NCERT Textbook - Force & Pressure UPSC Notes | EduRev

General Science(Prelims) by IRS Divey Sethi

Class 8 : NCERT Textbook - Force & Pressure UPSC Notes | EduRev

 Page 1


FORCE AND PRESSURE
FORCE AND PRESSURE
I
n Class VII, you have learnt how
objects move. Do you recall how we
can decide whether an object is
moving faster than the other? What does
the distance moved by an object in unit
time indicate? You also know that a
moving object like a ball rolling on the
ground slows down. Sometimes it may
change its direction of motion. It is also
possible that the ball may slow down
and also change its direction. Have you
ever wondered what makes an object
slow down or go faster, or change its
direction of motion?
Let us recall some of our everyday
experiences. What do you do to make
a football move? What do you do to
make a moving ball move faster? How
does a  goalkeeper stop a ball? A
hockey player changes the direction
of the moving ball with a flick of the
stick. How do fielders stop a ball hit
by a batsman? (Fig. 11.1). In all these
situations the ball is either made to
move faster or slower or its direction
of motion is changed.
We often say that a force has been
applied on a ball when it is kicked,
pushed, thrown or flicked. What is a
force? What can it do to bodies on which
it is applied? We shall seek answers to
such questions in this chapter.
11.1 Force – A Push or a Pull
Actions like picking, opening,
shutting, kicking, hitting, lifting,
flicking, pushing, pulling are often
used to describe certain tasks. Each
of these actions usually results in
some kind of change in the state of
motion of an object.  Can these terms
be replaced with one or more terms?
Let us find out.
(a) (a) (a) (a) (a) (b) (b) (b) (b) (b) (c) (c) (c) (c) (c)
Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : (a) (a) (a) (a) (a) A goal keeper saving a goal, (b) (b) (b) (b) (b) A hockey player flicking a ball, and
(c) (c) (c) (c) (c) A fielder stopping a ball
2020-21
Page 2


FORCE AND PRESSURE
FORCE AND PRESSURE
I
n Class VII, you have learnt how
objects move. Do you recall how we
can decide whether an object is
moving faster than the other? What does
the distance moved by an object in unit
time indicate? You also know that a
moving object like a ball rolling on the
ground slows down. Sometimes it may
change its direction of motion. It is also
possible that the ball may slow down
and also change its direction. Have you
ever wondered what makes an object
slow down or go faster, or change its
direction of motion?
Let us recall some of our everyday
experiences. What do you do to make
a football move? What do you do to
make a moving ball move faster? How
does a  goalkeeper stop a ball? A
hockey player changes the direction
of the moving ball with a flick of the
stick. How do fielders stop a ball hit
by a batsman? (Fig. 11.1). In all these
situations the ball is either made to
move faster or slower or its direction
of motion is changed.
We often say that a force has been
applied on a ball when it is kicked,
pushed, thrown or flicked. What is a
force? What can it do to bodies on which
it is applied? We shall seek answers to
such questions in this chapter.
11.1 Force – A Push or a Pull
Actions like picking, opening,
shutting, kicking, hitting, lifting,
flicking, pushing, pulling are often
used to describe certain tasks. Each
of these actions usually results in
some kind of change in the state of
motion of an object.  Can these terms
be replaced with one or more terms?
Let us find out.
(a) (a) (a) (a) (a) (b) (b) (b) (b) (b) (c) (c) (c) (c) (c)
Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : (a) (a) (a) (a) (a) A goal keeper saving a goal, (b) (b) (b) (b) (b) A hockey player flicking a ball, and
(c) (c) (c) (c) (c) A fielder stopping a ball
2020-21
SCIENCE 128
Activity 11.1
Table 11.1 gives some examples of familiar situations involving motion of objects.
You can add more such situations or replace those given here. Try to identify
action involved in each case as a push and/or a pull and record your observations.
One example has been given to help you.
S.
