NCERT Book Class 8 Science Chapter 9 Friction

FRICTION

Y 9.1 Force of Friction
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too, Activity 9.1
slow down your bicycle whenever Gently push a book on a table
needed by applying brakes. Have you [Fig. 9.2(a)]. You observe that it stops
ever thought why a vehicle slows after moving for some distance.
down when brakes are applied? Not Repeat this activity pushing the
only vehicles, any object, moving over book from the opposite direction
the surface of another object slows [Fig. 9.2, (b)]. Does the book stop
down when no external force is this time, too? Can you think of an
applied on it. Finally it stops. Have explanation? Can we say that a force
you not seen a moving ball on the must be acting on the book
ground stopping after some time? opposing its motion? This force is
Why do we slip when we step on a called the force of friction.
banana peel (Fig. 9.1)? Why is it
difficult to walk on a smooth and
wet floor?

(a)

(b)
Fig. 9.1 : A boy falls down when he steps on a Fig. 9.2 (a), (b) : Friction opposes relative
banana peel motion between the
surfaces of the book and
You will find the answers to such
the table
questions in this chapter.