NCERT Book - Friction Class 8 Notes | EduRev

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Class 8 : NCERT Book - Friction Class 8 Notes | EduRev

 Page 1


FRICTION
FRICTION
Y
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too,
slow down your bicycle whenever
needed by applying brakes. Have you
ever thought why a vehicle slows
down when brakes are applied? Not
only vehicles, any object, moving over
the surface of another object slows
down when no external force is
applied on it. Finally it stops. Have
you not seen a moving ball on the
ground stopping after some time?
Why do we slip when we step on a
banana peel (Fig. 12.1)? Why is it
difficult to walk on a smooth and wet
floor?
Fig. 12.1 : A boy falls down when he steps on a
banana peel
12.1 Force of Friction
Activity 12.1
Gently push a book on a table [Fig.
12.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 12.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
Fig. 12.2 (a), (b) : Friction opposes relative motion
between the surfaces of the
book and the table
(a)
(b)
You will find the answers to such
questions in this chapter.
Page 2


FRICTION
FRICTION
Y
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too,
slow down your bicycle whenever
needed by applying brakes. Have you
ever thought why a vehicle slows
down when brakes are applied? Not
only vehicles, any object, moving over
the surface of another object slows
down when no external force is
applied on it. Finally it stops. Have
you not seen a moving ball on the
ground stopping after some time?
Why do we slip when we step on a
banana peel (Fig. 12.1)? Why is it
difficult to walk on a smooth and wet
floor?
Fig. 12.1 : A boy falls down when he steps on a
banana peel
12.1 Force of Friction
Activity 12.1
Gently push a book on a table [Fig.
12.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 12.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
Fig. 12.2 (a), (b) : Friction opposes relative motion
between the surfaces of the
book and the table
(a)
(b)
You will find the answers to such
questions in this chapter.
SCIENCE 148
You saw that if you apply the force
along the left, friction acts along the
right. If you apply the force along the
right, the friction acts along the left
direction. In both cases the force opposes
the motion of the book. The force of
friction always opposes the applied force.
In the above activity, the force of
friction acts between the surface of the
book and the surface of the table.
Is the  friction the same for all the
surfaces? Does it depend on the
smoothness of the surfaces? Let us find
out.
12.2 Factors affecting
Friction
Activity 12.2
Tie a string around a brick. Pull the
brick by a spring balance (Fig.
12.3). You need to apply some force.
Note down the reading on the
spring balance when the brick just
begins to move. It gives you a
measure of the of force of friction
between the surface of the brick and
the floor.
for this difference? Repeat this
activity by wrapping a piece of jute
bag around the brick. What do you
observe?
Fig. 12.3 : A brick is being pulled by spring
balance.
Now wrap a piece of polythene
around the brick and repeat the
activity. Do you observe any
difference in the readings of the
spring balance in the above two
cases? What might be the reason
Spring Balance
Spring balance is a device used for
measuring the force acting on an
object. It consists of a coiled spring
which gets stretched when a force is
applied to it. Stretching of the spring
is measured by a pointer moving on
a graduated scale. The reading on the
scale gives the magnitude of the force.
Activity 12.3
Make an inclined plane on a smooth
floor, or on a table. You may use a
wooden board supported by bricks,
or books. [Fig. 12.4 (a)]. Put a mark
with a pen at any point A on the
inclined plane. Now let  a pencil cell
move down from this point. How far
does it move on the table before
coming to rest? Note down the
distance. Now spread a piece of cloth
Page 3


