NCERT Solutions Class 8 Science Chapter 5 Exploring Forces

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 8 · SCIENCE

NCERT Solutions

Chapter 5: Exploring Forces

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

62 – 79 27 67 English

Solutions, notes, sample papers & more at 60 pages

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 5: Exploring Forces
Complete NCERT Solutions for Class 8 Science Chapter 5 Exploring Forces from the NCERT textbook Curiosity.
Every question the chapter asks is answered — the Probe and ponder openers, all thirteen Activities (5.1 to
5.13), the A step further boxes, every in-text question from pages 62 to 76, all 10 questions of Keep the
curiosity alive and the Discover, design, and debate tasks — with the forces named, their directions stated
and the reasoning worked out in full.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 62 – 79

SECTIONS QUESTIONS

27 67

MEDIUM

English

Probe and ponder — Page 62
Chapter opener — the questions Sonali and Ragini's ride raises

PROBE AND PONDER

Q1 Why does it feel harder to pedal a bicycle when going uphill than on flat ground?

Because on a slope you are pedalling against the pull of the Earth, and on flat ground you are
not.
On level ground the Earth pulls the cycle and you straight down, into the road. That pull does
not oppose your forward motion at all — the only forces your muscles have to overcome are the
friction of the road and tyres and the friction of the air. On a hill, every turn of the pedal also
lifts you and the cycle a little higher, so now your muscular force has to work against gravity as
well. The same speed therefore needs a much bigger push on the pedals, and that is what feels
hard.

Why it happens: a force is needed to move an object; the steeper the road, the
larger the share of the Earth's pull that acts backwards along the slope and opposes
you. Add the fact that hill roads are often rough — greater friction — and pedalling
becomes harder still.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Check it yourself: notice what the story says next. Coming down the slope the girls
stop pedalling and still speed up. The same gravitational force that opposed them
going up now acts along their motion, so no muscular force is needed at all.

Q2 Why is it easier to slip on a wet surface?

Because water gets into the tiny irregularities of the two surfaces and keeps them slightly apart,
so the force of friction between your foot and the floor becomes very small.
Even a floor that looks perfectly smooth has a large number of minute bumps and hollows (Fig.
5.6), and so does the sole of your shoe. When they are pressed together these irregularities lock
into one another and oppose sliding — that opposition is friction, and it is what lets your foot
grip the floor and push backwards while you walk. A film of water fills the hollows and floats the
two surfaces apart. The irregularities can no longer interlock, friction drops sharply, your foot
slides instead of gripping, and you slip.

Did you know? This is exactly why bathroom tiles are made with a rough, grooved
surface and why tyres have a tread pattern cut into them — the grooves let water
escape so that the raised parts can still touch the road and provide friction.

Q3 Why do we feel ‘light’ or like we are ‘floating’ just after our swing reaches its
highest point and begins to come down?

Because at that moment you and the seat are falling together, so the seat stops pressing up on
you as hard as it usually does — and it is that push of the seat, not the Earth's pull, that we
actually feel.
Sitting still on a swing, the Earth pulls you down and the seat pushes you up by an equal
amount; the two balance and you feel your normal weight through the seat. At the highest
point the swing stops for an instant and then starts moving down. While it is moving down, the
seat is dropping away beneath you and its upward push on you becomes much smaller. Less
push under you is felt as lightness — the same sensation as in a lift that has just started going
down.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Why it happens — and what does not change: your mass is the same, and the
Earth's pull on you (your weight) is the same as always. Only the contact force from
the seat has changed. Feeling light is not the same thing as weighing less.

Q4 Share your questions

Frame questions that begin with why, how, what if or where — questions whose answer needs
a force to be named and its direction stated. Write them in your notebook and take them to your
class.
Sample questions:

Why does a cyclist have to pedal harder against the wind, when the wind is not touching the
cycle's wheels?
If friction always slows things down, how are we able to walk at all?
What would happen to the reading of a spring balance if the same object were weighed at
the top of a very high mountain?
A magnet attracts an iron nail without touching it. Does the nail also pull the magnet?
Why does a heavy iron ship float while a small iron nail sinks?

Tip: the best questions in this chapter are the ones you can test. Two of the five
above can be checked with a spring balance, a magnet and a bucket of water in your
own classroom.

Activity 5.1: Let us explore — Page 63
5.1 What Is a Force?

ACTIVITY

Q1 Take a large cardboard box. Try moving the box in as many different ways as you
can think of.

Every way you can think of turns out to be either a push or a pull.

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Class 8 Science Chapter 5 Exploring Forces
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WHAT YOU DO TO THE BOX PUSH OR WHICH TWO OBJECTS

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PULL? INTERACT

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Press it with both hands so that it slides along Push Your hands and the box

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Tie a rope to it and drag it towards you Pull The rope and the box

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Lift it up and carry it across the room Your arms and the box

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object is what we call a force in science.

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Did you move the box in any other way than shown in Fig. 5.1?
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Yes — many other ways are possible. Fig. 5.1 shows only three of them: (a) pushing, (b) pulling,

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Other ways include rolling the box over end by end, dragging it with a rope tied to one side,

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sliding it on a piece of cloth, sweeping it along with a stick, or kicking it.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

But whichever way you choose, you did only one of two things to the box: you pushed it or you
pulled it. Lifting is simply a pull in the upward direction. That is the point of the activity — no
matter how many methods you invent, they all reduce to a push or a pull, and the push or pull
applied on an object is called force.

Activity 5.2: Let us analyse — Page 64
5.2 What Can a Force Do to the Objects on Which It Is Applied? · Table 5.1

ACTIVITY

Q1 Think of situations where a force (push or pull) is applied and list them in Table 5.1.
Analyse each situation and write the effect of the force in Table 5.1. Some situations
and their effects are already listed for you.

Table 5.1: Different actions and their effects

S.NO. ACTION PUSH/PULL EFFECT

1. Your friend holding your moving Pull Stopping or decreasing the
bicycle from behind to stop it speed of the bicycle

2. Hitting a moving ball with a bat Push Changing the direction of a
moving ball

3. Pressing an inflated balloon Push Change in shape of the
balloon

……………… ……………… ………………

Here is Table 5.1 completed. The first three rows are the ones the book has already filled in; the
rest are situations from everyday life.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

S.NO. ACTION PUSH / PULL EFFECT

1. Your friend holding your moving Pull Stopping or decreasing the
bicycle from behind to stop it speed of the bicycle

2. Hitting a moving ball with a bat Push Changing the direction of a
moving ball

3. Pressing an inflated balloon Push Change in shape of the
balloon

4. Kicking a football lying on the ground Push Makes the ball move from
rest

5. Opening a drawer Pull The drawer moves from rest

6. Stretching a rubber band Pull Change in shape (it becomes
longer and thinner)

7. Rolling a chapati with a belan Push Change in shape of the dough

8. Turning the steering handle of an Push with one hand, Change in the direction of
autorickshaw pull with the other motion

9. Applying brakes on a moving bicycle Push on the brake lever Speed decreases and the cycle
finally stops

10. A fielder stopping a fast-moving ball Push (his palms push Speed of the ball becomes
back on the ball) zero

Tip: when you fill this table yourself, first ask in which direction is the force applied? If
it is away from the person, it is a push; if it is towards the person, it is a pull.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Q2 What do you conclude from these examples? Does a force cause a moving object to
stop? Can it change speed, or direction of motion, or change the shape of an object?

