FRICTIONFRICTION

You might have seen a driver of acar or a truck slowing down thevehicle at a traffic signal. You, too,

slow down your bicycle wheneverneeded by applying brakes. Have youever thought why a vehicle slowsdown when brakes are applied? Notonly vehicles, any object, moving overthe surface of another object slowsdown when no external force isapplied on it. Finally it stops. Haveyou not seen a moving ball on theground stopping after some time?Why do we slip when we step on abanana peel (Fig. 12.1)? Why is itdifficult to walk on a smooth and wetfloor?

Fig. 12.1 : A boy falls down when he steps on abanana peel

12.1 Force of Friction

Activity 12.1

Gently push a book on a table [Fig.12.2(a)]. You observe that it stopsafter moving for some distance.Repeat this activity pushing thebook from the opposite direction[Fig. 12.2, (b)]. Does the book stopthis time, too? Can you think of anexplanation? Can we say that a forcemust be acting on the bookopposing its motion? This force iscalled the force of friction.

Fig. 12.2 (a), (b) : Friction opposes relative motionbetween the surfaces of thebook and the table

(a)

(b)

You will find the answers to suchquestions in this chapter.

SCIENCE148

You saw that if you apply the forcealong the left, friction acts along theright. If you apply the force along theright, the friction acts along the leftdirection. In both cases the force opposesthe motion of the book. The force offriction always opposes the applied force.

In the above activity, the force offriction acts between the surface of thebook and the surface of the table.

Is the friction the same for all thesurfaces? Does it depend on thesmoothness of the surfaces? Let us findout.

12.2 Factors affectingFriction

Activity 12.2

Tie a string around a brick. Pull thebrick by a spring balance (Fig.12.3). You need to apply some force.Note down the reading on thespring balance when the brick justbegins to move. It gives you ameasure of the of force of frictionbetween the surface of the brick andthe floor.

for this difference? Repeat thisactivity by wrapping a piece of jutebag around the brick. What do youobserve?

Fig. 12.3 : A brick is being pulled by springbalance.

Now wrap a piece of polythenearound the brick and repeat theactivity. Do you observe anydifference in the readings of thespring balance in the above twocases? What might be the reason

Spring Balance

Spring balance is a device used formeasuring the force acting on anobject. It consists of a coiled springwhich gets stretched when a force isapplied to it. Stretching of the springis measured by a pointer moving ona graduated scale. The reading on thescale gives the magnitude of the force.

Activity 12.3

Make an inclined plane on a smoothfloor, or on a table. You may use awooden board supported by bricks,or books. [Fig. 12.4 (a)]. Put a markwith a pen at any point A on theinclined plane. Now let a pencil cellmove down from this point. How fardoes it move on the table beforecoming to rest? Note down thedistance. Now spread a piece of cloth

FRICTION 149

Fig. 12.5 : Surface irregularities

over the table. Make sure that thereare no wrinkles in the cloth. Try theactivity again [Fig. 12.4 (b)].

Does the distance covered depend onthe nature of the surface on which thecell moves?

Could the smoothness of the surfaceof the cell also affect the distancetravelled by it?

(a)

(b)

Repeat this activity by spreadinga thin layer of sand over the table.Maintain the same slopethroughout 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. 12.4 : The pencil cell covers differentdistances on different surfaces

I shall try the activityby wrapping a piece of

sandpaper aroundthe cell.

Friction is caused by theirregularities on the two surfaces incontact. Even those surfaces whichappear very smooth have a large numberof minute irregularities on them (Fig.12.5). Irregularities on the two surfaceslock into one another. When we attemptto move any surface, we have to apply aforce to overcome interlocking. Onrough surfaces, there are a largernumber of irregularities. So the force offriction is greater if a rough surface isinvolved.

Mark A

Mark A

SCIENCE150

We 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 — 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 NecessaryEvil

Recall 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 static

Fig. 12.7 : A nail is fixed in the wall due tofriction

You 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 to

Fig. 12.6 : You have to push on the box to keep itmoving

FRICTION 151

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. 12.8 : Soles of shoes wear out due tofriction

Fig. 12.10 : Stricking a match stick produces fireby friction

produces 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 feelwarm

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 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 run

for a few minutes. You can cite variousother examples in which friction

SCIENCE152

12.4 Increasing and ReducingFriction

As 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 reduce

Fig. 12.12 : Powder is sprinkled on the carromboard to reduce friction

friction 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 friction

Fig. 12.13 : Action of lubricant

Lubricant

FRICTION 153

Activity 12.4

Take a few pencils which arecylindrical in shape. Place themparallel to each other on a table.Place a thick book over it (Fig.12.15). Now push the book. Youobserve the pencils rolling as thebook moves. Do you feel it easier tomove the book in this way than toslide it? Do you think thatresistance to the motion of the bookhas been reduced? Have you seenheavy machinary being moved byplacing logs under it?

