SRIMATHI SUNDARAVALLI MEMORIAL SCHOOL

CHENNAI
2020 - 2021
PHYSICS
CLASS: IX
FORCE AND LAWS OF MOTION

In the previous assignments, we learnt about the motion of a body along a

straight line in terms of its velocity, acceleration and distance travelled by the
body in a particular time. In this assignment, we shall try to understand the
causes of motion.

Do all motions require a cause? If so, what is the nature of this cause? Why does
the speed of an object change (increase or decrease) with time? How can the
direction of a body be changed?

FORCE
It is a push or a pull. It is an agent which when applied on a body brings about a
change in its position or state or form.

EFFECTS OF FORCE
(i) It can change the state of motion of an object.
(ii) It can change the speed of a moving object.
(iii) It can change the direction of a moving body.
(iv) It can change the shape of an object.

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 Force is an external effort in the form of pushing, pulling, stretching,
compressing, etc. which may move a body at rest or stop a moving body
or change the speed of a body or change the direction of a moving body
or change the size and the shape of a body.

Force is a vector quantity. It is generally expressed with the magnitude

along with its direction of action.
The SI unit of force is newton (N).

TYPES OF FORCES

The forces that we come across in our day-to-day life can be divided broadly into
two types:
(i) Balanced forces
(ii) Unbalanced forces

i) BALANCED FORCES

If the resultant of all the forces acting on a body is zero, the

forces are said to be balanced.

(Reaction force)

Two equal forces are acting in the opposite direction. Since the forces are equal
and opposite, the net resultant force is zero.

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The balanced forces cannot change the state of rest or the state of uniform
motion or the direction of the body. It can change the shape and the size of
the body.

Similarly, a body moving uniformly along a straight line will continue to move
along the same line with the same speed even when an unbalanced force is
applied on the body. This is as if no force acts on the body.

ii) UNBALANCED FORCES

Two unequal forces are acting in the opposite direction. Here, the net
resultant force is not equal to zero. The resultant force will act in the
direction of the greater force.

Forces that cause a change in the motion of an object

are unbalanced forces.

The unbalanced forces can change the state of rest or state of uniform
motion or the direction of the body or the shape and the size of the body.

Examples of Balanced and Unbalanced forces

Example 1
Reaction force of the table
Let us consider a wooden block placed on a
X
horizontal table. A string X is attached to the
right face of the block as shown in the figure. Weight of the metal block

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When a small force P is applied to the string X, the block does not move. This is
because two pairs of balanced forces are acting on the block.
The first pair of forces is:
a) The weight of the block (mg) acting vertically downwards on the table.
b) The force of reaction (R) of the horizontal table acting on the block vertically
upwards.
R = mg (balanced forces)
The other pair of forces is:
a) The applied force P tending to move the block to the right.
b) The force of friction F between the block and the table top, acting to the left
and opposing the motion.
F = P (balanced forces)
If we go on increasing the applied force to the string X, a stage reaches when the
block begins to move on the table. At this stage the applied force P becomes
greater than the maximum value of opposing force of friction F. This is the
unbalanced force, which produces motion in the block.

Example 2

While riding a bicycle, when we stop pedalling, the bicycle begins to slowdown.
This is because of the friction between the tyre and the road, which opposes the
motion. To keep the bicycle moving, we have to pedal again. It appears as if an
object maintains its motion under the continuous application of an unbalanced
force. But the conclusion is wrong. In fact, an unbalanced force is needed only for
starting the motion in a body at rest or for changing the speed/direction of
motion. This will persist as long as the unbalanced force is acting.

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The moment the unbalanced force is removed, the object would continue to
move with the velocity it has acquired till then. There will be no further
change in velocity in the absence of the unbalanced force. It should be
clearly understood that an unbalanced force is needed to move a body from
the state of rest. But no such force is required to maintain the uniform
motion of the body.

Galileo’s Observations
Galileo Galilei, an Italian astronomer, physicist and engineer observed the motion
of objects on an inclined plane.

When a marble is rolled down an inclined plane, its velocity increases.

Galileo’s Arguments

When a marble is rolled down from the left, it will go up on the opposite side up to
the same height at which it is dropped.

If the inclination of the planes is equal, the marble would travel equal distances
while climbing up as travelled while rolling down.

If we decrease the angle of inclination of the right plane, the marble would travel
further until it reaches its original height.

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If the right side of the plane is made flat, the marble would travel forever to
achieve the same height.