Table 5.1: Different actions and their effects

S.NO. ACTION PUSH/PULL EFFECT

1. Your friend holding your moving Pull Stopping or decreasing the
bicycle from behind to stop it speed of the bicycle

2. Hitting a moving ball with a bat Push Changing the direction of a
moving ball

3. Pressing an inflated balloon Push Change in shape of the
balloon

……………… ……………… ………………

Yes to all of them. The examples show that a force applied on an object may

make an object move from rest (row 4, 5);
change the speed of an object if it is moving — including bringing it to a stop, which is
simply a change of speed to zero (rows 1, 9, 10);
change the direction of motion of an object (rows 2, 8);
bring about a change in the shape of an object (rows 3, 6, 7);
cause some or all of these effects together.

Why it happens: the conclusion runs the other way too, and that is the more useful
half of it — wherever you see a change in speed, in direction, or in shape, a force
must be acting. Nothing on this list happens on its own.

In-text Questions — Page 65
5.2 What Can a Force Do … · 5.3 Are Forces an Interaction Between Two or More Objects?

Q1 What effect can the application of force have on objects?

The force applied on an object may

make an object move from rest — a football lying still starts moving when it is kicked;

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

change the speed of an object if it is moving — brakes slow a cycle down, pedalling speeds
it up;
change the direction of motion of an object — a bat turns a moving ball back;
bring about a change in the shape of an object — a balloon flattens when pressed, dough
spreads under a belan;
cause some or all of these effects at once — a fielder's hands both slow a ball and turn it
aside.

Q2 Does this mean that whenever there is a change in speed or direction, or change in
shape, a force is acting on the object?

Yes — none of these take place without the action of a force.
So if you see a moving object speed up, slow down, stop, turn, or get squeezed or stretched, you
can say with certainty that some force is acting on it, even if you cannot see what is applying it.
The rolling ball that stops on its own is the standard example: something must be acting on it,
and in the next section we find that it is friction.

Careful: the statement does not work backwards. If nothing is changing, it does not
follow that no force acts — the forces on the object may simply be balancing one
another (see the A step further box on the same page).

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Class 8 Science Chapter 5 Exploring Forces
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A step further — Page 65

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

5.3 Are Forces an Interaction Between Two or More Objects?

A STEP FURTHER

Q1 Suppose an object is at rest. Does it mean that no force is acting on this object?

No. It means that the forces acting on the object are balancing one another.
Take a book lying on your table. The Earth pulls it downwards — that pull is its weight, and it
certainly has not switched off. At the same time the table pushes the book upwards by exactly
as much. The two forces act on the same book in opposite directions and cancel out, so the
book does not move. Being at rest tells you the forces are balanced; it does not tell you there
are none.

Did you know? The same idea explains a tug-of-war in which neither team moves —
both are pulling hard, but equally hard and in opposite directions. You will study
balanced and unbalanced forces properly in higher grades.

A step further — Page 66
5.4 What Are the Different Types of Forces? · 5.4.1 Contact forces

A STEP FURTHER

Q1 When you pushed the table with your hand, did you feel a force on your hand too?

Yes — your palm feels a push from the table for exactly as long as you push the table, and the
moment you stop pushing, the force on your hand disappears.
This follows directly from the definition of a force. A force is the result of an interaction
between two objects, and an interaction cannot be one-sided. Whenever two objects interact,
each object experiences a force from the other. As soon as the interaction ceases, the two
objects no longer experience the force.

Try this: press your palm against a wall and then against a soft sponge. The harder
you press, the harder each pushes back on you — you feel it in your palm. Push a
friend while both of you stand on skates and watch both of you move apart.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

In-text Questions — Page 67
5.4.1 Contact forces — Friction

Q1 Is there any other contact force?

Yes — the force of friction. Muscular force is not the only contact force.
Friction acts between the two surfaces that are touching each other whenever one moves, or
tries to move, over the other. It is a contact force because it arises only from two surfaces in
contact — no touching surfaces, no friction. A ball rolling on the ground, a cycle coasting on a
level road and a lunch box pushed across a table all slow down and stop because of it.

Q2 A ball rolling on a flat ground stops on its own after some time … What causes the
change in the speed of objects in such situations? … Is it possible that some force is
indeed acting on them? Which force is that?

Yes, a force is certainly acting — the force of friction, and it acts in the direction opposite to
the motion.
We already know that a force is essential to change the speed of an object. The ball and the
coasting bicycle both lose speed, so a force must be acting on them, even though nobody
appears to be touching them. That force acts between the surfaces in contact — the ball and the
ground, the tyre and the road — and it acts backwards, against the motion. That is why the
speed falls steadily until the object comes to rest.

Why it happens: notice the clue the book gives. On a rough road the cycle stops
sooner than on a smooth one. Whatever this force is, it depends on the nature of the
surfaces in contact — which is exactly what you would expect of a force born at the
meeting of two surfaces.

Activity 5.3: Let us investigate — Page 67

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

5.4.1 Contact forces — Friction

ACTIVITY

Q1 Gently push it and observe (Fig. 5.5). Does it stop after travelling some distance? Is
there a force acting on it which brings it to rest?

Fig. 5.5, page 68 — redrawn sketch: a hand gives the closed lunch box a gentle push
along the table top.

Yes on both counts. The lunch box slides a certain distance and comes to rest, and yes, a force
brings it to rest — the force of friction between the base of the box and the table.
Your hand pushed the box only at the start; after that nothing is touching it except the table. Yet
its speed keeps falling. Since speed cannot change without a force, a force must be acting, and
it must act opposite to the direction in which the box is sliding — a backward force is the
only kind that can reduce speed.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

direction of motion
lunch box

force of friction
table top — the two surfaces in contact

Friction acts along the surfaces in contact, in the direction opposite to the motion, and brings the box
to rest.

Why it happens: even a table top that feels smooth carries a large number of
minute irregularities, and so does the base of the box (Fig. 5.6). Pressed together,
these irregularities lock into each other and oppose any effort to move one surface
over the other.

Q2 Now repeat by pushing the object in the opposite direction. Does it stop again after
travelling some distance?

Yes, it stops again — and after roughly the same distance, because it is sliding on the same
pair of surfaces.
This is the useful half of the activity. Friction did not act towards a fixed side of the room; when
you reversed the motion, friction reversed too. This tells us that friction has no direction of its
own — it always acts opposite to the direction in which the object is moving or trying to move.

Check it yourself: now push the box very gently, so gently that it does not move at
all. Friction is still acting — this time opposing your attempt to move it. That is why
the definition says moves or tries to move.

Activity 5.4: Let us explore — Page 68

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Class 8 Science Chapter 5 Exploring Forces
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5.4.1 Contact forces — Friction depends on the nature of the surfaces
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ACTIVITY

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No. Give the same object the same gentle push,
distance before stopping.
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Nothing about the object has changed — it is the same box with the same base. Only the
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SURFACE HOW FAR THE OBJECT WHAT THIS TELLS US ABOUT

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Glass, ceramic Longest distance Very smooth — least friction

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Sand Stops almost at once Roughest — greatest friction

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In-text Questions — Page

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

5.4.1 Contact forces — irregularities of the surfaces

Q1 Does this mean that the force of friction will be greater if the surfaces are rough?

Yes. Friction is greater on rough surfaces, and Activity 5.4 shows it: on sand the box stops
almost at once, on glass it slides far.

Why it happens: friction arises from the irregularities of the two surfaces in contact.
Even surfaces that appear smooth have a large number of minute bumps and
hollows (Fig. 5.6). When two surfaces are placed in contact, these irregularities lock
into each other and oppose any effort to move one surface over the other. A rough
surface has bigger and deeper irregularities, so more of them interlock and more
firmly — the opposition to sliding, that is the friction, is therefore greater.

