This document discusses Newton's laws of motion and key concepts in mechanics including force, inertia, momentum, and conservation of momentum. It provides examples of balanced and unbalanced forces. The three Newton's laws are explained: 1) an object remains at rest or in uniform motion unless acted upon by an unbalanced force, 2) the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, and 3) for every action, there is an equal and opposite reaction. Examples are given to illustrate momentum, inertia, and conservation of momentum. Multiple choice and short answer questions from an NCERT textbook are also included.

1. Force and Newton’s laws : Force and Motion,
Newton’s Laws of Motion, Action and
Reaction
forces, Inertia of a body, Inertia and mass,
Momentum, Force and Acceleration.
Elementary
idea of conservation of Momentum
Revised syllabus
CORONA PANDEMIC
2020
Force and Laws of
Motion

2. In our everyday life we observe that some effort is
required to put a stationary object into motion or
to stop a moving object.
Force and Laws of Motion

4. What role does force plays?
• By pushing, hitting and pulling objects we are
bringing objects in motion. They move because
we make a force act on them.
• We know that force can be used to provide
acceleration.
• We also know that a force can change the
shape and size of objects.
By blowing air into the balloon we are applying
force , which causes change in its shape

6. Balanced and Unbalanced forces

7. Balanced or Unbalanced force?

9. IMPORTANT POINTS
• If net addition of all the forces on an object is
zero then the forces acting on it are called
Balanced Forces
• If net addition of all the forces is not zero
then the forces are termed as unbalanced
forces.
• If an unbalanced force is applied on object it
will have acceleration. And the acceleration will
be there as long as the force is there.
• After the force is removed it will continue to
move with the velocity it has gained till then.

10. EXAMPLES

11. What is the force of friction?
• It is a force extended when two surfaces are in
contact with each other. It always acts in a
direction opposite to the direction of motion
of the object.

13. • An object remains in a state of rest or of
uniform motion in a straight line unless
compelled to change that state by an
applied force.
• The tendency of an object to remain at its
natural position, rest or uniform motion, is
because of its property called Inertia
First Law of Motion

14. NEWTON’S LAWS OF
MOTION

16. EXAMPLES

17. INERTIA IS THE PROPERTY OF ANY
OBJECT TO RESIST OR OPPOSE THE
CHANGE IN IT’S STATE ……
MASS IS ACCOUNTABLE FOR INERTIA

18. Inertia & Mass
• The inertia of an object is dependent upon its
mass.
• Lighter objects have less inertia, that is, they can
easily change their state of rest or motion.
• Heavier objects have large inertia and therefore they
show more resistance.
• Hence ‘Mass’ is called a measure of the inertia of
an object.
it is easier for a person to push
the bucket that is empty rather
than the one that is filled with
sand. This is because the mass of
an empty bucket is less than
that of the bucket filled with
sand.

19. WHICH IS HAVING MORE
INERTIA
ELEPHANT
OR
BALL

22. IDENTIFY…..WHICH INERTIA?

26. The Second Law of Motion
The impact produced by a moving object depends upon its mass and velocity.
For Example, a small bullet fired at a high velocity can kill a person.
Momentum – The product of mass and velocity is called Momentum. It is a
vector quantity. Its direction is same as that of the object’s velocity.
Denoted by – p SI unit – kg metre per second
p = mv, where m is the mass of the object, v is the velocity of the object
The momentum of a stationary object –
Let the mass of a stationary object be ‘m’,
Let the velocity of a stationary object be ‘v’,
The stationary object has no velocity, so v = 0,
Therefore, p = m*v = m*0 = 0
So, the momentum of a stationary object is zero.
We know that the velocity of an object can be changed by applying an unbalanced
force on to it. Similarly, the momentum of an object can change by applying an
unbalanced force.
According to the second law of motion –
The rate of change of momentum of an object is directly proportional to the
applied unbalanced force on the object in the direction of the force

27. F=ma

28. Example
• A cricketer when
catches a ball pulls his
hands in the backward
direction to give some
time to decrease the
velocity of the ball. As
the acceleration of the
ball decreases the force
exerted on catching the
moving ball also
decreases. If the
cricketer would try to
stop a moving ball
suddenly he would have
to apply larger force.
.

29. Mathematical Formulation of the
Second Law of Motion
-

30. • with help of the second law of motion we
can evaluate the amount of force that is
being exerted on any object. From the
formula stated above, we can see that the
force is directly proportional to
acceleration. So the acceleration of an
object can change depending upon the
change in force applied.
• Force = ma
• SI Unit: kg-ms-2 or N (Newton)

31. The Third Law of Motion
Action and Reaction Forces
• Two forces acting from
opposite directions are
called Action and Reaction
Forces.
• For Example, a ball when
hits the ground (action)
bounces back with a
certain force reaction
The Third Law of Motion States that –every action has an
equal and opposite reaction.

32. EXAMPLE
• when a bullet is
fired from a gun,
the gun only
moves a little
backwards (recoils)
while the bullet
can travel a large
distance. This is
because of the
difference in the
mass of the bullet
and the gun.

35. Conservation of Momentum
• As per the law of conservation of
momentum, the sum of momenta of two
objects before the collision and after
collision remains the same given that no
external unbalanced force acts upon them.
In another way, collision conserves the
total momentum of two objects.

36. HOW IS THE MOMENTUM
CONSERVED?

37. DERIVATION

38. • Two balls A and B having a certain initial velocities collide
with each other. Conditions before the collision-
• There is no unbalanced force acting upon them
• The initial velocity of A is greater than initial velocity of B
• The figure below explains how the momentum of the balls is
conserved after the collision.

39. NUMERICAL

42. NCERT Textbook for Class 9 Science –
Page 118
• Question 1. Which of the following has more inertia:
• (a) a rubber ball and a stone of the same size?
• (b) a bicycle and a train?
• (c) a five-rupees coin and a one-rupee coin?
• Answer:
• (a) A stone of the same size
• (b) a train
• (c) a five-rupees coin
• As the mass of an object is a measure of its inertia, objects with
more mass have more inertia.

43. Question 2. In the following example, try to identify the number of
times the velocity of the ball changes.
“A football player kicks a football to another player of his team who
kicks the football towards the goal The goalkeeper of the opposite team
collects the football and kicks it towards a player of his own team”.
Also identify the agent supplying the force in each case.
The velocity of football changed four times.

44. Question 3. Explain why some of the leaves
may get detached from a tree if we
vigorously shake its branch
• When the tree’s branch is shaken vigorously the
branch attain motion but the leaves stay at rest.
Due to the inertia of rest, the leaves tend to remain
in its position and hence detaches from the tree to
fall down.

45. IDETIFY THE DIRECTION OF FORCE ,
ACCELERATION AND VELOCITY

46. ONE MARKER
• Fill in the following blanks with suitable words :
• (a) .................... is a measure of the inertia of a
body.
• (b) When a running car stops suddenly, the
passengers are jerked ............
• (c) When a stationary car starts suddenly, the
passengers are jerked ..............
• (d) Newton’s first law of motion is also called
Galileo’s law of .....................
MASS
FORWARD
BACKWARDS
INERTIA