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Chapter 12 : Friction
You might have seen a driver of a car or a truck slowing down the vehicle at a traffic signal. You, too, slow down your bicycle whenever needed by applying brakes. Have you ever thought why a vehicle slows down when brakes are applied? Not only vehicles, any object, moving over the surface of another object slows down when no external force is applied on it. Finally it stops. Have you not seen a moving ball on the ground stopping after some time? Why do we slip when we step on a banana peel (Fig. 12.1)? Why is it difficult to walk on a smooth and wet floor?
12.1 Force Of Friction
Activity 12.1
Gently push a book on a table [Fig.
12.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 12.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
12.2 Factors Affecting Friction
Activity 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.
Now wrap a piece of polythene
around the brick and repeat the
activity. Do you observe any
difference in the readings of the
spring balance in the above two
cases? What might be the reason
12.3 Friction : A Necessary Evil
Recall now some of your experiences. Is it easier to hold a kulhar (earthen pot) or a glass tumbler? Suppose the outer surface of the tumbler is greasy, or has a film of cooking oil on it; would it become easier or more difficult to hold it? Just think : would it be possible to hold the glass at all if there is no friction?
12.4 Increasing And Reducing Friction
As you have seen in the previous section,
friction is desirable in some situations.
Have you ever thought why the sole
of your shoes is grooved [Fig. 12.11 (a)]?
It is done to provide the shoes better
grip on the floor, so that you can move
safely. Similarly, the treaded tyres of
cars,trucks and bulldozers provide better
grip with the ground.
12.5 Wheels Reduce Friction
Activity 12.4
Take a few pencils which are
cylindrical in shape. Place them
parallel to each other on a table.
Place a thick book over it (Fig.
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
slide it? Do you think that
resistance to the motion of the book
has been reduced? Have you seen
heavy machinary being moved by
placing logs under it?
12.6 Fluid Friction
You know that air is very light and thin. Yet it exerts frictional force on objects moving through it. Similarly, water and other liquids exert force of friction when objects move through them. In science, the common name of gases and liquids is fluids. So we can say that fluids exert force of friction on objects in motion through them.