No
Description of
the situation
Action : (pushing/ pulling/picking/
hitting/lifting/ lowering/flying/
kicking/ throwing/shutting/
flicking )
Action can be
grouped as a
Push Pull
1. Moving a book
placed on a table
2. Opening or
shutting  a door
3. Drawing a bucket
of water from a
well
4. A football player
taking a penalty
kick
5. A cricket ball hit
by a batsman
6. Moving a loaded
cart
7. Opening a
drawer
Pushing Pulling Lifting — Yes Yes
Table 11.1 : Identifying Actions as Push or Pull
I learnt in Class VI that a
magnet attracts a piece of
iron towards it. Is attraction
also a pull?   What about
repulsion between similar
poles of two magnets? Is it a
pull or a push?
Do you notice that each of the actions
can be grouped as a pull or a push or
both? Can we infer from this, that to move
an object, it has to be pushed or pulled?
In science, a push or a pull on an
object is called a force. Thus, we can
say that the motion imparted to objects
was due to the action of a force. When
does a force come into play? Let us
find out.
2020-21
Page 3


FORCE AND PRESSURE
FORCE AND PRESSURE
I
n Class VII, you have learnt how
objects move. Do you recall how we
can decide whether an object is
moving faster than the other? What does
the distance moved by an object in unit
time indicate? You also know that a
moving object like a ball rolling on the
ground slows down. Sometimes it may
change its direction of motion. It is also
possible that the ball may slow down
and also change its direction. Have you
ever wondered what makes an object
slow down or go faster, or change its
direction of motion?
Let us recall some of our everyday
experiences. What do you do to make
a football move? What do you do to
make a moving ball move faster? How
does a  goalkeeper stop a ball? A
hockey player changes the direction
of the moving ball with a flick of the
stick. How do fielders stop a ball hit
by a batsman? (Fig. 11.1). In all these
situations the ball is either made to
move faster or slower or its direction
of motion is changed.
We often say that a force has been
applied on a ball when it is kicked,
pushed, thrown or flicked. What is a
force? What can it do to bodies on which
it is applied? We shall seek answers to
such questions in this chapter.
11.1 Force – A Push or a Pull
Actions like picking, opening,
shutting, kicking, hitting, lifting,
flicking, pushing, pulling are often
used to describe certain tasks. Each
of these actions usually results in
some kind of change in the state of
motion of an object.  Can these terms
be replaced with one or more terms?
Let us find out.
(a) (a) (a) (a) (a) (b) (b) (b) (b) (b) (c) (c) (c) (c) (c)
Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : (a) (a) (a) (a) (a) A goal keeper saving a goal, (b) (b) (b) (b) (b) A hockey player flicking a ball, and
(c) (c) (c) (c) (c) A fielder stopping a ball
2020-21
SCIENCE 128
Activity 11.1
Table 11.1 gives some examples of familiar situations involving motion of objects.
You can add more such situations or replace those given here. Try to identify
action involved in each case as a push and/or a pull and record your observations.
One example has been given to help you.
S.
No
Description of
the situation
Action : (pushing/ pulling/picking/
hitting/lifting/ lowering/flying/
kicking/ throwing/shutting/
flicking )
Action can be
grouped as a
Push Pull
1. Moving a book
placed on a table
2. Opening or
shutting  a door
3. Drawing a bucket
of water from a
well
4. A football player
taking a penalty
kick
5. A cricket ball hit
by a batsman
6. Moving a loaded
cart
7. Opening a
drawer
Pushing Pulling Lifting — Yes Yes
Table 11.1 : Identifying Actions as Push or Pull
I learnt in Class VI that a
magnet attracts a piece of
iron towards it. Is attraction
also a pull?   What about
repulsion between similar
poles of two magnets? Is it a
pull or a push?
Do you notice that each of the actions
can be grouped as a pull or a push or
both? Can we infer from this, that to move
an object, it has to be pushed or pulled?