Galileo's Inference

An unbalanced external force is required to initiate the motion (from the state of
rest), but no unbalanced net force is needed to sustain the uniform motion,
i.e. the objects move with a constant speed along a straight line when no force
acts on them.
In actual practice, it is difficult to achieve the state of zero unbalanced
force, because of the presence of friction which opposes the motion.
Therefore, in the presence of the frictional force some external force is
required to move the objects with constant speed along a straight line.
CONTACT FORCE

When a force is applied on an object by direct or indirect physical contact, then

the applied force is called a contact force. One such force is FRICTIONAL FORCE.
When two objects are in contact, then frictional force comes into existence.
Our bicycle will not move unless we pedal it. To move forward, we should
overcome the existing resisting force which opposes the motion of the bicycle. The
MUSCULAR FORCE that we exert on the pedal makes the frictional force as an
unbalanced force. The force of friction always acts in the direction opposite
to the direction of motion.

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NEWTON’S FIRST LAW OF MOTION

Newton has stated in his FIRST LAW OF MOTION that the bodies will remain
in their own state unless and until they are compelled by an external force.

So, the law clearly states that the state of a body can be changed by
applying an external force.

Let us learn a new term but a known concept, INERTIA.

Every object in this universe prefers to remain in their own state of motion or rest
which is called INERTIA. In other words, the tendency of an object to remain in its
own state without getting disturbed is called inertia.

The inability of a body to change by itself its state of rest or state of

uniform motion along a straight line is called the inertia of a body. Thus,
the Newton’s first law of motion defines inertia and is rightly called the
law of inertia.

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Inertia is of three types:
(a) Inertia of Rest
(b) Inertia of Motion
(c) Inertia of Direction

Examples of Inertia of Rest

• A passenger standing in a bus leans backwards when the bus starts all of a
sudden.
• The fruits fall down when the branches of a tree are shaken.
• The dust particles on a carpet falls when we beat the carpet with a stick.

Examples of Inertia of Motion

• A passenger standing in a moving bus leans forward when the bus stops all
of a sudden.
• A man alighting carelessly from a moving train leans forward.

Examples of Inertia of Direction

• The water particles sticking to the cycle tyres are found to fly off
tangentially.
• Whenever a driver is negotiating a curve, the passengers experience a force
acting away from the centre of the curve.

Let us see a few examples from our day to day life.

Example 1
When we play carrom, we arrange the coins in the form of a vertical pile. When we
strike the bottom coin using a fast-moving striker, the bottom coin alone is
pocketed, while most of the time the remaining pile of coins stand still.
Reasons:
(i) The bottom coin alone experiences the force. Hence, it has changed its state
from the inertia of rest to the inertia of motion.
(ii) But the remaining coins do not experience any force. Hence, they prefer to
remain at the same state.

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Example 2
When a person jumps out of a moving bus, he used to run along with the bus in
the direction of motion to keep his body in the inertia of motion before coming to
rest.
Reasons:
(i) Currently his body is in inertia of motion. If he tries to stand and not run
while getting out of a moving bus, then he will experience an unbalanced force
and fall down.
(ii) Even if he tries to get down in the opposite direction of motion, it is
dangerous. Because, his body is aligned to one particular direction and a
sudden change in this will trip him down.

Inertia of a body depends upon its mass. That is, massive objects possess
more inertia than lighter ones.

INERTIA DEPENDS ON MASS

Imagine a huge rock is blocking the

road. Is it easy to put it aside?
No, because its inertia of rest is great.
In order to move this, a greater force is
required. Hence, we can say that,
inertia depends on the mass of the
object.

It is easy to change the direction of a

car rather than that of a road roller or a
bulldozer. Because, the mass of a road
roller or a bulldozer is huge.

While playing cricket, it is easier to catch a rubber ball than a cricket ball. It is
because of the mass of the cricket ball.
All these examples clearly show that, heavier or massive objects offer larger
inertia.
We can also say that the mass of the object is the measure of its INERTIA.

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MOMENTUM

The first law of motion clearly tells us about how an unbalanced force brings
about changes in the body. Let us see, how are we going to measure this force?

An unbalanced force brings changes in the velocity of a moving body with time,
which means acceleration. An accelerating body depends on two important
physical quantities. What are they?

Let us see some examples.

A small pebble thrown at a great velocity can shatter a glass window.

A bullet fired from a gun can pierce through human flesh.
It is easy to stop a cycle rolling down a hill.
What if it is a huge truck? Then, it would be impossible even if they were rolling
down with the same speed.
Similarly, if an object is to be accelerated, we know that a greater force is required
to give greater velocity.

