NCERT Textbook - Force & Pressure UPSC Notes | EduRev

General Science(Prelims) by IRS Divey Sethi

Created by: Divey Sethi

UPSC : 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. Did you
ever wonder what makes an object to
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? How
do fielders stop a ball hit by a
batsman? A hockey player changes
the direction of the moving ball with
a flick of the stick (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 motion of
an object.  Can these terms be
replaced with one or more terms? Let
us find out.
(a) (b) (c)
Fig. 11.1 : (a) A goal keeper saving a goal (b) A hockey player flicking a ball
(c) A fielder stopping a ball
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. Did you
ever wonder what makes an object to
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? How
do fielders stop a ball hit by a
batsman? A hockey player changes
the direction of the moving ball with
a flick of the stick (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 motion of
an object.  Can these terms be
replaced with one or more terms? Let
us find out.
(a) (b) (c)
Fig. 11.1 : (a) A goal keeper saving a goal (b) A hockey player flicking a ball
(c) A fielder stopping a ball
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.
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. Did you
ever wonder what makes an object to
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? How
do fielders stop a ball hit by a
batsman? A hockey player changes
the direction of the moving ball with
a flick of the stick (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 motion of
an object.  Can these terms be
replaced with one or more terms? Let
us find out.
(a) (b) (c)
Fig. 11.1 : (a) A goal keeper saving a goal (b) A hockey player flicking a ball
(c) A fielder stopping a ball
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.
FORCE AND PRESSURE 129
11.2 Forces are due to an
Interaction
Suppose a man is standing behind a
stationary car (Fig.11.2). Will the car
move due to his presence? Suppose the
man now begins to push the car, that
is, he applies a force on it. The car may
begin to move in the direction of the
applied force. Note that the man has to
push the car to make it move.
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
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
Fig 11.3 (c) : Who is pulling whom?
Fig.11.2 : A car being pushed by a man
Fig11.3  (a) : Who is pushing whom? Fig 11.3 (b) : Who is pulling whom ?
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. Did you
ever wonder what makes an object to
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? How
do fielders stop a ball hit by a
batsman? A hockey player changes
the direction of the moving ball with
a flick of the stick (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 motion of
an object.  Can these terms be
replaced with one or more terms? Let
us find out.
(a) (b) (c)
Fig. 11.1 : (a) A goal keeper saving a goal (b) A hockey player flicking a ball
(c) A fielder stopping a ball
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.
FORCE AND PRESSURE 129
11.2 Forces are due to an
Interaction
Suppose a man is standing behind a
stationary car (Fig.11.2). Will the car
move due to his presence? Suppose the
man now begins to push the car, that
is, he applies a force on it. The car may
begin to move in the direction of the
applied force. Note that the man has to
push the car to make it move.
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
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
Fig 11.3 (c) : Who is pulling whom?
Fig.11.2 : A car being pushed by a man
Fig11.3  (a) : Who is pushing whom? Fig 11.3 (b) : Who is pulling whom ?
SCIENCE 130
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.
11.3 Exploring Forces
Let us try to learn more about forces.
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 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.
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?
Next 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
Fig. 11.5 : The rope may not move if the two teams pull at it with equal force
(a)
(b)
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. Did you
ever wonder what makes an object to
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? How
do fielders stop a ball hit by a
batsman? A hockey player changes
the direction of the moving ball with
a flick of the stick (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 motion of
an object.  Can these terms be
replaced with one or more terms? Let
us find out.
(a) (b) (c)
Fig. 11.1 : (a) A goal keeper saving a goal (b) A hockey player flicking a ball
(c) A fielder stopping a ball
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.
FORCE AND PRESSURE 129
11.2 Forces are due to an
Interaction
Suppose a man is standing behind a
stationary car (Fig.11.2). Will the car
move due to his presence? Suppose the
man now begins to push the car, that
is, he applies a force on it. The car may
begin to move in the direction of the
applied force. Note that the man has to
push the car to make it move.
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
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
Fig 11.3 (c) : Who is pulling whom?
Fig.11.2 : A car being pushed by a man
Fig11.3  (a) : Who is pushing whom? Fig 11.3 (b) : Who is pulling whom ?
SCIENCE 130
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.
11.3 Exploring Forces
Let us try to learn more about forces.
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 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.
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?
Next 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
Fig. 11.5 : The rope may not move if the two teams pull at it with equal force
(a)
(b)
FORCE AND PRESSURE 131
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?
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?
Does it mean that the net
force on an object is zero if the
two forces acting on it in
opposite directions are equal?
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. 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.
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.
Fig. 11.6  : A ball at rest begins to move
when a force is applied on it
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
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