Did you know? This is why the sole of a new sports shoe is deeply patterned and
why a well-worn one slips. It is also why we sprinkle sand on a wet slope and why
carrom players smooth the board with boric powder — to increase friction where we
want a grip, and reduce it where we do not.

A step further — Page 68
5.4.1 Contact forces — friction in liquids and gases

A STEP FURTHER

Q1 Does the force of friction act only if the objects are moving on solid surfaces? What
about objects moving through liquids and gases?

No, friction is not limited to solid surfaces. Air, water and other liquids also exert a force of
friction on the objects moving through them.
Anything moving through air or water has to push that air or water out of its way, and the air or
water pushes back — opposite to the motion, exactly as friction does between two solid
surfaces. You feel it yourself when you cycle fast into the wind, or when you try to run in waist-
deep water at a pond.
Because this friction wastes fuel and limits speed, aeroplanes, ships, boats and high-speed
trains are designed with specific shapes — narrow and tapering — so that the air or water
flows past them smoothly and the force of friction is reduced.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Did you know? Fish and birds have the same tapering shape, and for the same
reason. Their bodies have been shaped by nature for the very problem engineers
solve when they design a train nose.

In-text Questions — Page 69
5.4.2 Non-contact forces — Magnetic force

Q1 Is it essential for an object applying force on another object to always be in contact
with it?

No, it is not. There are forces whose effect can be experienced even if the objects are not in
contact. These are called non-contact forces.
Muscular force and friction need touching — they are contact forces. But a magnet pulls an iron
nail across a gap, a rubbed comb lifts bits of paper without touching them, and the Earth pulls a
falling mango although nothing joins the two. The three non-contact forces studied in this
chapter are the magnetic force, the electrostatic force and the gravitational force.

Q2 Can you recall that a magnet could exert force on another magnet or a magnetic
material without being in contact with it?

Yes. We learnt this in the chapter ‘Exploring Magnets’ in Curiosity, Grade 6.

A magnet attracts objects made of magnetic materials, such as iron pins, without
touching them.
When two magnets are brought close, like poles (North–North, South–South) repel each
other and unlike poles (North–South) attract each other.
In an earlier chapter of this book we also learnt about electromagnets, which behave like
magnets.

Attraction and repulsion are themselves a pull and a push — that is, a force. And the gap
between the two magnets shows that this force does not need contact.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Q3 Are there more such forces which act from a distance?

Yes — two more are studied in this chapter.

NON-CONTACT EXERTED BY ATTRACTIVE OR REPULSIVE?
FORCE

Magnetic force A magnet, on another magnet or on a Either — like poles repel, unlike poles
magnetic material attract

Electrostatic force A charged body, on another charged or Either — like charges repel, unlike
uncharged body charges attract

Gravitational force The Earth (and every other planet), on Always attractive
every object

Activity 5.5: Let us test — Page 69
5.4.2 Non-contact forces — Magnetic force (Fig. 5.7)

ACTIVITY

Q1 Now insert the second ring magnet above it such that the like poles of the two
magnets face each other. Does the second magnet stay floating above the first
magnet?

Yes. The upper ring magnet stays floating in the air, a little above the lower one, with only the
wooden stick passing through it.
The Earth pulls the upper magnet downwards, so something must be pushing it up by exactly as
much, or it would come down. Nothing is touching it. That upward push is the magnetic force of
repulsion between the two like poles that face each other, and it is acting across a gap.

Q2 Try pushing the second magnet down gently. Do you feel a force on it?

Yes — you feel a springy push resisting your finger, and the closer you press the upper magnet
to the lower one, the harder that push becomes.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

The moment you let go, the magnet rises back to where it was floating. This shows two things
clearly: the repulsion between like poles is a real force, and its strength increases as the
magnets come nearer.

Q3 Now, reverse the poles of both the magnets. Does the second magnet still remain
floating?

No. With the poles reversed, unlike poles now face each other, so the two magnets attract
instead of repelling. The upper magnet slides straight down the stick and sticks to the lower
one.

Why it happens: the magnetic force did not vanish — it changed direction. Facing
like poles, it pushed the upper magnet up and balanced the Earth's pull. Facing
unlike poles, it pulls the upper magnet down, adding to the Earth's pull instead of
opposing it. This is the proof that a magnet can exert a force on another magnet
without being in contact with it — and that the magnetic force is a non-contact force
which can be either attractive or repulsive.

Activity 5.6: Let us experiment — Page 70

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Class 8 Science Chapter 5 Exploring Forces
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5.4.2 Non-contact forces — Electrostatic force (Fig. 5.8)
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ACTIVITY

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a s em bring it close to the small pieces of paper placed on a table, taking care not to
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touch the paper pieces (Fig. 5.8). Do you notice something surprising?

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Fig. 5.8, page 70 — redrawn sketch: the rubbed plastic scale held just above the table,

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se The tiny pieces of paper jump up from the table and cling to the plastic scale or straw —

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mbrought close to paper pieces. Why does this happen?glas e
Q2 The paper pieces get pulled towards the plastic scale/straw and stick to it when it is

. c o a
a s em
agl ANSWER
.c
Because rubbing has made the scale a charged object, and a charged object exerts an
s e m
. c om
electrostatic force on uncharged objects near it.
a g la
s e m are rubbed together, electrical charges build up on
gla static charges, because they do not move by themselves; the
When two objects of certain materials

a
their surfaces. These are called
object that acquires them is said to be a charged object. A charged object attracts — that is,

com
m .
m ase
.co


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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

exerts a force on — uncharged objects made of certain materials, such as small pieces of paper.
This force comes into play even when the objects are not in contact, which is why the paper
jumps up before it touches the scale.

Tip: the activity fails on a damp day, and it fails if you touch the rubbed part with
your hand or a metal object. That is why the book warns you not to — the charge
leaks away and the scale becomes ordinary again.

Activity 5.7: Let us experiment — Page 70
5.4.2 Non-contact forces — Electrostatic force (Fig. 5.9)

ACTIVITY

Q1 Rub both balloons with the woollen cloth and release them. Be careful not to touch
the rubbed balloons with your fingers. What do you observe?

The two hanging balloons move away from each other, as if they are repelling each other (Fig.
5.9b), and the threads slant outwards.
Before rubbing they hung quietly side by side (Fig. 5.9a). Nothing has been added to them
except the rubbing — and now they push each other apart across a gap. So rubbing has put
something on both balloons that makes them repel.

Q2 Now bring the woollen cloth used for rubbing the balloons close to one of the
rubbed balloons. What happens?

The balloon and the woollen cloth move towards each other, as if they are attracting each
other. The balloon swings out of its vertical position to meet the cloth.
This is the opposite of what the two balloons did to each other — and the cloth is the very object
that was used to rub them.

Q3 What do we infer from these observations?

Three conclusions follow, in order:

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

1. Both balloons were charged in the same way, so they must have acquired similar charges.
Since they repelled each other, similar (like) charges repel each other.
2. The charged balloon and the woollen cloth attracted each other. Both the rubbing object and
the rubbed object get charged, but they acquire the opposite kind of charge. Their
attraction shows that opposite kind (unlike) charges attract each other.
3. Therefore there are exactly two kinds of static charges, and they are called ‘positive’ and
‘negative’.

The force exerted by a charged body on another charged body or an uncharged body is called
the electrostatic force. It acts across a gap, so it is a non-contact force.

