CLASS VIII

CHAPTER 12

“FRICTION”
 Definition of Friction
Friction is defined as the force which opposes the
motion of an object over a surface.
 Frictional force arises when two objects are in
contact with each other.
 Friction always works in a direction opposite to
the direction in which the object is moving or
trying to move.
 E.g. Moving of a car on the road and then
applying brake to slow down the speed
 Factors affecting friction
There are three factors that can directly affect
friction between two objects or surfaces.

 Nature of area/surface
 Weight of the bodies
 Area of contact
 (a) Nature of the area
Riding your bicycle/bike on the road is much
easier than riding over the sand or over snow.
It is due to the nature of surface, i.e. The
surface of road is a little bit smoother than the
surface of sand/snow. This is how the nature
of the surface influences the frictional force.
 (b) Weight of the objects
It is easy to ride your bicycle without any load.
But when you take your friend for a ride or
when ride your bicycle with a load, it becomes
difficult for you to pedal. This is because as
weight increases, the friction also increases.
Since friction opposes the motion of the
object, it becomes difficult for us to pedal or
we have to apply more force.
 (c) Area of Contact
If the area of contact is bigger, the friction will
be more. Whereas, if the area of contact is
less/smaller, the friction will be comparatively
lesser.
Microscopic view of interlocking of
irregularities
 Causes of Friction
Friction is caused by the interlocking of irregularities
present in the surfaces of two objects which are in
contact with each other.
To move one object over the other, we have to apply a
force to overcome the interlocking of the
irregularities/irregular projections in their surfaces.
More the roughness of a surface, larger is the number
of minute irregularities on the surface and greater
will be the force of friction.
 Types of Friction
 Static friction
 Kinetic friction
 Sliding friction
 Rolling friction
Static Friction
 Definition of Static friction
The friction acting between two surfaces in
contact when one object just tend to move or
slip over the surface of the other object is
called “Static friction”.
In this case, the object just tends to start
moving, but it does not actually move.
Eg. If you push a table gently, it will not move.
 Kinetic Friction
The frictional force acting between two surfaces
in contact when there is a relative motion
between them is called “Kinetic friction”.
 Sliding Friction
The friction acting between two surfaces in
contact when one object slides over the
surface of another object is called “Sliding
friction”.
The sliding friction is smaller than the static
friction.
Since sliding friction is smaller than the
static friction, it is easier to keep an
object moving which is already in motion
than to move the same object from rest.
 Rolling Friction
The friction acting between two surfaces in
contact when one object rolls over the surface
of other object is called “Rolling friction”.
It is always easier to roll an object than to slide
over another object. So rolling friction is much
less than the sliding friction.
 Since rolling friction is much less, it is very
easy and convenient :
 To pull heavy luggage fitted with rollers
 Heavy machines/objects can be easily moved
from one place to another by placing round
logs of wood under them and then pushing
with the force of hands.
 Advantages of Friction
 Due to friction, we can walk on the floor.
 Due to friction, we can write.
 Due to friction, we are able to hold objects i.e.
books, copies etc. in our hands. Without friction,
the objects would slip off from our hands.
 Due to friction produced between the brake
pad and the brake disc when we apply brakes,
we can reduce the speed of the vehicle (car,
bike, scooty, bus,truck etc.) and MANY
MORE........
 Disadvantages of Friction
 It can cause wear and tear of the tyres of the
vehicles. The tyres of vehicles wear out
gradually due to friction.
 It also can cause wear and tear of machines.
 It reduces the life expectancy of the moving
parts of vehicles.
 Friction produces heat which may damage
machines thereby it reduces the efficiency of
machines.
 Friction is a necessary evil......
What do you think about this?
 Methods to minimize friction......
 Friction can be reduced by making the surface
smooth.
 Friction can be reduced by applying lubricants
such as castor oil, grease, graphite power,
boric powder etc. on the surface.
 Friction can be also reduced by using wheels
to move objects.
 Friction can be reduced by using ball bearings
between the moving parts of machines,
vehicles...
 Friction can also be reduced by making the
body structure of some vehicles streamlined.