FRICTION
FRICTION
Y
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too,
slow down your bicycle whenever
needed by applying brakes. Have you
ever thought why a vehicle slows
down when brakes are applied? Not
only vehicles, any object, moving over
the surface of another object slows
down when no external force is
applied on it. Finally it stops. Have
you not seen a moving ball on the
ground stopping after some time?
Why do we slip when we step on a
banana peel (Fig. 12.1)? Why is it
difficult to walk on a smooth and wet
floor?
Fig. 12.1 : A boy falls down when he steps on a
banana peel
12.1 Force of Friction
Activity 12.1
Gently push a book on a table [Fig.
12.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 12.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
Fig. 12.2 (a), (b) : Friction opposes relative motion
between the surfaces of the
book and the table
(a)
(b)
You will find the answers to such
questions in this chapter.
SCIENCE 148
You saw that if you apply the force
along the left, friction acts along the
right. If you apply the force along the
right, the friction acts along the left
direction. In both cases the force opposes
the motion of the book. The force of
friction always opposes the applied force.
In the above activity, the force of
friction acts between the surface of the
book and the surface of the table.
Is the  friction the same for all the
surfaces? Does it depend on the
smoothness of the surfaces? Let us find
out.
12.2 Factors affecting
Friction
Activity 12.2
Tie a string around a brick. Pull the
brick by a spring balance (Fig.
12.3). You need to apply some force.
Note down the reading on the
spring balance when the brick just
begins to move. It gives you a
measure of the of force of friction
between the surface of the brick and
the floor.
for this difference? Repeat this
activity by wrapping a piece of jute
bag around the brick. What do you
observe?
Fig. 12.3 : A brick is being pulled by spring
balance.
Now wrap a piece of polythene
around the brick and repeat the
activity. Do you observe any
difference in the readings of the
spring balance in the above two
cases? What might be the reason
Spring Balance
Spring balance is a device used for
measuring the force acting on an
object. It consists of a coiled spring
which gets stretched when a force is
applied to it. Stretching of the spring
is measured by a pointer moving on
a graduated scale. The reading on the
scale gives the magnitude of the force.
Activity 12.3
Make an inclined plane on a smooth
floor, or on a table. You may use a
wooden board supported by bricks,
or books. [Fig. 12.4 (a)]. Put a mark
with a pen at any point A on the
inclined plane. Now let  a pencil cell
move down from this point. How far
does it move on the table before
coming to rest? Note down the
distance. Now spread a piece of cloth
FRICTION 149
Fig. 12.5 : Surface irregularities
over the table. Make sure that there
are no wrinkles in the cloth. Try the
activity again [Fig. 12.4 (b)].
Does the distance covered depend on
the nature of the surface on which the
cell moves?
Could the smoothness of the surface
of the cell also affect the distance
travelled by it?
(a)
(b)
Repeat this activity by spreading
a thin layer of sand over the table.
Maintain the same slope
throughout the activity.
In which case is the distance covered
the minimum? Why is the distance
covered by the pencil cell different every
time. Try to reason why? Discuss the
result.
Fig. 12.4 : The pencil cell covers different
distances on different surfaces
I shall try the activity
by wrapping a piece of
sandpaper around
the cell.
Friction is caused by the
irregularities on the two surfaces in
contact. Even those surfaces which
appear very smooth have a large number
of minute irregularities on them (Fig.
12.5). Irregularities on the two surfaces
lock into one another. When we attempt
to move any surface, we have to apply a
force to overcome interlocking. On
rough surfaces, there are a larger
number of irregularities. So the force of
friction is greater if a rough surface is
involved.
Mark A
Mark A
Page 4