PAIR BROUGHT CLOSE WHAT IS WHAT IT PROVES
OBSERVED

Two rubbed balloons They move apart Like charges repel

Rubbed balloon and the woollen cloth They move together Unlike charges attract

Rubbed scale and plain paper bits Paper is pulled up A charged body attracts an
(Activity 5.6) uncharged one

In-text Questions — Page 71
5.4.2 Non-contact forces — the two kinds of charge

Q1 Does this indicate that the charge on the balloon is of a different kind from the
charge on the woollen cloth?

Yes, it does. The two similarly charged balloons repelled each other, while the balloon and the
woollen cloth attracted each other. Since the same kind of charge repels, the attraction can only
mean that the cloth carries a different kind of charge from the balloon.
Both the rubbing object (the cloth) and the rubbed object (the balloon) get charged, but they
acquire opposite kinds of charge.

Q2 Does it mean that there are two kinds of electrical charges?

Yes. The two kinds of static charges are said to be ‘positive’ and ‘negative’.
The whole of the rubbing experiment is explained by two simple rules:

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Like (similar) charges → repel each other

Unlike (opposite) charges → attract each other

Rubbing never creates one charge alone. It always produces the two kinds together, one on
each object — which is why the balloon and the cloth end up attracting each other, and why two
balloons rubbed the same way end up pushing each other apart.

Did you know? Compare this with magnets, where the two poles are North and
South, and the same two rules apply. The big difference comes next: gravitational
force has no such pair — it is always attractive.

Activity 5.8: Let us observe — Page 71
5.4.2 Non-contact forces — Gravitational force

ACTIVITY

Q1 Take a ball and throw it vertically upwards. Does it come down?

Yes. The ball rises, slows down, stops for an instant at the top and then falls back to the ground
— every single time.
While going up the ball is slowing down, so a force must be acting on it, and it must act
downwards, opposite to the motion. That same downward force then brings the ball back and
makes it speed up as it comes. Nothing is touching the ball while it is in the air, so this is a non-
contact force — the pull of the Earth.

Q2 Now throw it again, but this time harder. Does it still fall back down to the ground?

Yes. Throwing harder makes the ball go higher and stay in the air longer, but it still comes back
to the ground.
The strength of your throw decides only how high the ball gets. It cannot switch the Earth's pull
off. Fig. 5.10 shows the same thing happening to a basketball, a leaping athlete, a fruit on a tree
and a diver — whatever is thrown up in any direction finally falls or comes back to the ground.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Why it happens: since all objects fall towards the Earth, it means the Earth attracts
(pulls) them. The force with which the Earth attracts objects towards itself is called
the gravitational force — also called the force of gravity, or simply gravity.

In-text Questions — Page 72
5.4.2 Non-contact forces — Gravitational force · 5.5 Weight and Its Measurement

Q1 Why do all the objects fall towards the Earth?

Because the Earth attracts (pulls) every object towards itself. That pull is the gravitational
force.
A dropped object takes a straight vertical path downwards (Fig. 5.11a) because that is the
direction in which the Earth pulls it — towards the Earth. An object thrown vertically upwards
moves up straight, slows down, stops momentarily at the top, and then takes a straight vertical
path downwards (Fig. 5.11b). Going up, its speed keeps decreasing; coming down, its speed
keeps increasing. Both changes are produced by the same downward pull. Motion of this kind,
under the influence of the gravitational force, is called vertical motion.

Q2 Is there any force which acts on them? What exerts this force?

Yes — the gravitational force, and it is exerted by the Earth.
The gravitational force exerted by the Earth is also called the force of gravity, or simply gravity.
Two things about it are worth fixing in mind:

It acts without contact with the object it attracts, so it is a non-contact force.
It is always an attractive force, unlike magnetic force or electrostatic force, which can be
either attractive or repulsive.

Q3 Does the Earth pull every object with equal force?

No. Different objects are pulled with different forces, and Activity 5.9 shows it directly.

Page 23 of 60

as e
Class 8 Science Chapter 5 Exploring Forces
a g l AglaSem · NCERT Solutions

Hang a spring from a nail and suspend a pencil box from it — the spring stretches. Change the
co m
se
object and the stretch changes. Since the stretch is produced by the Earth's pull on the hanging
m.
o m l a
gsame as saying that
.c itself is what we call the weight of that object, so this is the
object, an unequal stretch means an unequal pull. The force with which the Earth pulls an

m a
seobjects have different weights.
object towards

l a
ag
different

co m
em
. ag
s
Activity 5.9: Let us explore — Page 73
a
agl
5.5 Weight and Its Measurement (Fig. 5.12)

ACTIVITY

co m
se m.
o m g l a
.c and observe the spring (Fig. 5.12b). Does the springastretch?
Q1 Hang one end of the spring from a nail (Fig. 5.12a). From the other end, hang an

se m
object
a
agl
m a s
m.co agl
l a se
a g

com
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m
(a)

l a se
ag
se m
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ase
agl

co m
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se
(b)

o m l a
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Fig. 5.12, page 73 — (a) a hanging spring; (b) two different

g l a spring.
a c
m .
m a s e
e m . co agl
as

a g l
Yes, the spring stretches as soon as the object is hung from it, and it stays stretched as long as
the object hangs there.

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m .
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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

The spring is being pulled down at its lower end. Nothing is touching the object except the
spring, so the downward pull on it comes from the Earth — the gravitational force, that is, the
object's weight. The stretch of the spring is simply the visible effect of that pull.

Q2 Now hang the other objects, one by one and notice the stretch in the spring each
time. Is the stretch caused by each object the same?

No. The stretch is different for different objects — the stone stretches the spring more than
the pencil box, and the tiffin box stretches it by a different amount again.
The spring has not changed, so a bigger stretch can only mean a bigger pull. This indicates that
the Earth pulls different objects with different forces, that is, the weight of different
objects is different.

Q3 Can we use the spring to measure the weight of an object?

Yes — and that is exactly what a spring balance is.
The stretch of the spring grows with the pull on it, so the stretch can stand in for the weight. A
spring balance is a simple device used to measure weight (force). It consists of a spring fixed at
one end, with a hook attached at the other end. When we hang an object from the hook, the
spring stretches, and the amount of stretching gives the weight of the object. There is a scale on
the balance marked to show the weight (force) in newton, and usually a second scale showing
the corresponding values of mass in gram (g).

Careful: those gram markings are made on the assumption that the balance is
being used on the Earth, with the Earth's gravitational force attracting the object.
The instrument really measures a force; the mass scale is only a convenience.

Activity 5.10: Let us observe — Page 73

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5.5 Weight and Its Measurement — getting to know the spring balance (Fig. 5.13)

ACTIVITY

Q1 Look at the spring balance shown in Fig. 5.13 carefully. What is the maximum
weight it can measure?

