The document discusses the topic of friction. It defines friction as a force that opposes the motion of moving objects or prevents stationary objects from moving. It notes there are different types of friction including static, sliding, rolling, and fluid friction. The document discusses factors that affect friction such as the nature of the surfaces in contact and the mass of the moving object. It also covers advantages and disadvantages of friction, as well as methods to increase and decrease friction.

1. Friction
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Subject: Science
Grade: VIII
Topic:Friction
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What is Friction?
You have 90 seconds !

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“Friction is a force that slows down moving objects
or prevents stationary objects from moving.”
Or
“Friction is a force that comes into play when two
surfaces come in contact with each other.”
Friction

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Friction
 The force of friction always opposes the motion of an
object.
 It tends to stop a moving object.
 It also tends to prevent a stationary object moving.
 It is measured in Newton.

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Walking
Writing
Skating
Lighting a matchstick
Driving of the vehicle on a surface
Applications of breaks in the vehicle to stop it
Flight of aeroplanes
Drilling a nail into the wall
Sliding on a garden
Examples of friction in daily life:
Can you explain the role of friction in each of these?

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Types of Friction
 Static friction
 Sliding friction
 Rolling friction
 Fluid friction

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Static friction is friction that occurs
when the surfaces in contact are
at rest (not in motion).
Examples:
A book resting on a desk.
A potted plant sitting on a
sidewalk.
Static Friction

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Sliding friction is a friction force
that opposes the direction of
motion of an object that occurs
when solid surfaces slide over
one another.
Examples:
Writing – pencil point and
paper
Combing your hair – surface
of comb and strands of hair
Sliding Friction

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Rolling Friction
Rolling friction is friction that
occurs between surfaces in
motion that are rolling in
which one of the surfaces is a
wheel, roller, or ball.
Examples:
Riding a bike – tires and
ground
Bowling – ball and lane

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Types of Friction
Rolling Friction < Sliding Friction < Static Friction

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Fluid Friction
The force that tries to slow
objects down when they
move through a liquid or a
gas. It's also known as "drag",
or "air resistance".
All gases and liquids are fluids.
Fluid Friction increases as the
speed of the object increases.
An airplane and a swimmer
both experience fluid friction.

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Causes of Friction
What are the causes of friction?
Microscopic view of Contact between two surfaces

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1. The irregularities of a surface: If we move an object which
has an irregular or rough surface on another surface which is
also irregular, the force of friction will be high in this case and
the movement of the object would be restricted.
2. The regularity of a surface or its smoothness: If the surfaces
of either the objects are smooth, the force of friction would
be less and the object would move easily over the surface.
Even smooth surfaces have a certain irregularity.
3. If two surfaces are pressed hard: The force of friction
increases between two surfaces, if they are pressed hard and
hence the movement of the object becomes restricted.
However, if there is no pressure the object can easily move
Causes of Friction

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 Friction depends on the nature of surfaces in contact.
 Friction depends on the mass of the moving object.
Factors that affect the frictional force

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Advantages of Friction

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Advantages of Friction
Frictional force is necessary for various purposes in our
daily lives such as:
 It allows us to walk on the earth surface.
 It allows us to write with a pen on a surface or a paper.
 It allows us to fix a nail in the wall.
 A moving object would never be able to come to the
state of rest without the frictional force.
 It would not be possible to drive any automobiles on the
road without the frictional force.
 It would not be possible to light a matchstick by rubbing
it against the matchbox, if there is no friction.

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Disadvantages of Friction

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Disadvantages of Friction
 Friction causes wastage of energy. This is because any object
that moves, has to overcome the force of friction.
 Frictional force results in wear and tear of objects such as the
moving parts of a machine, the tyres of a vehicle, sole of the
shoes etc.
 It also results in the production of heat. In the case of
machines, the production of heat leads to wastage of energy.
 The frictional force also leads to a decrease in the speed of a
moving object or some time stops it.
 It can lead to noise pollution in certain cases. For instance,
aircrafts produce loud sound due to the resistance of the air.

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Is it beneficial to
reduce or increase
friction? Why or
Why Not?

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Methods to increase Friction
 By making the surfaces rough: Friction can be
increased by increasing the roughness of the
surfaces in contact.
 By making grooves: Tyres of vehicles are
grooved for increasing friction.
 By using brake pads: In bicycles and
automobiles, friction is increased by using
brake pads in the brake system.
 By pressing the two surfaces together: Friction
between two surfaces can also be increased by
pressing them harder towards each other.

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Methods to increase Friction

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Methods to decrease Friction
 By polishing: Polishing a rough surface smoothen it and reduces
friction.
The powder is sprinkled over the carrom board to decrease the
friction between the board’s surface and the striker. In this way,
the surface of the carrom board becomes smooth.
 Grease is used in bicycles and other motors or different parts of a
machine to reduce friction and increase their efficiency.
 Oil is applied on the hinges of the door so that they can move
easily.

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By lubrication:
Applying substances like oil, grease or powder allows in
smooth movement as they block the irregularities of a surface.
The substances that can reduce the amount of Friction
between different objects are called lubricants.

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By using wheels and ball bearings – rolling friction
Rolling Friction is also used in the parts of machines that are moving continuously.
In this way, the sliding Friction is removed with the help of rolling friction and
therefore the adverse effects of friction such as the production of heat or wear and
tear of the mechanical parts are also reduced.
For instance, ball bearings are being used in fans and bicycles to reduce friction.

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By streamlining to reduce fluid friction: Fluids such as water
and air also exert friction on objects moving from them. This
frictional force that fluids exert on objects is called drag.
Friction is minimum for streamlined shape, which is rounded in
the front and narrows at the back.

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Choose the correct option:
1. Friction always:
a. helps the motion
b. opposes the motion
2. Which one of these characteristics does a smooth surface has?
a. Less frictional force
b. More frictional force
3. Friction is a:
a. Contact force
b. Non-contact force
4.Fluid are____
a. both solids and gases
b. both gases and liquids
5. Lubricants ________
a. Increase friction
b. Reduce friction
QUIZ

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QUIZ Answers
1.b
2.a
3.a
4.b
5.b