In science, a push or a pull on an
object is called a force. Thus, we can
say that the motion imparted to objects
was due to the action of a force. When
does a force come into play? Let us
find out.
2020-21
FORCE AND PRESSURE 129
Fig 11.3 (b) : Who is pulling whom ?
11.2 Forces are due to an
Interaction
Suppose a man is standing behind a
stationary car [Fig.11.2(a)]. Will the car
move due to his presence? Suppose the
man now begins to push the car
[Fig.11.2(b)], that is, he applies a force on
it. The car may begin to move in the
Fig. 11.3 shows three situations that
may be familiar to you. Can you decide
who is pulling and who is pushing in
these cases? In Fig. 11.3 (a), both the
girls appear to push each other while
Fig.11.2 (b) : A car being pushed by a man
Fig11.3  (a) : Who is pushing whom?
direction of the applied force. Note that the
man has to push the car to make it move.
Fig.11.2(a) : A man standing behind a stationary car
Fig 11.3 (c) : Who is pulling whom?
the pair of girls in Fig. 11.3 (b) are trying
to pull each other. Similarly, the cow
and the man in Fig. 11. 3(c) appear to
pull each other. The girls in the two
situations shown here are applying force
on each other. Is it also true for the man
and the cow?
From these examples, we can infer
that at least two objects must interact
for a force to come into play. Thus, an
interaction of one object with another
object results in a force between the
two objects.
2020-21
Page 4


FORCE AND PRESSURE
FORCE AND PRESSURE
I
n Class VII, you have learnt how
objects move. Do you recall how we
can decide whether an object is
moving faster than the other? What does
the distance moved by an object in unit
time indicate? You also know that a
moving object like a ball rolling on the
ground slows down. Sometimes it may
change its direction of motion. It is also
possible that the ball may slow down
and also change its direction. Have you
ever wondered what makes an object
slow down or go faster, or change its
direction of motion?
Let us recall some of our everyday
experiences. What do you do to make
a football move? What do you do to
make a moving ball move faster? How
does a  goalkeeper stop a ball? A
hockey player changes the direction
of the moving ball with a flick of the
stick. How do fielders stop a ball hit
by a batsman? (Fig. 11.1). In all these
situations the ball is either made to
move faster or slower or its direction
of motion is changed.
We often say that a force has been
applied on a ball when it is kicked,
pushed, thrown or flicked. What is a
force? What can it do to bodies on which
it is applied? We shall seek answers to
such questions in this chapter.
11.1 Force – A Push or a Pull
Actions like picking, opening,
shutting, kicking, hitting, lifting,
flicking, pushing, pulling are often
used to describe certain tasks. Each
of these actions usually results in
some kind of change in the state of
motion of an object.  Can these terms
be replaced with one or more terms?
Let us find out.
(a) (a) (a) (a) (a) (b) (b) (b) (b) (b) (c) (c) (c) (c) (c)
Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : (a) (a) (a) (a) (a) A goal keeper saving a goal, (b) (b) (b) (b) (b) A hockey player flicking a ball, and
(c) (c) (c) (c) (c) A fielder stopping a ball
2020-21
SCIENCE 128
Activity 11.1
Table 11.1 gives some examples of familiar situations involving motion of objects.
You can add more such situations or replace those given here. Try to identify
action involved in each case as a push and/or a pull and record your observations.
One example has been given to help you.
S.
No
Description of
the situation
Action : (pushing/ pulling/picking/
hitting/lifting/ lowering/flying/
kicking/ throwing/shutting/
flicking )
Action can be
grouped as a
Push Pull
1. Moving a book
placed on a table
2. Opening or
shutting  a door
3. Drawing a bucket
of water from a
well
4. A football player
taking a penalty
kick
5. A cricket ball hit
by a batsman
6. Moving a loaded
cart
7. Opening a
drawer
Pushing Pulling Lifting — Yes Yes
Table 11.1 : Identifying Actions as Push or Pull
I learnt in Class VI that a
magnet attracts a piece of
iron towards it. Is attraction
also a pull?   What about
repulsion between similar
poles of two magnets? Is it a
pull or a push?
