laws of motion class-XI

THIS PPT IS RELATED TO LAWS OF MOTION

1. LAWS OF MOTION
SUBMITTED TO- MR.DC
BHATT

2. AcknowledgementNewt on’s e
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I made this project under the guidance of
my mathematics teacher MR. D.C BHATT
CLASS
X-A

3. 1) Force :-
Force is an external effort which may move a body at rest or stop
a moving body or change the speed of a moving body or change the
direction of a moving body or change the shape and size of a body.
Effects of force :-
i) Force can move a body at rest.
ii) Force can stop a moving body.
iii) Force can change the speed of a moving body.
iv) Force can change the direction of a moving body.
v) Force can change the shape and size of a body.

6. 2) Balanced and unbalanced forces :-
i) Balanced forces :-
If two forces act on a body in opposite direction and if both the forces are
equal, then the resultant force acting on the body is zero. Such forces are called
balanced forces.
Balanced forces cannot change the state of rest or motion of a body.
ii) Unbalanced forces :-
If two forces act on a body in opposite direction and if one force is greater
than the other, then the resultant force is not equal to zero. Such forces are called
unbalanced forces.
Unbalanced forces changes the state of rest or the motion of a body.
F
1
F
2
F
1
= F
2
F
1
F
2
F
1
> F
2

7. 3) Force of friction :-
Force of friction is the force which opposes the motion of an object over a
surface.
Eg :- A ball rolling on ground gradually slows down and
comes to a stop due to force of friction.
If we stop pedalling a bicycle, it gradually slows
down and comes to a stop due to force of friction.

An object with uniform motion will continue to move with uniform motion if
the forces acting on it ( pushing force and frictional force ) are balanced.
If an unbalanced force acts on the moving body, then its speed or direction of
motion changes.
If the unbalanced force is removed, then it will continue to move with the
speed it had acquired till then.

8. 4) Galileo’s experiment of motion of an object on an
inclined plane :-

h h

When a marble rolls down an inclined plane, its velocity increases and when it goes up on
the second inclined plane, its velocity decreases. If the inclinations of both the planes are
equal, then the marble will reach the same height which it rolled down. If the inclination of
the second plane is decreased, it will travel more distance to reach the original height. If
the inclination of the second plane is made horizontal, the marble will travel forever trying
to reach the same height. An unbalanced force is required to change the motion of the
marble but no force is needed to sustain the uniform motion of the marble.

10. Newton’s Laws of MotionNewton’s Laws of Motion
•11
stst
Law Law – An object at rest will stay at rest,
and an object in motion will stay in
motion at constant velocity, unless acted
upon by an unbalanced force.
•22
ndnd
Law Law – Force equals mass times
acceleration.
•33
rdrd
Law Law – For every action there is an
equal and opposite reaction.

11. 5) Newton’s laws of motion :-
Newton’s first law of motion states that :- ‘An object remains in a
state of rest or in uniform motion in a straight line unless compelled
to change that state by an applied force.’
Inertia :- The natural tendency of objects to remain in a state of
rest or in uniform motion is called inertia.
This is why the first law of motion is also known as the law of
inertia.

13. 11
stst
Law of Motion Law of Motion
(Law of Inertia) (Law of Inertia)
An object at rest will stay at
rest, and an object in motion
will stay in motion at
constant velocity, unless acted
upon by an unbalanced force.

14. 1
st
Law
•Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
motionless.
These pumpkins will not move unless
acted on by an unbalanced force.

15. 1
st
Law
•Once airborne,
unless acted on
by an
unbalanced
force (gravity
and air – fluid
friction), it
would never
stop!

16. 1
st
Law
•Unless acted
upon by an
unbalanced
force, this golf
ball would sit on
the tee forever.

17. Examples of inertia :-
i) If a striker hits a pile of coins on a carrom board, the lowest coin moves out
and due to inertia of rest, the other coins fall down.
ii) If a coin placed on a playing card over a tumbler is flicked with the finger, due
to inertia of rest, the coin falls down into the tumbler.
iii) When we travel in a car and the driver applies the brakes suddenly, we tend to
fall forward due to inertia of motion.
iv) When we are standing in a bus and the bus begins to move suddenly, we tend
to fall backward because our feet in contact with the floor moves forward but the
upper part of the body continues to remain at rest due to inertia of rest.

18. Don’t let this be you. Wear seat belts.Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped in their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.by the brick wall, your body keeps moving at 80 m/hour.

19. 6) Inertia and Mass :-
A body at rest continues to be at rest and a body in motion
continues to be in motion. This property of a body is called its
inertia.
The inertia of a body is measured by the magnitude of force
required to change the state of the body. The force required to
change the state of a heavier body is more than the force required
to change the state of the lighter body. This is because the mass of
the heavier body is more than the mass of the lighter body.
So ‘The mass of a body is a measure of its inertia.’

20. 7) Momentum of a body :-
The momentum of a body is the product of its mass and velocity.
Momentum = mass x velocity
p = mv where p is the momentum of a body
m is the mass of the body
v is the velocity of the body
If a body is at rest its velocity is zero and so its momentum is also
zero.
The SI unit of momentum is kilogram metre per second or kg m/s
or kg ms
-1

Eg :- A truck moving at a very low speed can kill a person standing in its path
because of the heavy mass of the truck.
A bullet of small mass when fired from a gun can kill a person because of
the large velocity of the bullet.
So the impact of a body depends upon its mass and velocity.

21. 2
nd
Law

22. 2
nd
Law
The net force of an object is
equal to the product of its mass
and acceleration, or F=ma.

23. 2
nd
Law
•When mass is in kilograms and acceleration is
in m/s/s, the unit of force is in newtons (N).
•One newton is equal to the force required to
accelerate one kilogram of mass at one
meter/second/second.