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Page 27 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

SPRING SCALE

GRAMS NEWTONS
0 0

100 01

200 02

300 03

400 04

500 05

600 06

700 07

800 08

900 09

1000 10

Page 28 of 60

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Class 8 Science Chapter 5 Exploring Forces
a g l AglaSem · NCERT Solutions

co m
em.
m l as
.co a g
a s emFig. 5.13, page 74 — the spring balance and its scale: GRAMS down the left, NEWTONS
a gl down the right.

com
m . ag
l a se
agis 10 N. Thus, this scale has a range of 0 to 10 N.
The maximum weight it can measure

m
Read it off the instrument itself: the NEWTONS scale on the right of Fig. 5.13 is marked 0, 01, 02,
co
m.
… up to 10, and 10 is the last mark on it. Alongside it runs a GRAMS scale marked 0, 100, 200, …

m as e
.co l
up to 1000, so the same balance can show masses from 0 to 1000 g.

a g
a s emthe range is the first thing to check on any instrument, exactly as you did with
gl
Tip:
a the thermometer in ‘Temperature and Its Measurement’ in Curiosity, Grade 6.

m
Hanging an object heavier than 10 N from this balance can damage the spring
a s
permanently.
m.co agl
l a se
a g
Activity 5.11: Let us calculate — Page 74
co m
5.5 Weight and Its Measurement — the smallest weight the balance can read m.

o m l a se
.c a g
se m
ACTIVITY

g l a
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How much is the weight difference indicated between the two bigger marks?
se
Q1

com g l a
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ase

agl
The weight difference indicated between 0 and 01 N, or between 01 N and 02 N, is 1 N.
On this balance the bigger marks are numbered 0, 01, 02, … 10, and those numbers stand for 0

co m
.
N, 1 N, 2 N, … 10 N. So every step from one big mark to the next is a step of one newton.

em
m l as
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a s emQ2
gl
How many divisions (shown by smaller marks) are there between these two bigger
a c
.
marks?

s e m
m a

e m . co agl
l as
There are 5 divisions between two bigger marks.
g
a

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m .
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.co


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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Count the gaps, not the lines. Between 0 and 01 N there are four small marks, and those four
marks cut the space into five equal divisions.

Q3 How much weight does one small division indicate?

One small division stands for one-fifth of the 1 N between two big marks.

Weight between two bigger marks = 1 N

Number of divisions between them = 5

One small division = 1 N ÷ 5 = 0.2 N

So the smallest value that this spring balance can read is 0.2 N. This smallest readable value is
called the least count of the instrument.

0
Between two big marks = 1 N
Small divisions in between = 5

01 N One division = 1 N ÷ 5 = 0.2 N
Least count = 0.2 N

02 N
The newton scale of the spring balance of Fig. 5.13

Four small marks cut the 1 N space into five equal divisions, so each division is worth 0.2 N.

Check it yourself: the balance in your school laboratory may have a different range
and a different smallest division, so work its least count out the same way before
you use it. Do the same for the mass scale of Fig. 5.13 — there, 100 g is divided into
5 parts, so one division reads 100 g ÷ 5 = 20 g.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Activity 5.12: Let us measure — Page 74

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

5.5 Weight and Its Measurement · Table 5.2 (Fig. 5.14)

ACTIVITY

Q1 Take a spring balance and a few objects. Keep in mind that the objects should not
be heavier than the maximum value of weight the spring balance can measure,
otherwise it may get damaged. Suspend the objects one by one from the hook (Fig.
5.14). Read the scale for weight carefully and record your observations in the Table
5.2.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Page 33 of 60

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Class 8 Science Chapter 5 Exploring Forces
a g l AglaSem · NCERT Solutions

co m
e m.
m l as
m .co a g
l a se
a g

o m
SPRING
e m . c SCALE ag
s
gla
aGRAMS NEWTONS
0 0 co m
em.
m l as
.co g
100 01
m a
l a se
a g 200 02
300 03
a s
. com agl
em
400
s 04
gla
a 500 05

co m
600 06
m .
e
.co
m 700 07 glas
m a
ase
agl 800 08
900 09 se m
com g l a
. a
a em
s1000 10
agl

co m
m .
m as e
.co a g l
se m
g l a
a c
m .
m a s e
e m . co agl
g l as
a

com
m .
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.co


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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Fig. 5.14, page 74 — redrawn sketch: an object suspended from the hook of a spring
balance.

Table 5.2: Measuring weight using a spring balance

S.NO. OBJECT WEIGHT (N)

1. Pencil Box

2. Partially filled water bottle

How to take the reading correctly

1. Hold the balance hanging freely and check that the pointer reads zero before anything is
attached.
2. Suspend one object from the hook and wait until it stops swinging.
3. Read the newton scale with your eye level with the pointer, so that you do not read a mark
above or below the true one.
4. Read to the nearest division — for this balance, to the nearest 0.2 N. Write the unit N with
every entry.

Sample answer — your own readings will differ, because your objects are not the same as
these.

S.NO. OBJECT WEIGHT (N)

1. Pencil box 1.6 N

2. Partially filled water bottle 5.0 N

3. Geometry box 2.4 N

4. A small stone 3.2 N

Why the warning matters: a spring balance works only as long as the spring
returns to its original length when the load is removed. Overload it and the spring
stays permanently stretched — after that every reading it gives is wrong, and no
amount of care in reading will fix it.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Tip: repeat Activities 5.10 to 5.12 using the mass scale on the left of the balance (Fig.
5.13) and you will have measured the mass of the same objects, in grams. Notice
that you did the same experiment both times — the instrument really measures the
pull of the Earth, and the gram scale simply converts it for you.

In-text Questions — Page 75
5.5 Weight and Its Measurement — mass and weight

Q1 What is the difference between weight and mass?

Mass is the amount of matter in an object. Weight is the gravitational force with which
the Earth (or another planet) pulls that object. They are different quantities and are
measured in different units.

MASS WEIGHT

What it is The amount of matter in an object The gravitational force with which the Earth
pulls the object

Is it a force? No Yes — weight is a force

Unit gram (g) or kilogram (kg) newton (N)

Measured with A beam balance (or the mass scale of A spring balance
a spring balance)

Does it change from place No — its value remains the same at Yes — it can change, because the
to place? every place gravitational force varies

The table the book gives makes the difference concrete. The same object is carried to different
worlds:

EARTH MOON MARS VENUS JUPITER

Mass of the object 1 kg 1 kg 1 kg 1 kg 1 kg

Weight of the object 10 N 1.6 N 3.8 N 9N 25.4 N

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Why it happens: the object still contains exactly the same matter wherever you take
it, so its mass cannot change. Its weight is not a property of the object alone — it is
the strength of the pull between the object and the world it is on. Change the world
and you change the pull. Even on the Earth the gravitational force varies very slightly
from place to place, so weight varies very slightly too.

Tip: in the market we say ‘the weight of this wheat bag is 10 kg’. Scientifically that is
wrong on both counts — 10 kg is a mass, and weight would have to be given in
newton. What we are really interested in when we buy wheat is the amount of
matter, that is, the mass. Use the correct terms with their correct units, even when
everyday language is casual.

In-text Questions — Page 76
5.6 Floating and Sinking

Q1 While taking out water from a bucket filled with water using a mug, do you notice
that the mug feels lighter when it is inside water?

Yes — and it feels heavier the moment you lift it clear of the water surface.
The Earth's pull on the mug is the same in both positions; nothing has been added to or taken
out of the mug. What changes is that while the mug is inside the water, the water pushes it
upwards. That upward push takes over part of the load, so your hand has to supply less, and
the mug feels lighter. The force applied by a liquid on an object in the upward direction is called
upthrust or buoyant force.

Q2 If we place some objects on water, some of them float, while others fall to the
bottom. The gravitational force of the Earth is acting on all objects, then why don’t
all objects fall to the bottom?

Because gravity is not the only force acting on an object in water. The water pushes back up with
the buoyant force, and what happens depends on which of the two is larger.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Gravitational force more than buoyant force → the object sinks

Gravitational force equal to buoyant force → the object floats

So the objects that float are not escaping gravity — the Earth pulls them just as hard as it pulls
the ones that sink. They float because the water beneath them happens to push up on them by
exactly as much as the Earth pulls them down, and the two forces balance.

Did you know? One of the factors on which the buoyant force depends is the
density of the liquid — which is why it is easier to float in very salty water than in a
freshwater pond. You will learn about density in a later chapter of this book.

