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1. CHAPTER - 12
FRICTION
CLASS – 8TH
MADE BY – BHAVESH KHANDELWAL
INSTITUTE – NEW WAY COACHING
CLASSES

2. FORCE OF FRICTION
Force of friction is the force which opposes the motion
of an object over a surface.
The force of friction acts between the object and the
surface.
Eg :- A ball rolling on ground gradually slows down
and comes to
rest due to force of friction between the ball and the
ground.
If we stop pedalling a bicycle, it gradually slows down
and comes to a stop due to force of friction between
the wheel and the road.

4. FACTORS AFFECTING FRICTION
Friction depends upon two factors. They are :-
i) Nature of the surfaces in contact. ( The smoothness of
the surfaces).
ii) How hard the surfaces press together.
Friction is less on a smooth surface.

5. Friction is more on a
rough surface.

6. Friction is more if the surfaces are
pressed harder…………….?
Friction is less if the surfaces are
not pressed harder…………..?

7. CAUSE OF FRICTION
Friction is caused due to the interlocking of
irregularities between the two surfaces in contact.
Smooth surfaces have minute irregularities between the
two surfaces.
Rough surfaces have larger irregularities between the
two surfaces.
So force of friction is more if the surfaces are rough.

9. TYPES OF FRICTION
There are three main types of friction. They are static friction,
sliding friction and rolling friction.
a) Static friction is the friction exerted on a object at rest. .
b) Sliding friction is the friction exerted when an object slides over
a
surface.
c) Rolling friction is the friction exerted when an object rolls over
a
surface.
Sliding friction is slightly less than static friction.
Rolling friction is less than sliding friction and static friction.

11. FRICTION IS
SOMETIMES
USEFUL
 Friction is useful for many of our daily activities.
Friction between the feet and ground helps us to walk on the ground.
It is possible to write with a pen or pencil on a paper due to friction
between the pen or pencil and the paper.
It is possible to write on a blackboard due to the friction between the
chalk and black board.

12. Friction between the tyres and the road helps
automobiles to move on roads.
Friction between the bricks helps in the construction of
buildings.

14. FRICTION IS SOMETIMES
UNDESIRABLE
 Soles of shoes wear out due to friction.
 Tyres of bicycles and automobiles wear out due to
friction.
 Steps of staircases and foot over bridges in railway
stations wear out due to friction.
 Knives and razors lose their sharp edges due to friction.

16. INCREASING
FRICTION
Friction can be increased by increasing the roughness of the
surfaces in contact.
Eg :-
1.The soles of shoes are grooved to have a better grip on the floor.
2.The tyres of vehicles are treaded to increase the grip on the road.
3.The break pads of vehicles are rough to stop moving vehicles when the
breaks are applied.
4.Gymnasts apply some coarse substance on their hands for a better
grip.

18. REDUCING FRICTION
FRICTION CAN BE REDUCED BY :-
 Using lubricants like powders or oils and grease.
 Using rollers or wheels.
 Using ball bearings.
 We sprinkle powder on a carrom board to reduce friction.
 Oil or grease is applied between moving parts of machines to
reduce friction.
 Rollers are used in luggage bags to reduce friction.
 Wheels are used in vehicles to reduce friction.
 Ball bearings are used in ceiling fans, bicycles and vehicles
to reduce friction.

20. FLUID FRICTION
Fluid friction is the force of friction exerted by liquids and
gases on objects moving through them.
Fluid friction depends upon :-
1.The speed of the object.
2.Shape of the object.
3.The nature of the fluid.
Eg:-
Birds flying in air have streamlined body to reduce fluid
friction.
Fishes living in water have streamlined body to reduce fluid
friction.
Aeroplanes and spacecrafts have streamlined body to
reduce fluid friction.