CAPSULE NOTE
STANDARD 8
PHYSICS
(CBSE SYLLABUS)
As per Rationalised NCERT & Latest CBSE Syllabus 2023-24
CHAPTER 9
FRICTION
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FORCE OF FRICTION
Friction is the resistance that one surface or object encounters when moving over
another. It acts on both surfaces.
Force of friction always acts in a direction opposite to the direction of motion of
object.
Motion
Force applied
Force of friction
FACTORS AFFECTING FRICTION
Force of friction depends on:
 The nature of two surfaces in contact.
Force of friction is not same for all the surfaces. It depends on the roughness
or smoothness of surfaces in contact.
 When the surfaces are smooth, then the friction is very less.
 When the surfaces are rough, the friction increases.
 The force with which two surfaces are pressed together.
Interlocking of irregularities on the two surfaces in contact causes friction.
That is,
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 Irregularities on the two surfaces lock into one another.
 When we attempt to move any surface, we have to apply a force to
overcome this interlocking.
 On rough surfaces, there are a large number of irregularities.
 The force of friction is greater if a rough surface is involved.
Friction is independent of surface area of two objects.
TYPES OF FRICTION
STATIC FRICTION
The force required to overcome friction at the instant an object starts moving from
rest is called Static friction.
 Static friction is more powerful than other frictions.
 It is the friction exerted when an object is at rest.
Example:
Consider a girl pushes a heavy box, but the box does not move. She then apply more
force on the box and it starts moving. At that instant, the frictional force between the
floor and the box is the static friction.
SLIDING FRICTION
When one body slides over the surface of other, the resistance to its motion is called
Sliding friction.
 The force required to keep the object moving with the same speed is a measure
of sliding friction.
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ROLLING FRICTION
When one body rolls over the surface of another body, the resistance to its motion is
called Rolling friction.
 Rolling reduces friction.
 It is always easier to roll than to slide a body over another.
Example:
 When a wheel rolls over a road, the rolling friction comes into play.
 The direction of rolling friction is opposite to the direction of turning of
wheels.
Sliding friction < Static friction
Rolling friction < Sliding friction
Static friction > Sliding friction > Rolling friction
FRICTION: ADVANTAGES AND DISADVANTAGES
Advantages:
 We can walk easily on the ground due to friction.
 Friction enables us to fix the nail into the wall.
 We can light a matchstick due to friction.
 The brakes slows down or stops, a moving bicycle. This is due to friction.
 We can write with our pen due to friction.
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 If there were no frictions between tyres of vehicles and road, then the wheels
would spin at the same place and the vehicles do not move.
 A belt drives the machines in a factory due to friction.
 Friction enables a person to climb a tree or pole.
 We can hold objects in our hand.
Disadvantages
 Soles of shoes wears away due to friction.
 Worn out steps of staircases.
 Heat produces due to friction.
 The efficiency of machines gets reduced due to friction.
Friction can be reduced by:
 With the help of polishing the surface friction can be reduced.
 By applying lubricants (like grease or oil) to the rubbing surfaces.
Lubricants: The substances which reduce friction are called lubricants.
 Powder is sprinkled on carrom board to reduce friction.
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 Rollers are used in luggage bags and wheels are used in vehicles to
reduce friction.
 Ball bearing:
Ball bearings are designed to make moving parts of machines to roll
over each other. It consists of metal balls fitted in between metal rings.
 In bicycles, motorbike, etc. the ball bearing is inserted between
the hub of wheel and the axle. When the wheel revolves, the balls
of ball bearing roll and reduce friction. Hence the wheel rolls
smoothly over the axle.
Friction can be increased by:
 Friction can be increased by making a surface rough.
 The sole of shoes are made up of grooves to increase friction
 The tyres of the vehicle treaded to increase friction.
 Kabaddi players usually rub their hands with dry soil to increase
friction and to get a better grip on their opponent.
 Gymnasts apply some coarse substance on their hands to increase
friction for better grip.
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FLUID FRICTION
Substance which can flow easily are called fluids. Fluids does not have any definite
shape. Liquids and gases are fluids.
The friction experienced whenever an object moves through a fluid is called fluid
friction.
The frictional force exerted by fluid is also called drag.
Frictional force depends on:
i.
Speed of the object with respect to fluid
Higher the speed of an object moving through a fluid, greater will be the fluid
friction.
ii.
Size of the object
Larger the size of an object moving through a fluid, greater will be the fluid
friction acting on it.
iii.
Shape of the object
The streamline shaped objects face less fluid friction while moving through a
fluid.
Streamlined: A body shape that offers a small resistance to the flow of fluid
around it is called streamlined.
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iv.
Nature of fluid or viscosity
Higher the viscosity of fluid, greater will be the frictional force acting on the
fluid.
Disadvantages of fluid friction
When objects moves through fluids, they loses some of their energy in overcoming
the fluid friction. This decreases their efficiency.
Example:
 When a car is running on the road, it loses some of its energy in overcoming
the friction of air which opposes its motion.
Reducing fluid friction:
The fluid friction can be decreased by giving streamlined shape to the objects
moving through fluids.
 Cars are built with streamlined body shape to reduce air resistance.
 An aeroplane has streamlined shape similar to that of a bird in flight to reduce
air resistance.
 Fishes are streamlined by nature due to which they move easily in the water.
 The objects like speedboats, ships and submarines are built with streamline
shape to reduce the frictional force of water.
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