FRICTION
FRICTION
Y
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too,
slow down your bicycle whenever
needed by applying brakes. Have you
ever thought why a vehicle slows
down when brakes are applied? Not
only vehicles, any object, moving over
the surface of another object slows
down when no external force is
applied on it. Finally it stops. Have
you not seen a moving ball on the
ground stopping after some time?
Why do we slip when we step on a
banana peel (Fig. 12.1)? Why is it
difficult to walk on a smooth and wet
floor?
Fig. 12.1 : A boy falls down when he steps on a
banana peel
12.1 Force of Friction
Activity 12.1
Gently push a book on a table [Fig.
12.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 12.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
Fig. 12.2 (a), (b) : Friction opposes relative motion
between the surfaces of the
book and the table
(a)
(b)
You will find the answers to such
questions in this chapter.
SCIENCE 148
You saw that if you apply the force
along the left, friction acts along the
right. If you apply the force along the
right, the friction acts along the left
direction. In both cases the force opposes
the motion of the book. The force of
friction always opposes the applied force.
In the above activity, the force of
friction acts between the surface of the
book and the surface of the table.
Is the  friction the same for all the
surfaces? Does it depend on the
smoothness of the surfaces? Let us find
out.
12.2 Factors affecting
Friction
Activity 12.2
Tie a string around a brick. Pull the
brick by a spring balance (Fig.
12.3). You need to apply some force.
Note down the reading on the
spring balance when the brick just
begins to move. It gives you a
measure of the of force of friction
between the surface of the brick and
the floor.
for this difference? Repeat this
activity by wrapping a piece of jute
bag around the brick. What do you
observe?
Fig. 12.3 : A brick is being pulled by spring
balance.
Now wrap a piece of polythene
around the brick and repeat the
activity. Do you observe any
difference in the readings of the
spring balance in the above two
cases? What might be the reason
Spring Balance
Spring balance is a device used for
measuring the force acting on an
object. It consists of a coiled spring
which gets stretched when a force is
applied to it. Stretching of the spring
is measured by a pointer moving on
a graduated scale. The reading on the
scale gives the magnitude of the force.
Activity 12.3
Make an inclined plane on a smooth
floor, or on a table. You may use a
wooden board supported by bricks,
or books. [Fig. 12.4 (a)]. Put a mark
with a pen at any point A on the
inclined plane. Now let  a pencil cell
move down from this point. How far
does it move on the table before
coming to rest? Note down the
distance. Now spread a piece of cloth
FRICTION 149
Fig. 12.5 : Surface irregularities
over the table. Make sure that there
are no wrinkles in the cloth. Try the
activity again [Fig. 12.4 (b)].
Does the distance covered depend on
the nature of the surface on which the
cell moves?
Could the smoothness of the surface
of the cell also affect the distance
travelled by it?
(a)
(b)
Repeat this activity by spreading
a thin layer of sand over the table.
Maintain the same slope
throughout the activity.
In which case is the distance covered
the minimum? Why is the distance
covered by the pencil cell different every
time. Try to reason why? Discuss the
result.
Fig. 12.4 : The pencil cell covers different
distances on different surfaces
I shall try the activity
by wrapping a piece of
sandpaper around
the cell.
Friction is caused by the
irregularities on the two surfaces in
contact. Even those surfaces which
appear very smooth have a large number
of minute irregularities on them (Fig.
12.5). Irregularities on the two surfaces
lock into one another. When we attempt
to move any surface, we have to apply a
force to overcome interlocking. On
rough surfaces, there are a larger
number of irregularities. So the force of
friction is greater if a rough surface is
involved.
Mark A
Mark A
SCIENCE 150
We see that the friction is caused by
the interlocking of irregularities in the
two surfaces. It is obvious that the force
of friction will increase if the two
surfaces are pressed harder. You can
experience it by dragging a mat when
nobody is sitting on it, and when a
preson is sitting on it.
Recall your experience when last time
you moved a heavy box from one place
to another. If you have no such
experience, get that experience now.
What is easier — to move the box from
rest, or to move it when it is already in
motion?
The force required to overcome
friction at the instant an object starts
moving from rest is a measure of static
friction. On the other hand, the force
required to keep the object moving with
the same speed is a measure of sliding
friction.
friction and you find it somewhat easier
to move the box already in motion than
to get it started.
12.3 Friction : A Necessary
Evil
Recall now some of your experiences. Is
it easier to hold a kulhar (earthen pot)
or a glass tumbler? Suppose the outer
surface of the tumbler is greasy, or has
a film of cooking oil on it; would it
become easier or more difficult to hold
it? Just think : would it be possible to
hold the glass at all if there is no
friction?
Recall also how difficult it is to move
on a wet muddy track, or wet marble
floor. Can you imagine being able to walk
at all if there were no friction?
When the box starts sliding, the
contact points on its surface, do not get
enough time to lock into the contact
points on the floor. So, the sliding
friction is slightly smaller than the static
Fig. 12.7 : A nail is fixed in the wall due to
friction
You could not write with pen or
pencil if there were no friction.  When
your teacher is writing with chalk on
the blackboard, its rough surface rubs
off some chalk particles which stick to
Fig. 12.6 : You have to push on the box to keep it
moving
Page 5