12.5 Wheels Reduce Friction

You 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.

Can we reduce friction tozero by polishing surfacesor using large amount of

lubricants?

Friction can never beentirely eleminated. No

surface is perfectly smooth.Some irregularities are

always there.

Fig. 12.15 : Motion of the book on rollers

When one body rolls over the surfaceof another body, the resistance to itsmotion is called the rolling friction.Rolling reduces friction. It is alwayseasier to roll than to slide a body overanother. That is the reason it isconvenient to pull the luggages fittedwith rollers. Can you now understandwhy wheel is said to be one of thegreatest inventions of mankind?

Since the rolling friction is smallerthan the sliding friction, sliding isreplaced in most machines by rollingby the use of ball bearings. Commonexamples are the use of ball bearingsbetween hubs and the axles of ceilingfans and bicycles (Fig. 12.16).Fig. 12.14 : Rolling reduces friction

SCIENCE154

12.6 Fluid Friction

You know that air is very light and thin.Yet it exerts frictional force on objectsmoving through it. Similarly, water andother liquids exert force of friction whenobjects move through them. In science,the common name of gases and liquids

is fluids. So we can say that fluids exertforce of friction on objects in motionthrough them.

The frictional force exerted by fluidsis also called drag.

The frictional force on an object in afluid depends on its speed with respectto the fluid. The frictional force alsodepends on the shape of the object andthe nature of the fluid.

It is obvious that when objects movethrough fluids, they have to overcomefriction acting on them. In this processthey lose energy. Efforts are, therefore,made to minimize friction. So, objectsare given special shapes. Where do youthink the scientists get hints for thesespecial shapes? From nature, of course.Birds and fishes have to move about influids all the time. Their bodies musthave evolved to shapes which wouldmake them not to lose much energy inovercoming friction. You read aboutthese shapes in Class VI. Look carefullyat the shape of an aeroplanes (Fig.12.17). Do you find any similarity in itsshape and that of a bird? In fact, allvehicles are designed to have shapeswhich reduce fluid friction.

Fig. 12.17 : Similarity in shapes of an aeroplane and a bird

Fig. 12.16 : Ball bearings reduce friction

FRICTION 155

KEYWORDS

BALL BEARING

DRAG

FLUID FRICTION

FRICTION

INTERLOCKING

LUBRICANTS

ROLLING FRICTION

SLIDING FRICTION

STATIC FRICTION

WHAT YOU HAVE LEARNT

� Friction opposes the relative motion between

two surfaces in contact. It acts on both the

surfaces.

� Friction depends on the nature of surfaces in

contact.

� For a given pair of surfaces friction depends

upon the state of smoothness of those

surfaces.

� Friction depends on how hard the two surfaces

press together.

� Static friction comes into play when we try to

move an object at rest.

� Sliding friction comes with play when an

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 contactwith each other.

(b) Friction depends on the _____________ of surfaces.

SCIENCE156

(c) Friction produces __________.

(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 slidingfrictions 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, newspaperand towel spread on the floor. The force of friction acting on the car ondifferent 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 slidingdown. Show the direction of frictional force acting on it.

5. You spill a bucket of soapy water on a marble floor accidently. Would itmake 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 heavierbox 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.

E X

E R

C I

S E

S

FRICTION 157

Extended Learning — Activities and Projects

1. What role does friction play in the sport of your choice? Collectsome pictures of that sport in action where friction is eithersupporting it or opposing it. Display these pictures with propercaptions on the bulletin board of your classroom.

2. Imagine that friction suddenly vanishes. How would the life beaffected. List ten such situations.

3. Visit a shop which sells sports shoes. Observe the soles of shoesmeant for various sports. Describe your deservations.

4. A toy to play with:

Take an empty match box. Take out its tray. Cut a used refill of a ballpen of the same width as the tray as shown in the figure below. Fixthe refill with two pins on the top of the tray as shown in Fig. 12.18.Make two holes on the opposite sides of the tray. Make sure that theholes are large enough to allow a thread to pass through them easily.Take a thread about a metre long and pass it through the holes asshown. Fix beads at the two ends of the thread so that it does notcome out. Insert the tray in the outer cover of the matchbox.

Suspend the match box by the thread. Leave the thread loose.The match box will start falling down due to gravity. Tighten thethread now and observe what happens.

Explain your observation. Can you relate it to friction?

Fig. 12.18

You can read more on the related topic on the following websites :� http://www.school-for-champions.com/science/friction.htm� http//hyperphysics.phy-astr.gsu.edu/hbase/firct2.html

SCIENCE158

A riddle for you

In some situations, I oppose the motion

in other situations, I facilitate the motion

but, I always oppose

the relative motion

between two moving surfaces.

Put some lubricant, and

I become small there.

Make the moving surfaces rough

I make the movement tough.

I may be—

static, sliding or rolling

but whenever two surfaces are in motion

I am always there,

Tell me who I am!