24. 2
nd
Law (F = m x a)
•How much force is needed to accelerate a
1400 kilogram car 2 meters per second/per
second?
•Write the formula
•F = m x a
•Fill in given numbers and units
•F = 1400 kg x 2 meters per second/second
•Solve for the unknown
•2800 kg-meters/second/second or 2800 N

25. 8) Newton’s second law of motion :-
Newton’s second law of motion states that :- ‘ The rate of change of
momentum of an object is proportional to the applied force in the
direction of force.’
Mathematical formulation of Second law of motion :-
If an object of mass m is moving along a straight line with initial velocity u and
is accelerated to velocity v in time t by applying a force F, then
Initial momemtum p
1
= mu
Final momentum p
2 = mv
Change in momentum p
2
– p
1
= mv – mu
= m (v – u )
Rate of change of momentum = m (v – u )
t
Or the applied force F α m (v – u ) or F = k m (v – u ) but (v – u ) = a
t t t
So F = kma where k is a constant of proportionality
Or F = ma
The SI unit of mass is kg and acceleration is m/s
2
or ms
-2
so the unit of Force is
kg ms
-2
or newton . It’s symbol is N

26. Newton’s 2
nd
Law proves that different masses
accelerate to the earth at the same rate, but
with different forces.
•We know that objects
with different masses
accelerate to the
ground at the same
rate.
•However, because of
the 2
nd
Law we know
that they don’t hit the
ground with the same
force.
F = maF = ma
98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s
F = maF = ma
9.8 N = 1 kg x 9.8 9.8 N = 1 kg x 9.8
m/s/sm/s/s

28. 3
rd
Law
•For every action, there is an
equal and opposite reaction.

29. 3
rd
Law
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.

30. 3
rd
Law
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.

31. Newton’s 3rd Law in Nature
•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).

32. 3
rd
Law
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.

33. •Consider the flying motion of birds. A bird flies by use
of its wings. 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.

35. 10) Newton’s third law of motion :-
Newton’s third law of motion states that :- ‘To every action there is an equal
and opposite reaction and they act on two different bodies.’
To prove that action and reaction are equal and opposite :-
Take two spring balances A and B connected together. Fix the spring balance B to
a rigid support. When a force is applied by pulling the free end of the spring
balance A, both the spring balances show the same readings. This shows that the
force exerted by the spring balance A on B is equal but opposite in direction to the
force exerted by spring balance B on A . The force exerted by the spring balance A
on B is action and the force exerted by the spring balance B on A is reaction.
AB

36. Examples of action and reaction :-
i) When a bullet is fired from a gun, it exerts a forward force (action) on the bullet
and the bullet exerts an equal and opposite force on the gun (reaction) and the
gun recoils.
ii) When a sailor jumps out of a boat, he exerts a backward force of the boat
(action) and the boat exerts an equal and opposite force on the sailor (reaction)
and the sailor jumps forward.
Recoil force
on the gun
Accelerating force
on the bullet
Action
Reaction

37. iii) When an air filled balloon is released, the force of the air coming out of the
balloon (action) exerts an equal and opposite force on the balloon (reaction) and
it moves upward.
iv) When a rocket is fired, the force of the burning gases coming out (action)
exerts an equal and opposite force on the rocket (reaction) and it moves upward.

38. Other examples of Newton’s Third
Law
•The baseball forces the
bat to the left (an
action); the bat forces
the ball to the right (the
reaction).

39. 3
rd
Law
•Consider the motion of
a car on the way to
school. A car is
equipped with wheels
which spin backwards.
As the wheels spin
backwards, they grip
the road and push the
road backwards.

40. 3
rd
Law
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.

41. 11) Conservation of momentum :-
The Law of conservation of momemtum states that :-
‘The sum of momenta of two objects before collision is equal to the sum of
momenta after collision provided there is no unbalanced forces acting on them.’
This means that the total momentum of the two objects is unchanged or
conserved by collision.
u
A
u
B
v
A
v
B
F
BA
F
AB
If two balls A and B of masses m
A
and m
B
are travelling in a straight line with
initial velocities u
A
and u
B
and if u
A
> u
B
, the two balls will collide with each other.
During collision at a time t, ball A exerts a force F
AB
on ball B and ball B exerts a
force F
BA
on ball A. If v
A
and v
B
are the velocities of balls A and B after collision,
The momenta of ball A before and after collision are m
A
u
A
and m
A
v
A
and the
momenta of ball B before and after collision are m
B
u
B
and m
B
v
B
.
A B A B BA
m
A
m
Am
B
m
B

42. Change in momentum of ball A during collision = m
A
v
A
– m
A
u
A
(v
A
- u
A
)
Rate of change of momentum of ball A (F
AB
) = m
A
t
Change in momentum of ball B during collision = m
B
v
B
– m
B
u
B

(v
B
- u
B
)
Rate of change of momentum of ball B (F
BA
) = m
B
t
According to Newton’s third law of motion the force F
AB
exerted by
ball A on ball B is equal and opposite to the force F
BA
exerted by ball B
on ball A.
Therefore F
AB
= - F
BA

(v
A
- u
A
) (v
B
- u
B
)
or m
A
= = - m
B

t t
or m
A
v
A
- m
A
u
A
= - m
B
v
B
+ m
B
u
B
or - m
A
u
A
- m
A
u
A
= - m
A
v
A
- m
B
v
B

or m
A
u
A
+ m
B
u
B
= m
A
v
A
+ m
B
v
B

Momentum of the two balls before collision is equal to the
momentum of the two balls after collision.

43. 02/24/17

44. 02/24/17