FRICTION
FRICTION
Y
ou might have seen a driver of a
car or a truck slowing down the
vehicle at a traffic signal. You, too,
slow down your bicycle whenever
needed by applying brakes. Have you
ever thought why a vehicle slows
down when brakes are applied? Not
only vehicles, any object, moving over
the surface of another object slows
down when no external force is
applied on it. Finally it stops. Have
you not seen a moving ball on the
ground stopping after some time?
Why do we slip when we step on a
banana peel (Fig. 12.1)? Why is it
difficult to walk on a smooth and wet
floor?
Fig. 12.1 : A boy falls down when he steps on a
banana peel
12.1 Force of Friction
Activity 12.1
Gently push a book on a table [Fig.
12.2(a)]. You observe that it stops
after moving for some distance.
Repeat this activity pushing the
book from the opposite direction
[Fig. 12.2, (b)]. Does the book stop
this time, too? Can you think of an
explanation? Can we say that a force
must be acting on the book
opposing its motion? This force is
called the force of friction.
Fig. 12.2 (a), (b) : Friction opposes relative motion
between the surfaces of the
book and the table
(a)
(b)
You will find the answers to such
questions in this chapter.
SCIENCE 148
You saw that if you apply the force
along the left, friction acts along the
right. If you apply the force along the
right, the friction acts along the left
direction. In both cases the force opposes
the motion of the book. The force of
friction always opposes the applied force.
In the above activity, the force of
friction acts between the surface of the
book and the surface of the table.
Is the  friction the same for all the
surfaces? Does it depend on the
smoothness of the surfaces? Let us find
out.
12.2 Factors affecting
Friction
Activity 12.2
Tie a string around a brick. Pull the
brick by a spring balance (Fig.
12.3). You need to apply some force.
Note down the reading on the
spring balance when the brick just
begins to move. It gives you a
measure of the of force of friction
between the surface of the brick and
the floor.
for this difference? Repeat this
activity by wrapping a piece of jute
bag around the brick. What do you
observe?
Fig. 12.3 : A brick is being pulled by spring
balance.
Now wrap a piece of polythene
around the brick and repeat the
activity. Do you observe any
difference in the readings of the
spring balance in the above two
cases? What might be the reason
Spring Balance
Spring balance is a device used for
measuring the force acting on an
object. It consists of a coiled spring
which gets stretched when a force is
applied to it. Stretching of the spring
is measured by a pointer moving on
a graduated scale. The reading on the
scale gives the magnitude of the force.
Activity 12.3
Make an inclined plane on a smooth
floor, or on a table. You may use a
wooden board supported by bricks,
or books. [Fig. 12.4 (a)]. Put a mark
with a pen at any point A on the
inclined plane. Now let  a pencil cell
move down from this point. How far
does it move on the table before
coming to rest? Note down the
distance. Now spread a piece of cloth
FRICTION 149
Fig. 12.5 : Surface irregularities
over the table. Make sure that there
are no wrinkles in the cloth. Try the
activity again [Fig. 12.4 (b)].
Does the distance covered depend on
the nature of the surface on which the
cell moves?
Could the smoothness of the surface
of the cell also affect the distance
travelled by it?
(a)
(b)
Repeat this activity by spreading
a thin layer of sand over the table.
Maintain the same slope
throughout the activity.
In which case is the distance covered
the minimum? Why is the distance
covered by the pencil cell different every
time. Try to reason why? Discuss the
result.
Fig. 12.4 : The pencil cell covers different
distances on different surfaces
I shall try the activity
by wrapping a piece of
sandpaper around
the cell.
Friction is caused by the
irregularities on the two surfaces in
contact. Even those surfaces which
appear very smooth have a large number
of minute irregularities on them (Fig.
12.5). Irregularities on the two surfaces
lock into one another. When we attempt
to move any surface, we have to apply a
force to overcome interlocking. On
rough surfaces, there are a larger
number of irregularities. So the force of
friction is greater if a rough surface is
involved.
Mark A
Mark A
SCIENCE 150
We see that the friction is caused by
the interlocking of irregularities in the
two surfaces. It is obvious that the force
of friction will increase if the two
surfaces are pressed harder. You can
experience it by dragging a mat when
nobody is sitting on it, and when a
preson is sitting on it.
Recall your experience when last time
you moved a heavy box from one place
to another. If you have no such
experience, get that experience now.
What is easier — to move the box from
rest, or to move it when it is already in
motion?
The force required to overcome
friction at the instant an object starts
moving from rest is a measure of static
friction. On the other hand, the force
required to keep the object moving with
the same speed is a measure of sliding
friction.
friction and you find it somewhat easier
to move the box already in motion than
to get it started.
12.3 Friction : A Necessary
Evil
Recall now some of your experiences. Is
it easier to hold a kulhar (earthen pot)
or a glass tumbler? Suppose the outer
surface of the tumbler is greasy, or has
a film of cooking oil on it; would it
become easier or more difficult to hold
it? Just think : would it be possible to
hold the glass at all if there is no
friction?
Recall also how difficult it is to move
on a wet muddy track, or wet marble
floor. Can you imagine being able to walk
at all if there were no friction?
When the box starts sliding, the
contact points on its surface, do not get
enough time to lock into the contact
points on the floor. So, the sliding
friction is slightly smaller than the static
Fig. 12.7 : A nail is fixed in the wall due to
friction
You could not write with pen or
pencil if there were no friction.  When
your teacher is writing with chalk on
the blackboard, its rough surface rubs
off some chalk particles which stick to
Fig. 12.6 : You have to push on the box to keep it
moving
FRICTION 151
the black board. Could it happen if there
were no friction between the chalk and
the board?
If an object started moving, it would
never stop if there were no friction. Had
there been no friction between the tyres
of the automobiles and the road, they
could not be started or stoped or turned
to change the direction of motion. You
could not fix a nail in the wall (Fig. 12.7)
or tie a knot. Without friction no building
could be constructed.
Fig. 12.8 : Soles of shoes wear out due to
friction
Fig. 12.10 : Stricking a match stick produces fire
by friction
produces heat. In fact, when a machine
is operated, heat generated causes much
wastage of energy. We shall discuss the
ways of minimising friction in the
following section.
Fig. 12.9 : Rubbing of your palms makes you feel
warm
On the other hand, friction is an evil,
too. It wears out the materials whether
they are screws, ball bearings or soles
of shoes (Fig. 12.8). You must have seen
worn-out steps of foot over-bridges at
railway stations.
Friction can also produce heat.
Vigorously rub your palms together for
a few minutes (Fig. 12.9). How do you
feel? When you strike a matchstick
against the rough surface, it catches fire
(Fig. 12.10).
You might have observed that the jar
of a mixer becomes hot when it is run
for a few minutes. You can cite various
other examples in which friction
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