When we throw a cricket ball and a tennis ball, it is easy to catch a tennis ball.
Because, its mass is small compared to a cricket ball. At the same time, when we
throw two cricket balls of the same mass but with different velocities, then more
force is required to stop that cricket ball which is moving with higher speed than
the one with the lower speed.
From the above observations, we can say that the acceleration of a body
depends on both mass of the body and its velocity.

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There is a new term called “momentum” which combines these two physical
quantities. This term was coined by NEWTON.
The momentum of a body P, is the product of its mass and velocity.
So, P = mv
Its SI unit is kg m/s.
Momentum is a vector quantity and its direction will be the same as that of the
velocity.
The change in momentum can be written as Pf – Pi
where, Pf – final momentum and Pi – initial momentum
So, Pf = mv
Pi = mu (v = final velocity, u = initial velocity)
Change in momentum = Final momentum – Initial momentum
= mv – mu
= m (v – u)

Momentum, in a simple way, is a quantity of motion. Thus, the quantity of

motion in a body depends on both mass of the body and its velocity. Here,
the quantity is measurable, because if an object is moving and has mass, then it
has momentum. A body at rest has no momentum.

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 SRIMATHI SUNDARAVALLI MEMORIAL SCHOOL
CHENNAI
2020 - 2021
PHYSICS
ASSIGNMENT - 3
CLASS: IX
CHAPTER: FORCE AND LAWS OF MOTION

FILL IN THE BLANKS:

1. ______________________and ________________________ are examples of contact

forces.
2. A number of forces acting on a body change the velocity of the body. It
indicates that the forces are ______________.
3. To accelerate the motion of an object, an __________________ force is
required.
4. The measure of motion of a body depends on both ____________and its
_______________.
5. The SI unit of momentum P is ______.

CHOOSE THE CORRECT ANSWER:

6. A body is accelerating in a straight line. The unbalanced force acts

____________________.
a) in the direction of motion of the body
b) in a direction opposite to the direction of motion
c) in a direction perpendicular to the direction of motion of the body
d) None of the above

7. If a body is acted upon by a constant force, then it will have a uniform

__________.
a) speed
b) momentum
c) velocity
d) acceleration
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8. Two bodies A and B are thrown upwards simultaneously with the same
velocity. If the mass of A is greater than that of B and suppose the air
exerts a constant and equal force of resistance on both the bodies, then
______________________.
a) A will go higher than B
b) B will go higher than A
c) A & B will go to same height
d) None of the above

9. If A and B are two objects with masses 10 kg and 40 kg respectively,

then ________________.
a) A has more inertia than B
b) B has more inertia than A
c) Both A & B have the same inertia
d) None of the two have inertia

10. The body at rest does not possess ___________.

a) mass
b) weight
c) momentum
d) None of the above
11. _________________ is the momentum of an object of mass m, moving with a

velocity v.

a) (mv)2
b) mv2
c) ½ mv2
d) m v
12. A body whose momentum is constant, must have constant ____________.
a) force
b) velocity
c) acceleration
d) all of the above
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 13. Assertion: Mass is a measure of inertia of the body in motion.

Reason: The greater the mass, the greater is the force required to change

its state of rest or motion.

a) Both assertion and reason are true and reason is the correct explanation of
assertion.
b) Both assertion and reason are true but reason is not the correct
explanation of assertion.
c) Assertion is true but reason is false.
d) Both assertion and reason are false.

ANSWER THE FOLLOWING QUESTIONS:

14. Three bricks are piled up on a table, each of mass 1.5 kg. Calculate the
amount of force acting on the table. (Take g = 10 m/s2)

15. A book is lying on the table. Name the two forces acting on it.

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16. What is the resultant force acting on the block given below?

5N
25 N

15 N

17. Write any two differences between balanced and unbalanced forces.

18. When you kick a ball on a level ground, it rolls

through a certain distance and then comes to

rest. Why?

19. Which of these two have more inertia, a Maruti 800 car or a Toyota
Innova? Give reason.

20. Name the strongest and the weakest forces in nature.

21. Why is it advised to wear seat belts while travelling in a car?

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22. A bowler in a cricket match always runs

for some distance before bowling the

ball. Why?

23. State Newton’s first law of motion.

24. A rider on a horse falls back when the horse starts running. Why?

25. An object of mass 200 kg is accelerated uniformly from a velocity of

10 m/s to 15 m/s. Calculate the change in momentum.

26. A bullet of mass 10 g is fired from a gun with a velocity of 150 m/s.
Calculate the change in momentum.

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