1. Done by,
R.Sri Krishna
IXth CBSE
Roll no :- 21
Nalanda Talent School

3. A Brief Introduction…….
 Newton's laws of motion consist of three physical
laws that form the basis for classical mechanics. They
describe the relationship between the forces acting on
a body and its motion due to those forces. They have
been expressed in several different ways over nearly
three centuries.
 The three laws of motion were first compiled by Sir
Isaac Newton in his work ”Philosophiæ Naturalis
Principia Mathematica”, first published on July 5,
1687. Newton used them to explain and investigate
the motion of many physical objects and systems

4. Newton's First and
Second laws, in Latin,
from the original 1687
edition of the Principia
Mathematica.

6. First law of motion
 First law of motion :- Every body remains in a state of rest or
uniform motion (constant velocity) unless it is acted upon
by an external unbalanced force. This means that in the
absence of a non-zero net force, the center of mass of
a body either remains at rest, or moves at a
constant speed in a straight line.
 This tendency is also known as inertia!

7. Inertia
 Inertia is the
tendency of an object
to resist changes in its
velocity. whether in
motion or not
The ball doesn't move unless it is acted upon
by an external unbalanced force.

8.  Don’t let this be you. Wear seat belts.
 Because of inertia, objects (including you) resist changes in their
motion. When the car going 80 km/hour is stopped by the brick wall,
your body keeps moving at 80 m/hour, so it moves forward!!!
Newton’s 1st law and you

9.  Now we can get a doubt that –
 What is this unbalanced force that acts on an object in
motion?
 It is none other than friction!! Once airborne or in
motion, unless acted on by an unbalanced force
(gravity and air – fluid friction), it would never
stop!

10.  For example --- Unless acted upon by an unbalanced
force, this golf ball move forever. But because
friction and gravity are acting on it the ball is
stopping!

12. Momentum
 Momentum of a body is defined as the
product of its mass and velocity
 P(momentum) = M(mass) x
V(velocity)

13. Second law of motion
 The rate of change of momentum of an object is
proportional to the applied unbalanced force in the
direction of the force

14.  This tell us that :-

15. Units
 When mass is in kilograms and acceleration is in
m/s*s the unit of force is in Newton (N). One Newton
is equal to the force required to accelerate one
kilogram of mass at one meter/second square

16. If mass remains constant, doubling the acceleration, doubles the force. If force
remains constant, doubling the mass, halves the acceleration.

17.  We know that
objects with
different masses
accelerate to the
ground at the same
rate.
 However, because
of the 2nd Law we
know that they
don’t hit the
ground with the
same force.
Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with
different forces
F = ma
98 N = 10 kg x 9.8 m/s/s
F = ma
9.8 N = 1 kg x 9.8
m/s/s

18. Newton’s
3rd law of
motion

19. Third law of motion
 For every action, there is an equal and opposite
reaction.
 This may be the most popular law in the universe !
 This law is sometimes referred to as the action-
reaction law, with F called the "action" and −F the
"reaction". The action and the reaction are
simultaneous.

20. Conservation of momentum
 According to Newton's third law every action has an
equalant reaction in the opposite direction .
 So, when 2 bodies of masses M1 and M2 collide with
initial velocities U1 and U2 in a time period t ,Then -
 Force of A on B = -(Force of B on A )
 => M1(V1-U1)/t = M2(V2-U2)/t
 => M1U1 +M2U2 = -(M1V1 + M2V2)

21. Examples
According to
Newton, whenever
objects A and B
interact with each
other, they exert
forces upon each
other. When you sit
in your chair, your
body exerts a
downward force on
the chair and the
chair exerts an
upward force on
your body.

22. There are two forces
resulting from this
interaction - a force on
the chair and a force on
your body. These two
forces are called action
and reaction forces.

23.  Consider the propulsion of a fish through the
water. A fish uses its fins to push water backwards.
In turn, the water reacts by pushing the fish
forwards, propelling the fish through the water.
 The size of the force on the water equals the size
of the force on the fish; the direction of the force
on the water (backwards) is opposite the direction
of the force on the fish (forwards).

25. Flying gracefully through the air, birds
depend on Newton’s third law of
motion. As the birds push down on
the air with their wings, the air pushes
their wings up and gives them lift.
The wings of a bird push air
downwards. In turn, the air reacts by
pushing the bird upwards.
The size of the force on the air equals
the size of the force on the bird; the
direction of the force on the air
(downwards) is opposite the direction
of the force on the bird (upwards).
Action-reaction force pairs make it
possible for birds to fly.

26. The reaction of a rocket is an
application of the third law of motion.
Various fuels are burned in the engine,
producing hot gases.
The hot gases push against the inside
tube of the rocket and escape out the
bottom of the tube. As the gases move
downward, the rocket moves in the
opposite direction.

28. Bibliography :-
NCERT science text book of 9th class
Wikipedia
S.chand super simplified physics for
class 9

29. R.Sri kishna
IX CBSE