Do you notice that each of the actions
can be grouped as a pull or a push or
both? Can we infer from this, that to move
an object, it has to be pushed or pulled?
In science, a push or a pull on an
object is called a force. Thus, we can
say that the motion imparted to objects
was due to the action of a force. When
does a force come into play? Let us
find out.
2020-21
FORCE AND PRESSURE 129
Fig 11.3 (b) : Who is pulling whom ?
11.2 Forces are due to an
Interaction
Suppose a man is standing behind a
stationary car [Fig.11.2(a)]. Will the car
move due to his presence? Suppose the
man now begins to push the car
[Fig.11.2(b)], that is, he applies a force on
it. The car may begin to move in the
Fig. 11.3 shows three situations that
may be familiar to you. Can you decide
who is pulling and who is pushing in
these cases? In Fig. 11.3 (a), both the
girls appear to push each other while
Fig.11.2 (b) : A car being pushed by a man
Fig11.3  (a) : Who is pushing whom?
direction of the applied force. Note that the
man has to push the car to make it move.
Fig.11.2(a) : A man standing behind a stationary car
Fig 11.3 (c) : Who is pulling whom?
the pair of girls in Fig. 11.3 (b) are trying
to pull each other. Similarly, the cow
and the man in Fig. 11. 3(c) appear to
pull each other. The girls in the two
situations shown here are applying force
on each other. Is it also true for the man
and the cow?
From these examples, we can infer
that at least two objects must interact
for a force to come into play. Thus, an
interaction of one object with another
object results in a force between the
two objects.
2020-21
SCIENCE 130
Does it mean that the net
force on an object is zero if the
two forces acting on it in
opposite directions are equal?
rope in their direction. Sometimes the rope
simply does not move. Is it not similar to
the situation shown in Fig. 11.3 (b)? The
team that pulls harder, that is, applies a
larger force, finally wins the game.
What do these examples suggest
about the nature of force?
Forces applied on an object in the
same direction add to one another. Now
recall what happened when you and
your friend pushed the heavy box in the
same direction in Activity 11.2.
If the two forces act in the opposite
directions on an object, the net force acting
on it is the difference between the two
forces. What did you observe in Activity
11.2 when both of you were pushing the
heavy box from opposite directions?
Recall that in the tug-of-war when
two teams pull equally hard, the rope
does not move in any direction.
So, we learn that a force could be larger
or smaller than the other or equal to each
other. The strength of a force is usually
expressed by its magnitude. We have also
to specify the direction in which a force acts.
Also, if the direction or the magnitude of the
applied force changes, its effect also changes.
Activity 11.2
Choose a heavy object like a table
or a box, which you can move only
by pushing hard. Try to push it all
by yourself. Can you move it? Now
ask one of your friends to help you
in pushing it in the same direction
[Fig.11.4(a)]. Is it easier to move it
now? Can you explain why?
Now push the same object, but
ask your friend to push it from the
opposite side [Fig.11.4 (b)]. Does the
object move? If it does, note the
direction in which it moves. Can
you guess which one of you is
applying a larger force?
Fig. 11.4 : Two friends pushing a heavy load
(a) in the same direction, (b) in
opposite direction
(a)
(b)
11.3 Exploring Forces
Let us try to learn more about forces.
Fig. 11.5 : The rope may not move if the two
teams pull at it with equal force
Have you ever seen a game of tug-of
war? In this game two teams pull at a
rope in opposite directions (Fig. 11.5).
Members of both the teams try to pull the
2020-21
Page 5


FORCE AND PRESSURE
FORCE AND PRESSURE
I
n Class VII, you have learnt how
objects move. Do you recall how we
can decide whether an object is
moving faster than the other? What does
the distance moved by an object in unit
time indicate? You also know that a
moving object like a ball rolling on the
ground slows down. Sometimes it may
change its direction of motion. It is also
possible that the ball may slow down
and also change its direction. Have you
ever wondered what makes an object
slow down or go faster, or change its
direction of motion?