Activity 5.13: Let us investigate — Page 76

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Class 8 Science Chapter 5 Exploring Forces
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5.6 Floating and Sinking (Fig. 5.15)
co m
e m.
m as
ACTIVITY

.co a g l
a s emthe bottle in the water (Fig. 5.15). Do you feel an upward push?
gl
Push
a
Q1

co m
e m . ag
g l as
a

co m
em.
m l as
m .co a g
l a se
a g
m a s
m .co agl
l a se
a g

co m
m .
m as e
.co a g l
se m
g l a
a
se m
com
Fig. 5.15, page 76 — redrawn sketch: a closed empty bottle in a bucket full of water.
g l a
m . a
ase
agl

co m
.
Yes. As you press the closed empty bottle down into the bucket, your hand feels the water

em
as
pushing the bottle upwards, against you — and the deeper you push it, the stronger that push
m l
.co g
becomes.
a
emThis upward push is the upthrust, or buoyant force. It is applied by the water on the bottle, and
a s
agl it acts in the direction opposite to the way you are pressing.
.c
s e m
m a
e m . co agl
g l as
a

co m
m .
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.co


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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Q2 Release the bottle. Does it bounce up?

Yes. The moment you let go, the bottle shoots back up and bobs on the surface of the water.

Why it happens: once your hand is removed, only two forces are left on the bottle
— the Earth's pull downwards and the water's upthrust upwards. Held under, the
upthrust on the closed, air-filled bottle is far greater than its weight, so the bottle is
driven upwards until enough of it is out of the water for the two forces to balance. It
then floats. In fact, all liquids apply a similar force on objects placed in them.

Try this: repeat with the same bottle filled to the brim with water and capped. Now
it barely rises at all. The bottle occupies the same space either way — what has
changed is the weight of the bottle, and so which of the two forces wins.

Keep the curiosity alive — Page 77

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Chapter-end exercise, Questions 1–10

KEEP THE CURIOSITY ALIVE

Q1 Match items in Column A with the items in Column B. Column A (Type of force): (i)
Muscular force (ii) Magnetic force (iii) Frictional force (iv) Gravitational force (v)
Electrostatic force. Column B (Example): (a) A cricket ball stopping on its own just
before touching the boundary line (b) A child lifting a school bag (c) A fruit falling
from a tree (d) Balloon rubbed on woollen cloth attracting hair strands (e) A
compass needle pointing North.

COLUMN A COLUMN B WHY THIS PAIR

(i) Muscular force (b) A child lifting a school bag The upward pull comes from the child's arm
muscles contracting — a contact force

(ii) Magnetic (e) A compass needle pointing North The magnetised needle is turned by a magnetic
force force acting on it without contact

(iii) Frictional (a) A cricket ball stopping on its own just Friction between the ball and the ground acts
force before touching the boundary line opposite to the ball's motion and stops it

(iv) Gravitational (c) A fruit falling from a tree The Earth attracts the fruit towards itself — a
force non-contact, always attractive force

(v) Electrostatic (d) Balloon rubbed on woollen cloth Rubbing charges the balloon; a charged body
force attracting hair strands attracts uncharged hair across a gap

Tip: sort the five by contact first. Only (i) and (iii) need touching, and only one of the
examples involves a person's muscles — that settles two rows immediately.

Q2 State whether the following statements are True or False. (i) A force is always
required to change the speed of motion of an object. (ii) Due to friction, the speed of
the ball rolling on a flat ground increases. (iii) There is no force between two
charged objects placed at a small distance apart.

(i) True. Speed cannot change on its own. Whether an object starts moving, speeds up, slows
down or stops, a force must be acting on it — that is the central conclusion of Table 5.1.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

(ii) False. Friction acts opposite to the direction in which the ball is rolling, so it decreases the
speed and finally brings the ball to rest. It can never increase the speed. (A rolling ball on a
rough ground stops sooner than on a smooth one — the very opposite of what this statement
claims.)
(iii) False. Electrostatic force is a non-contact force, so it acts across the gap: like charges repel
and unlike charges attract. Two rubbed balloons hanging apart push each other away (Activity
5.7) without ever touching.

Q3 Two balloons rubbed with a woollen cloth are brought near each other. What would
happen and why?

They will move away from each other — they repel.
Both balloons were rubbed with the same woollen cloth, so both have been charged in the
same way and have therefore acquired similar (like) charges. Like charges repel each other, so
each balloon pushes the other away and the threads holding them slant outwards (Fig. 5.9b).

Why it happens: rubbing does not create charge on one object alone. Charges build
up on both the rubbing object and the rubbed object, and they are of opposite
kinds. So the two balloons end up alike (and repel), while either balloon and the
cloth are unlike (and attract). Bring the woollen cloth near one of the balloons and
you will see it swing towards the cloth.

Check it yourself: the effect is a force, so it obeys the rules of a force — it acts on
both balloons, and it acts without contact, which is why we call it an electrostatic
force, a non-contact force.

Q4 When you drop a coin in a glass of water, it sinks, but when you place a bigger
wooden block in water, it floats. Explain.

Both objects have two forces on them in water — the gravitational force pulling them down
and the buoyant force (upthrust) of the water pushing them up. Which one wins decides
whether the object sinks or floats.

The coin: a coin is small, so it pushes very little water aside, and the upthrust it receives is
small. But it is made of metal, so even that small coin is heavy. Here the gravitational force
is more than the buoyant force, and the coin sinks to the bottom.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

The wooden block: the block is much bigger, so as it settles into the water it pushes a large
amount of water aside and receives a large upthrust. Wood is light for its size, so the block
does not have to sink far before the upthrust has grown equal to its weight. At that point the
two forces are equal, and the block floats with part of it above the surface.

gravitational force buoyant force (upthrust)

Coin Wooden block
gravity > upthrust → sinks gravity = upthrust → floats

The arrows are drawn to scale: on the coin the downward pull is the longer arrow, on the block the
two arrows are equal.

Why the size is misleading: it is tempting to say the block floats because it is wood
and the coin sinks because it is metal — but the real comparison is between the
weight of the object and the upthrust it can gather, and the upthrust depends on
how much water the object pushes aside. The bigger block gets a bigger upthrust
precisely because it is bigger. This is Archimedes' Principle: the upward force on an
immersed object equals the weight of the liquid it displaces.

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Class 8 Science Chapter 5 Exploring Forces
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co m
m.
If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to
se
Q5

o m l a
the ground. Name the forces acting on the ball and specify their directions. (i)

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

The Earth's pull never changes direction; the friction of the air flips over, because it always opposes
whichever way the ball is going.

Why the ball stops at the top: going up, both forces act downwards, so the speed
falls steadily until it becomes zero. Gravity does not stop acting at that instant — it is
still pulling downwards — so the ball cannot stay there. Its direction of motion
changes and it begins to fall, gaining speed all the way down.

Tip: if the question asks only for the force named in this chapter's section on gravity,
the gravitational force alone is enough. Mentioning air friction is the fuller answer,
since the book has told us that air also exerts a force of friction on objects moving
through it.

Page 45 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Q6 A ball is released from the point P and moves along an inclined plane and then
along a horizontal surface as shown in the Fig. 5.16. It comes to stop at the point A
on the horizontal surface. Think of a way so that when the ball is released from the
same point P, it stops (i) before the point A (ii) after crossing the point A.

P

A

Fig. 5.16, page 78 — the ball is released at P at the top of the inclined plane and comes to
rest at A on the horizontal surface.

