1) The document discusses Newton's laws of motion and related concepts like balanced and unbalanced forces, inertia, momentum, and impulse. 2) Newton's first law states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. 3) Newton's second law states that the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

1. FORCE AND LAWS OF MOTION
Balanced and unbalanced forces and its examples
NEWTON’S FIRST LAW,SECOND LAW AND THIRD LAW AND ITS EXAMPLES

2. Balanced force
When two forces of equal magnitude but acting in opposite
directions on an object simultaneously then the object
continues in its state of rest of uniform motion in a straight
line. Such forces acting on the object are known as balance
force
 When we push a wall, the wall does not move at all i.e., it remains at rest.
 When we try to push a heavy box on a rough surface, it does not move.

3. Unbalanced force
 When two forces of unequal magnitudes act in opposite
directions on an object simultaneously then the object move
in the direction of a large force. These forces acting on the
object are known as unbalanced forces
 When a boy drags a box on the floor, then an unbalanced
force is acting on the box
 A bicycle will slow down if the rider stop pedalling it.

5. Newton’s first law of motion
 A body at rest remains at rest unless an external unbalanced force acts
on it to change its state of rest.
 Ex. Consider a wooden block kept on a horizontal surface at rest. It will
remain at rest unless somebody moves it

6. Galileo’s law of inertia
 Galileo’s Law of Inertia Galileo studied motion of objects on an inclined plane. He found that
 i) Objects moving down a smooth inclined plane accelerate.

7. ii) Objects moving up a smooth inclined plane retard
 I
iii)objects moving on a frictionless horizontal plane move with a constant velocity, having neither
acceleration nor retardation.

8. EXPERIMENT
 In another experiment using a double inclined plane, Galileo observed that
 i) A ball released from rest on one smooth inclined plane rolls down and climbs up the
other smooth inclined plane. He found that’ In ideal situation, when there is no friction,
the final height of the ball is the same as its initial height. In actual practice, when some
friction is there, final height is somewhat less than the initial height. When the slopes of
the two planes are same, distance covered in rolling down one incline is the same as the
distance’ covered in climbing up the other incline. This is shown in fig

9.  ii)When the slope of second smooth inclined plane is decreased, and the
experiment is repeated, the ball still reaches the same final height. But in doing so,
it travels a larger distance as shown in fig.
 iii)When the slope of second smooth inclined plane is made zero (i.e., the second
plane is made horizontal), the ball travels an infinity distance in the ideal situation
(when there is no friction).
 From his experiments, Galileo concluded that the state of rest and the state of
motion with constant velocity are equivalent. In both cases, no net force is acting
on the body. Galileo emphasized that it is incorrect to assume that a net force is
needed to keep a body in uniform motion along a straight line.

10. Inertia-The tendency of a body to oppose any
change in its state of rest or of uniform motion
is called inertia of the body.
Inertia of rest
Inertia of motion
Inertia of direction

11. Inertia of rest
The resistance offered by a body to change its
state of rest is called inertia of rest.
 EX: When a branch of a tree is shaken vigorously, ripe fruits get detached and fall.
This is because the branch comes in motion but the fruits at rest tend to remain at
rest due to inertia of rest and get detached. After the fruits get detached, gravity
plays its role in making the fruits fall.

12. Inertia of motion
The resistance offered by a body to change its
state of uniform motion is called inertia of
motion
 When we switch off a fan, it continues to rotate for a while due to inertia of motion

13. Inertia of direction-The resistance offered by a body to
change its direction of motion is called inertia of
direction
 When a bus suddenly takes a turn, the passengers sitting
casually experience a jerk in the outward direction. This
happens because the passenger tends to remain in its original
direction of motion due to inertia of direction.

14. INERTIA AND MASS
 Consider two stationary objects say a small table and a big
table Thus, greater is the mass, greater is the inertia.
Hence, inertia of a body is equal to the mass of the body. •
The inertia of an object is measured by its mass, a large
mass, such as that of a freight train, indicating a large
inertia.
 • Inertia is the natural tendency of an object to remain at
rest or in motion at a constant speed along a straight line.
The mass of an object is a quantitative measure of inertia.

15. LINEAR MOMENTUM
DEFINITION : The product of mass and velocity of a body is called Linear
momentum, it is denoted by P. Linear momentum = mass × velocity
Momentum is a vector quantity. Magnitude of momentum, P = mass × speed or
P = mu Direction of momentum of a body is same as that of the direction of the
velocity of the body
Units of momentum
Momentum = mass × velocity
Unit of momentum = unit of mass × unit of velocity
S.I. unit of momentum is kg m/s
In CGS system, unit of momentum is g /cm. The other unit of momentum is N-s

16. QUESTIONS
1.Newton’s first law of motion describes the following
a)Energy b) Work c) Inertia d) Moment of inertia
2. When a bus suddenly takes a turn, the passengers are thrown outwards because of
a)Inertia of motion b) Acceleration of motion c) Speed of motion d) Inertia of Direction
3. When a horse pulls a cart, the force that helps the horse to move forward is the force exerted by
a)the cart on the horse b) the ground on the horse
c) the ground on the cart d) the horse on the ground
4. If no external force acts on a body, it will :
a) move with more speed
b) change its shape
c) break into pieces
d) either remain in its state of rest or in uniform motion.
5. Which of the following are vector quantities:
a) Momentum b) velocity c) force d) all of the above.