Let us recall some of our everyday
experiences. What do you do to make
a football move? What do you do to
make a moving ball move faster? How
does a  goalkeeper stop a ball? A
hockey player changes the direction
of the moving ball with a flick of the
stick. How do fielders stop a ball hit
by a batsman? (Fig. 11.1). In all these
situations the ball is either made to
move faster or slower or its direction
of motion is changed.
We often say that a force has been
applied on a ball when it is kicked,
pushed, thrown or flicked. What is a
force? What can it do to bodies on which
it is applied? We shall seek answers to
such questions in this chapter.
11.1 Force – A Push or a Pull
Actions like picking, opening,
shutting, kicking, hitting, lifting,
flicking, pushing, pulling are often
used to describe certain tasks. Each
of these actions usually results in
some kind of change in the state of
motion of an object.  Can these terms
be replaced with one or more terms?
Let us find out.
(a) (a) (a) (a) (a) (b) (b) (b) (b) (b) (c) (c) (c) (c) (c)
Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : Fig. 11.1 : (a) (a) (a) (a) (a) A goal keeper saving a goal, (b) (b) (b) (b) (b) A hockey player flicking a ball, and
(c) (c) (c) (c) (c) A fielder stopping a ball
2020-21
SCIENCE 128
Activity 11.1
Table 11.1 gives some examples of familiar situations involving motion of objects.
You can add more such situations or replace those given here. Try to identify
action involved in each case as a push and/or a pull and record your observations.
One example has been given to help you.
S.
No
Description of
the situation
Action : (pushing/ pulling/picking/
hitting/lifting/ lowering/flying/
kicking/ throwing/shutting/
flicking )
Action can be
grouped as a
Push Pull
1. Moving a book
placed on a table
2. Opening or
shutting  a door
3. Drawing a bucket
of water from a
well
4. A football player
taking a penalty
kick
5. A cricket ball hit
by a batsman
6. Moving a loaded
cart
7. Opening a
drawer
Pushing Pulling Lifting — Yes Yes
Table 11.1 : Identifying Actions as Push or Pull
I learnt in Class VI that a
magnet attracts a piece of
iron towards it. Is attraction
also a pull?   What about
repulsion between similar
poles of two magnets? Is it a
pull or a push?
Do you notice that each of the actions
can be grouped as a pull or a push or
both? Can we infer from this, that to move
an object, it has to be pushed or pulled?
In science, a push or a pull on an
object is called a force. Thus, we can
say that the motion imparted to objects
was due to the action of a force. When
does a force come into play? Let us
find out.
2020-21
FORCE AND PRESSURE 129
Fig 11.3 (b) : Who is pulling whom ?
11.2 Forces are due to an
Interaction
Suppose a man is standing behind a
stationary car [Fig.11.2(a)]. Will the car
move due to his presence? Suppose the
man now begins to push the car
[Fig.11.2(b)], that is, he applies a force on
it. The car may begin to move in the
Fig. 11.3 shows three situations that
may be familiar to you. Can you decide
who is pulling and who is pushing in
these cases? In Fig. 11.3 (a), both the
girls appear to push each other while
Fig.11.2 (b) : A car being pushed by a man
Fig11.3  (a) : Who is pushing whom?
direction of the applied force. Note that the
man has to push the car to make it move.
Fig.11.2(a) : A man standing behind a stationary car
Fig 11.3 (c) : Who is pulling whom?
the pair of girls in Fig. 11.3 (b) are trying
to pull each other. Similarly, the cow
and the man in Fig. 11. 3(c) appear to
pull each other. The girls in the two
situations shown here are applying force
on each other. Is it also true for the man
and the cow?