The ball is released from the same point P every time, so it always arrives at the bottom of the
incline in the same way. The only thing left that can decide where it stops is the friction between
the ball and the horizontal surface — so change that.
(i) To make it stop before the point A — increase the friction. Spread a rough material along
the horizontal surface: sand, a piece of cloth, a jute mat or coarse paper. The rougher surface
has bigger irregularities, so the force of friction on the ball is larger, its speed falls faster, and it
comes to rest short of A.
(ii) To make it stop after crossing the point A — reduce the friction. Make the horizontal
surface smoother: polish it, lay a sheet of glass or a smooth ceramic tile along it, or sprinkle a
little talcum powder. With smaller irregularities the force of friction is less, the ball loses speed
slowly, and it travels past A before stopping.

Page 46 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

P

(i) A (ii)

(i) Sand or cloth on the surface → more friction → the ball stops before A
(ii) Polish it or lay a glass sheet → less friction → the ball stops after A

Same starting point P, same ball — only the horizontal surface is changed, and with it the friction.

Careful: do not answer by releasing the ball from a higher point, or by giving it a
push. The question fixes the starting point at P, so the answer must lie in the surface,
not in the release.

Q7 Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.

Because on such surfaces the force of friction between our feet and the ground is very
small, and it is friction that normally gives our feet their grip.
Think about what happens in an ordinary step. Your foot presses backwards on the ground;
friction, acting opposite to that attempted backward slide, holds the foot in place and lets you
push yourself forward. Friction arises from the irregularities of the two surfaces locking into one
another — and ice and polished floors have very few and very shallow irregularities. There is
almost nothing for the sole of your shoe to lock into. So instead of gripping, your foot slides out
from under you and you slip.

Why it happens: friction depends on the nature of the surfaces in contact, and is
greater on rough surfaces (Activity 5.4). A smooth surface is simply the extreme case
of a low-friction surface. A wet polished floor is worse still, because the film of water
keeps the two surfaces apart and reduces the friction further.

Page 47 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Did you know? Friction is often called a nuisance, but this is the situation that shows
how badly we need it. We spread sand or ash on an icy path, and shoes are given
deep tread patterns, precisely to increase friction where a smooth surface has taken
it away.

Q8 Is any force being applied to an object in a non-uniform motion?

Yes. An object in non-uniform motion is one whose speed keeps changing — and a force is
always required to change the speed of motion of an object.
So if a bus is speeding up, slowing down, or moving faster on one stretch than another, some
force is acting on it. Friction is the commonest one: it is the reason a rolling ball or a coasting
bicycle keeps losing speed until it stops, without anyone touching it.

Why it must be so: the chapter's central conclusion works both ways. A force can
change the speed of an object — and, conversely, nothing changes an object's speed
except a force. If the motion is not uniform, some force must be at work, whether or
not you can see what is applying it.

Careful: the question is about non-uniform motion. If the object is moving in a curve
at a steady speed, its direction is changing instead — and that too needs a force,
since a force can change the direction of motion as well.

Q9 The weight of an object on the Moon becomes one-sixth of its weight on the Earth.
What causes this change? Does the mass of the object also become one-sixth of its
mass on the Earth?

The cause: weight is the gravitational force with which a body pulls an object towards itself. The
Moon's gravitational force is much weaker than the Earth's — about one-sixth as strong — so
the Moon pulls the same object with about one-sixth of the force. That smaller pull is the
object's smaller weight on the Moon.
No — the mass does not change at all. Mass is the amount of matter in an object. Carrying
the object to the Moon does not remove any matter from it, so its mass on the Moon is exactly
what it was on the Earth.

Page 48 of 60

as e
Class 8 Science Chapter 5 Exploring Forces
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co m
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On the Earth: mass = 1 kg, weight = 10 N
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On the Moon: mass = 1 kg (unchanged), weight = 10 N ÷ 6 ≈ 1.6 N
m a g
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The book's own table shows exactly this — the mass stays 1 kg on the Earth, the Moon, Mars,
Venus and Jupiter, while the weight reads 10 N, 1.6 N, 3.8 N, 9 N and 25.4 N.

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Q10 Three objects 1, 2, and 3 of the same size and shape but made of different
materials are placed in the water. They dip to different depths as shown in Fig.
5.17. If the weights of the three objects 1, 2, and 3 are w1, w2, and w3, respectively,
then (i) w1 = w2 = w3 (ii) w1 > w2 > w3 (iii) w2 > w3 > w1 (iv) w3 > w1 > w2

3
2
1

Fig. 5.17, page 79 — three objects of the same size and shape but of different materials,
dipping to different depths in water.

Option (ii): w1 > w2 > w3.

In Fig. 5.17 all three objects are floating, so for each of them the buoyant force of the water is
exactly equal to its weight. Reading the figure: object 1 dips the deepest, object 2 dips less, and
object 3 dips the least.

Deeper the object dips → more water it pushes aside → greater the upthrust on it

Object floats → upthrust = weight of the object

Therefore: deepest dip → greatest weight

Depth of dip: 1 > 2 > 3, so w1 > w2 > w3

Page 50 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

3
2
1

dips deepest dips less dips least
w1 largest w2 in between w3 smallest

Same size and shape, different materials: the one that settles deepest displaces the most water, so it
must be the heaviest.

Why the other options fail: (i) would need all three to dip to the same depth, which
the figure contradicts. (iii) and (iv) both make object 1 — the one sunk deepest —
lighter than another, which cannot be, since a floating object sinks in only far
enough to gather an upthrust equal to its own weight.

Did you know? This is Archimedes' Principle in use: the upward force on an
immersed object equals the weight of the liquid it displaces. The objects are of the
same size and shape, so the only thing that can differ is the material — and the
heaviest material makes the heaviest object, which must sink in furthest.

Discover, design, and debate — Page 79

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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Interdisciplinary projects — Science and Society

DISCOVER, DESIGN, AND DEBATE

Q1 Collect objects made of different materials, such as plastic, wool, silk, rubber,
polythene sheet, paper, and metals. Rub one material with another and check if it
attracts small pieces of paper or not, that is, whether it gets charged or not. Record
your observations in a systematic manner and write a research paper.

How to run it. Cut a few tiny bits of paper and keep them on a dry table. Take two materials at a
time, rub them together briskly for about half a minute, then hold each one just above the
paper bits without touching them. Note whether the bits jump up. Do not touch the rubbed part
with your hand or with any metal, and do the experiment on a dry day — on a damp day the
charge leaks away and nothing works.
Record it like this (a sample table, with the kind of result you can expect):

MATERIAL RUBBED PAPER BITS DID IT GET CHARGED?
RUBBED WITH ATTRACTED?

Plastic scale Polythene sheet Yes, strongly Yes

Balloon (rubber) Woollen cloth Yes Yes

Plastic comb Dry hair Yes Yes

Silk cloth Glass rod Yes, weakly Yes

Iron nail held in the Woollen cloth No No — the charge runs away through
hand your hand

Paper Paper No No — two identical materials do not
charge each other

Writing it up as a research paper. Use the sections a real paper uses: Question (which
materials get charged on rubbing?), Method (exactly what you did, so someone else can repeat
it), Observations (the table), Analysis (which pairs worked and which did not), Conclusion, and
Limitations (humidity, how long you rubbed, size of the paper bits).

What the results will show: charging works best between two different non-metals,
and both objects get charged — with opposite kinds of charge. A metal held in your
bare hand appears not to charge at all, because the charge escapes through you.

Page 52 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Q2 Imagine a scenario where the gravity disappears. Develop a story. Create a cartoon
strip to present your story.

