by wrappi, polythene sheet, the friction with floor decreases. On the other hand, when the surface of brick is made, more rough by wrapping a jute cloth, then the friction with floor increases. Thus, the friction depends on, the nature of two surfaces. That is, the friction depends on the smoothness or roughness of the two, , surfaces., er 7, , , , | Make an inclined plane on a smooth marble floor by keeping a wooden board in tilted position with the, “help of a brick placed behind the wooden board (see Figure 7). Mark a horizontal line AB with a ball pen, on the upper half of the inclined wooden board (as Inclined, shown in Figure 7). Hold a pencil cell on the line AB wooden, marked on the inclined wooden board and then release board, it. The pencil cell will move down rapidly from the, _inclined board and travel a certain distance on the |, | marble floor before coming to rest (or stopping) (see, | Figure 7). The moving pencil cell stops due to friction, | exerted by the marble floor. We note the distance, , J, , , , | covered by the pencil cell on the marble floor (from the | Brick Pant, | base of the inclined board). This distance will give us an “, | idea of the friction between marble floor and pencil cell, | _ ———, , (), Let us now put some water on the marble floor to Figure 7. The pencil cell travels different distances, make it wet. We repeat the above activity by releasing different surfaces (because the friction on differe™, , pencil cell from the same line AB of inclined board. surfaces is different)., Z = " eae Fh aegis gs ae, , AN, , —

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217, , , , ~~ cil cell will travel a certain di, a the ealed by the pencil cell oe on wet marble floor before coming to rest. We lapsed, marble floor. We will find that the pencil cell travels a larger, , we avel, ye the wet marble floor indicati, itt ceo" cating that the friction on wet marble floor is less (than that 0” dry, , i fal!, ie 00 the wet marble, emake the wet marble :, ‘ i — the above sage wien it. Let us now spread a sheet of newspaper on the dry, f poat and note the distance travel climate ie enelicelirany Metall marked line AB oF os, i i travels less distance on the news the pencil cell before coming to rest. We will find that the, crn exerted By newspaper is —e (than that on the dry marble floor) indicating that, » the ne’ in that exerted by dry marble floor., ot ale ee pisbs and spread a towel on henetie floor Repeat tie, fy jas ve il cell isi pi anne marked line AB on the inclined board and note t, veld by the pene lore coming (0 rest. We will find that the pencil cell travels the lea:, ihe towel indicating that the friction exerted b 5 p, Ne towel is even greater than that of newspaper, smooth by making, surface of marble, ases. Thus, the, , , , , , , , , , , , , , ole loo, , activity once again, he distance, , st distance, , frictio, (iil) Le, , , , sat heabouecastivity WE'see that when the surface:olimarble floor? de more, yet with ater, the friction with pencil cell decreases. On the other tani wl oni, sor is made MOTE rough by covering it with newspaper or towel then th : be a, , ds on the nature (smoothness or roughness) of the two vvfaces, oom, , tion on the Force With Which Two Surfaces are Pressed Together, , by the interlocking regulariti, Xe interlocking of irregularities of the two surfaces when one object is pl, greater force, then the, , force will increase the, , jt, , » pependence of Fric, laced, , Friction IS caused |, object. If the two surfaces of objects are pressed together harder by a, , (because pressing together two surfaces of objects with a greater, , yer anothe, | increase, , , , , , , , , , , , , , , , , snction il, artocking 17 the two surfaces). This will become more clear from the following activit, er - —, ACTIVITY, suppose we have two boxes « | the same size but one box is light and the other box is heavy (see Figure, y rwe push both the boxes on the floor, one by one, we will find that we have to apply only a small force, & 7, g ad Z, i - |, -# |, - Y fo, a) [tis easier to pus ht box (having (b) Itis difficult to push a heavy box (having more, less weight) because it presses the floor weight) because it presses the floor harder, with less force and hence friction is less (with greater force) and friction is much more, , Figure 8., put a much larger force has to be applied to make the heavier box, , ba yh r is less friction between the light box and floor but much more, tween the heavy box and the floor (see Figure 8). We know that the weight of a box is also a force, n), Now, because of its smaller weight, the light box presses on the floor, between lighter box and floor is less |see Figure 8(a)]. This lesser force, ily by applying a smaller push. On the other hand, because, “bor an e floor with a greater force. Since the surfaces of heavy box, much =e pressed together harder (with a greater force) the friction between them increases and becomes, Olving ter [see Figure 8(b)]. This greater friction does not allow the heavy box to be moved on floor b', vy, 82 small force. A much larger force of our push has to be applied to make the heavy box move i, , en mal force Amick Be, y |S, , omake the lichter | i ;, ake the lighter box move on the floor t, Move on, , € on the floor. This shows that there, , = - in the dow nward directio, we force and hence the friction, ohh tion allows the lighter box to be moved eas, , ‘seater weight, the heavy box presses on th