From these examples, we can infer
that at least two objects must interact
for a force to come into play. Thus, an
interaction of one object with another
object results in a force between the
two objects.
2020-21
SCIENCE 130
Does it mean that the net
force on an object is zero if the
two forces acting on it in
opposite directions are equal?
rope in their direction. Sometimes the rope
simply does not move. Is it not similar to
the situation shown in Fig. 11.3 (b)? The
team that pulls harder, that is, applies a
larger force, finally wins the game.
What do these examples suggest
about the nature of force?
Forces applied on an object in the
same direction add to one another. Now
recall what happened when you and
your friend pushed the heavy box in the
same direction in Activity 11.2.
If the two forces act in the opposite
directions on an object, the net force acting
on it is the difference between the two
forces. What did you observe in Activity
11.2 when both of you were pushing the
heavy box from opposite directions?
Recall that in the tug-of-war when
two teams pull equally hard, the rope
does not move in any direction.
So, we learn that a force could be larger
or smaller than the other or equal to each
other. The strength of a force is usually
expressed by its magnitude. We have also
to specify the direction in which a force acts.
Also, if the direction or the magnitude of the
applied force changes, its effect also changes.
Activity 11.2
Choose a heavy object like a table
or a box, which you can move only
by pushing hard. Try to push it all
by yourself. Can you move it? Now
ask one of your friends to help you
in pushing it in the same direction
[Fig.11.4(a)]. Is it easier to move it
now? Can you explain why?
Now push the same object, but
ask your friend to push it from the
opposite side [Fig.11.4 (b)]. Does the
object move? If it does, note the
direction in which it moves. Can
you guess which one of you is
applying a larger force?
Fig. 11.4 : Two friends pushing a heavy load
(a) in the same direction, (b) in
opposite direction
(a)
(b)
11.3 Exploring Forces
Let us try to learn more about forces.
Fig. 11.5 : The rope may not move if the two
teams pull at it with equal force
Have you ever seen a game of tug-of
war? In this game two teams pull at a
rope in opposite directions (Fig. 11.5).
Members of both the teams try to pull the
2020-21
FORCE AND PRESSURE 131
In general, more than one force may
be acting on an object. However, the
effect on the object is due to the net
force acting on it.
11.4 A Force can Change the
State of Motion
Let us now find out what happens when
a force acts on an object.
Activity 11.3
Take a rubber ball and place it on a
level surface such as a table top or
a concrete floor. Now, gently push
the ball along the level surface
(Fig. 11.6). Does the ball begin to
move? Push the ball again while it
is still moving. Is there any change
in its speed? Does it increase or
decrease?
Next, place your palm in front
of the moving ball. Remove your
palm as soon as the moving ball
touches it. Does your palm apply a
force on the ball? What happens to
the speed of the ball now? Does it
increase or decrease? What would
happen if you let your palm hold
the moving ball?
Fig. 11.6  : A ball at rest begins to move
when a force is applied on it
I have seen
children competing with one
another in moving a rubber tyre
or a ring by pushing it
(Fig. 11.7).  I now understand
why the speed of the tyre
increases whenever it is
pushed.
Fig. 11.7 : To move a tyre faster it has to be
pushed repeatedly
You might recall similar situations.
For example, while taking a penalty kick
in football, the player applies a force on
the ball. Before being hit, the ball was at
rest and so its speed was zero. The
applied force makes the ball move
towards the goal. Suppose, the
goalkeeper dives or jumps up to save the
goal. By his action the goalkeeper tries
to apply a force on the moving ball. The
force applied by him can stop or deflect
the ball, saving a goal being scored. If
the goalkeeper succeeds in stopping the
ball, its speed decreases to zero.
These observations suggest that a
force applied on an object may change
its speed. If the force applied on the object
is in the direction of its motion, the speed
of the object increases. If the force is
applied in the direction opposite to the
direction of motion, then it results in a
decrease in the speed of the object.
2020-21
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