What your story has to get right. Gravity is the force that keeps everything pressed to the
ground and gives every object its weight. Take it away and (a) nothing falls, (b) nothing has
weight, though every object still has its mass, (c) everything not tied down drifts, (d) water does
not pour and does not stay in a glass, (e) you cannot walk, because walking depends on friction,
and friction between your shoes and the floor needs your weight to press them together.
Sample story — The morning gravity took the day off

1. Panel 1: Meera wakes up and finds her bed sheet, her pillow and herself hanging in mid-air
near the ceiling fan.
2. Panel 2: She pours tea for her grandmother. The tea leaves the kettle in a wobbling silver ball
and floats across the kitchen.
3. Panel 3: She tries to run to school and only spins in place — her shoes cannot press on the
floor, so there is no friction to push against.
4. Panel 4: In the classroom the spring balance reads zero for every object. The teacher points
out that the pencil box still has the same 200 g of matter in it — only the pull has gone.
5. Panel 5: A cricket ball, once hit, never comes down. It sails away over the village and does not
stop.
6. Panel 6: Gravity returns at four o'clock. Everything comes down at once, and Meera decides
that a force she used to complain about while cycling uphill is the one holding her whole
world together.

Tip for the cartoon strip: draw a small arrow beside one object in each panel to
show which force is acting (or missing). It turns a funny strip into a piece of science,
and it is what makes panel 4 — mass unchanged, weight zero — land properly.

Q3 Organise a discussion in your class on the topic: Friction — a necessity or a
problem? Make a note of the discussion and state where friction is a necessity and
when it is a problem.

The conclusion the discussion should reach: friction is neither simply good nor simply bad. It
is a necessity wherever we need a grip, and a problem wherever we want easy motion —
and often both at once in the same machine.

Page 53 of 60

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Class 8 Science Chapter 5 Exploring Forces
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FRICTION IS A NECESSITY FRICTION IS A PROBLEM

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Walking — the foot grips the ground and pushes back
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How to run the discussion: divide the class into two groups, give each ten minutes to collect
agexamples from home, the road and the playground, then let them argue. Insist that every

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m a
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point.

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The point to close on: engineers
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hub, while the tread of the same wheel's tyre is designed to increase it on the road.
co m
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a s em Make your own spring balance with the help of your teacher and calibrate it using
a gl Q4
standard weights. Now measure the weights of different objects and calculate the
ratio of the weight and mass of different objects. Do you observe a pattern?
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Making and calibrating it. Hang a light spring from a rigid stand, fix a pointer to its lower end
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Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

OBJECT MASS WEIGHT MEASURED WEIGHT ÷ MASS

Standard mass 100 g = 0.1 kg 1N 10 N/kg

Pencil box 200 g = 0.2 kg 2N 10 N/kg

Small stone 300 g = 0.3 kg 3N 10 N/kg

Water bottle 500 g = 0.5 kg 5N 10 N/kg

What the pattern means: the ratio is the same whatever the object is made of —
about 10 N for every kilogram on the Earth. That is why a spring balance can carry
a gram scale alongside its newton scale at all: on the Earth the two readings always
stand in the same fixed ratio. Take the same balance to the Moon and the newton
readings would fall to about one-sixth, while the masses would be unchanged —
and the gram scale would then be wrong.

Careful: choose a spring soft enough to stretch visibly with 100 g but strong enough
not to be permanently stretched by your heaviest object. Check after each
measurement that the pointer returns to zero; if it does not, the spring has been
overloaded and the balance must be recalibrated.

Page 55 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

Q5 An electroscope is a device which can determine whether an object is electrically
charged. You can make your own electroscope (Fig. 5.18) in your class with the help
of your teacher, test the device. Explore in what other ways you may use this
electroscope.

Copper wire

Straw

Lid

Jar

Aluminium foil

Fig. 5.18, page 79 — the electroscope you can build: a copper wire wound into a flat
spiral at the top, passed down a straw through the lid of a jar, and ending in a hook that
carries two strips of aluminium foil.

Building it (Fig. 5.18). Take a glass jar with a plastic or cardboard lid. Push a straw through a
hole in the lid. Pass a copper wire down through the straw, wind the top end into a flat spiral,
and bend the lower end into a small hook inside the jar. Hang two thin strips of aluminium foil
side by side from the hook so that they touch each other and hang freely. Close the lid.
Testing it. Rub a plastic scale with polythene and touch it to the copper spiral. The two foil
strips fly apart. Charge has passed down the wire to both strips; both strips receive the same
kind of charge, and like charges repel — so the strips push each other away. Touch the spiral
with your finger and they fall back together, because the charge escapes through you.

Page 56 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

WHAT YOU CAN USE IT FOR WHAT TO LOOK FOR

Testing whether an object is charged at all Foil strips separate → charged; no movement → uncharged

Comparing how strongly two objects are The wider the strips spread, the greater the charge
charged

Finding which materials charge best on Test plastic, rubber, silk, wool, paper in turn and compare
rubbing the spread

Showing that charge leaks away Watch the strips slowly close on a humid day, or the moment
you touch the spiral

Showing that metals carry charge away but Touch the spiral with a metal spoon, then with a plastic one,
plastics do not and compare

Careful: keep the jar and the foil strips completely dry, and handle the rubbed
object only by its unrubbed end. Moisture and stray fingers are what make this
experiment fail.

Chapter at a glance
A force is a push or a pull on an object that comes out of the object's interaction with
another object. At least two objects must interact for a force to come into play. The SI unit
of force is the newton, symbol N.
A force can make a resting object move, change the speed of a moving object, change its
direction of motion, change its shape, or produce several of these effects together. None
of these changes happens without a force.
Contact forces act only when the surfaces touch — muscular force and the force of
friction. Friction arises from the irregularities of the two surfaces locking into each other,
always acts opposite to the motion (or attempted motion), and is greater on rough
surfaces.
Non-contact forces act across a gap — magnetic force, electrostatic force and
gravitational force. Like poles and like charges repel, unlike ones attract; gravitational
force is always attractive.
The weight of an object is the gravitational force with which the Earth pulls it. It is a force,
so it is measured in newton — with a spring balance. Mass is the amount of matter in the
object, measured in g or kg. Mass never changes; weight can change from place to
place.
A liquid pushes up on whatever is placed in it — the upthrust or buoyant force. If the
gravitational force on the object is greater than the buoyant force the object sinks; if the

Page 57 of 60

Class 8 Science Chapter 5 Exploring Forces AglaSem · NCERT Solutions

two are equal it floats.

Page 58 of 60

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Class 8 Science Chapter 5 Exploring Forces
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Quick revision
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TERM WHERE IT FROM THE

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from its interaction with another object cardboard box (Activity 5.1,

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weight 65
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Contact force A force that acts only when the two Section 5.4.1, Muscular force and the

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ag force

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Muscular force The force produced when muscles Section 5.4.1, Bullocks pulling a cart;
contract and elongate page 66 heart muscle circulating

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Friction The force that comes into play when an Section 5.4.1, A pushed lunch box slides

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object moves or tries to pages 67–68 some distance and stops
another surface; always (Activity 5.3)

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Magnetic, electrostatic and
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Magnetic The force a magnet exerts on another Section 5.4.2, A ring magnet floating
a magnet or on a magnetic material page 69 above another with like
poles facing (Activity 5.5)
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Electrostatic The force a charged body exerts Section 5.4.2, A rubbed plastic scale

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paper (Activity 5.6)

com back to the ground
Gravitational The force with which the Earth attracts Section 5.4.2, Every object thrown up
force (gravity) objects towards itself; it is always page 72
.
m (Activity 5.8, Fig. 5.10)
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