17. 6.When balanced forces act on a body, the body:
a)must remain in its state of rest
b) must continue moving with uniform velocity, if already in motion
c) must experience some acceleration
d) Both (a) and (b)
7. When unbalanced forces act on a body, the body:
a) must move with uniform velocity
b) must remain at rest
c) must experience acceleration
d) must move in a curved path
8. Which of the following are categorized into contact forces
a) Frictional force
b) Tension forces as applied through string
c) Force exerted during collision
d) All of these

18. NEWTON’S SECOND LAW OF MOTION
 The change in momentum of a body per unit time (i.e. rate of change of
momentum) is directly proportional to the unbalanced force acting on the body
and the change in momentum takes place in the direction of the unbalanced
force on the body
where, dp = change in momentum and dt = time taken for this change in momentum

19. DERIVATION OF SECOND LAW OF MOTION
 Consider a body of mass m moving with initial velocity u. Let a force F acts on the body for time t . Initial
momentum of the body, Pi = mu Final momentum of the body, Pf = mu Now, change in momentum of the
body = Pf – Pi = mu – mu = m(u – u) Time taken to change this momentum = (t – 0) = t
 we get F = ma Thus, force acting on the body is directly proportional to (i) its mass (m) and (ii) its
acceleration (a). Equation (i) gives the mathematical form of Newton’s second law of motion.

20. Newton’s second law of motion in vector
form
Newton’s first law of motion is a special case of Newton’s second law of motion

21. UNITS OF FORCE
SI UNIT OF FORCE
 Definition of newton (N) :The force is said to be 1 newton if it products acceleration
in a body of 1 kg mass
CGS UNITS OF FORCE

22. Newton’s Second Law in Terms of Linear Momentum
 DEFINITION : The rate of change of momentum of a body with respect to time is directly
proportional to the external force acting on the body and takes place in the direction of
force. Suppose a body of mass m is acted upon by an unbalanced external force F which
creates an acceleration a in the body. Let the initial velocity of the body be u. Let the
force continues to act for a time interval t and the final velocity of the body be v.

23. IMPULSE
 The forces which act on bodies for short time are called
impulsive forces.
 For example: i) In hitting a ball with a bat.
 ii) In driving a nail into a wooden block by a hammer.
 An impulsive force does not remain constant but changes first from zero to
maximum and then from maximum to zero. Thus it is not possible to measure
easily the value of impulsive force because it changes with time. In such cases, we
measure the total effect of the force, called impulse.
 Hence impulse of a force is a measure of total effect of the force. It is given by the
product of average force and the time for which the force acts on the body; i.e.,
Impulse = Change in linear momentum
 Impulse is a vector quantity SI unit of impulse is N-s and kg m/s

24. When a tennis ball hits the
racket, it is supplied with a
high magnitude and short
duration force that helps to
change the direction of
motion of the ball. The
force exerted by the player
acting on the ball is the
impulsive force.

25. Applications for the concept of impulse
 When a person falls from a certain height on a cemented floor, the floor does not yield.
The total change in linear momentum is produced in a smaller interval of time. The floor
exerts a much larger force. Due to it, a person receives more injury. On the other hand,
when a person falls on a heap of sand, the sand yields. The same change in linear
momentum is produced in much longer time and hence the person is not hurt.
 China wares and glasswares are wrapped in paper or straw pieces before packing. In the
event of fall, impact will take a longer time to reach the glass/chinawares through
paper/straw. As a result, the average force exerted on the china or glasswares is small
and chances of their breaking reduce.
 It is difficult to catch a cricket ball than to catch a tennis ball. The cricket ball being
heavier has much larger momentum and therefore, exerts a much larger force on the
hands during catch, in comparison to the force exerted by tennis ball.

26. NEWTON’S THIRD LAW OF MOTION
 DEFINITION : For each and every action, there is equal and opposite reaction.
 Action and reaction acts on different bodies hence they never cancel each
other
 Action and reactions forces occur simultaneously. It is wrong to think that
first action occurs and it is followed by reaction.

27. Examples of Newton’s third law
 While walking or running, you push the ground in the backward direction
with your feet. The ground simultaneously exerts a force of equal magnified
in the forward direction on feet. This force enables us to walk.
 When a man jumps from a boat, the boat also experiences a backward jerk.
This is due to the action-reaction pair .
 Inflate a balloon and leave it. You will observe that the balloon moves in
opposite direction to the opening in balloon through which the air is coming
out

29. Law of conservation of momentum
 If a moving body strikes a body at rest, the moving body slows down and the stationary body
starts moving. Whereas the first body loses momentum, the second body gains momentum.
We shall observe that the total momentum before impact is equal to total momentum after
impact. If two bodies of masses m1 , m2 are initially moving with velocities u1 , u2 and after
collision they start moving with velocities v1 and v2 respectively, then

30. Proof for the law of conservation of
momentum

31. Examples to illustrate the law of
conservation of momentum
 Rocket propulsion (Movement of a rocket in the upward direction) The movement of a
rocket in the upward direction can also be explained with the help of the law of
conservation of momentum. The momentum of a rocket before it is fired is zero. When the
rocket is fired, gases are produced in the combustion chamber of the rocket due to the
burning of fuel. These gases come out of the rear of the rocket with high speed. The
direction of the momentum of the gases coming out of the rocket is in the downward
direction. To conserve the momentum of the system (rocket + gases), the rocket moves
upward with a momentum equal to the momentum of the gases. The rocket continues to
move upward as long as the gases are ejected out of the rocket
 Inflated balloon lying on the surface of a floor moves forward when pierced with a pin. The
momentum of the inflated balloon before it is pierced with a pin is zero. When it is pierced
with a pin. air in it comes out with a speed in the backward direction. To conserve the
momentum, the balloon moves in the forward direction.

32. Questions
 1. A particle is moving with a constant speed along a straight line path. A force is not required to
a) Increase its speed b) Decrease the momentum
c) Change the direction d) Keep it moving with uniform velocity
 2. When a bus suddenly takes a turn, the passengers are thrown outwards because of
a) Inertia of motion b) Acceleration of motion
c) Speed of motion d) Inertia of Direction
 3. China wares are wrapped in straw or paper before packing. This is the application of concept of:
a) Impulse b) Momentum c) Acceleration d) Force
 4. The principle of conservation of linear momentum states that in a system it :
a) cannot be changed b) can be changed, if internal forces act on it
b) c) can be changed, if external forces act on it d) none of the above
5. A bird weighs 2 kg and is inside a closed cage of 1 kg. If it starts flying, then what is the weight of the bird and
cage assembly
a) 1.5 kg b) 2.5 kg c) 3 kg d) 4 kg

33.  6.A man is at rest in the middle of a pond on perfectly smooth ice. He can get himself to the shore
by making use of Newton’s
a) First law b) Second law c) Third law d) All the laws
 7. A machine gun fires a bullet of mass 40 g with a velocity 1200m/s. The man holding it can exert a
maximum force of 144 N on the gun. How many bullets can he fire per second at the most
a) One b) Four c) Two d) Three
 8. Action-reaction forces:
a) act on same body b) act on different bodies
c) act along different lines d) act in same direction
 9. A rocket works on the principle of:
a) conservation of energy b) conservation of linear momentum
c) conservation of inertia d) conservation of force
 10.Two bodies collide at the same time. Which of the following is conserved?
a) velocity b) momentum c) kinetic energy d) force
 11. Impulse is equal to :
a) the change in force b) the change in momentum
c) the change in velocity d) all the above

34.  12.What is the momentum of a person of mass 75kg when he walks with a uniform velocity
of 2m/s
a) 100 kg m/s b) 200 kg m/s c) 150 kg m/s d) 125 kg m/s
 13. A car of mass 200 kg is moving with a speed of 20 m/s, after 25 seconds the velocity
increased by 10 m/s, then what is the change in momentum?
a) 4000kg m/s b) 3000kg m/s c) 1000kg m/s d) 2000kg m/s
 14.A force of 5 N acts on a body of weight 9.8 N. What is the acceleration produced in
a) 49.00 b) 5.00 c) 1.46 d) 0.51

35. HW
 Why a fireman struggles to hold a hose-pipe ?
 SOLVE:
 A boy of mass 30 kg while running at constant velocity has a momentum .of 180 Ns. The
constant velocity of the boy is
a) 3 m/s b) 6 m/s c) 18 m/s d) 12 m/s
 A dish of mass 20 g is kept horizontally in air by firing bullets of mass 10 gm each at the
rate of 10 per sec. If the bullets rebounded with the same speed, what is the velocity with
which the bullets are fired?
a) 49 cm/sec b) 98 cm/sec c) 147 cm/sec d) 196 cm/sec
 A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg along a horizontal track. If
the engine exerts a force of 40,000 N and track offers a friction of 5000 N, then net
accelerating force acting on the system is :
a) 45,000 N b) 40,000 N c) 35,000 N d) none of the above