Force and laws of

motion

Made by
uttam kumar
class – 9 a’
th ‘
1
 force
 A force can be a push or a pull. For example, when you push
open a door you have to apply a force to the door. You also
have to apply a force to pull open a drawer.

 You cannot see a force but often you can see what it does.
Forces can change the speed of something, the direction it is
moving in or its shape. For example, an elastic band gets
2
longer if you pull it.
 Balance force
• When two forces acting on an object are equal in size
but act in opposite directions, we say that they
are balanced forces.
 If the forces on an object are balanced (or if there are
no forces acting on it) this is what happens:
• an object that is not moving stays still
• an object that is moving continues to move at the
same speed and in the same direction

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 Example
• Hanging objects
The forces on this
hanging crate are
equal in size but
act in opposite
directions. The
weight pulls down
and the tension in
the rope pulls up.
The forces on this
hanging crate are 4
 Unbalanced force
• When two forces acting on an object are
not equal in size, we say that they
are unbalanced forces.
If the forces on an object
are unbalanced this is what happens:
• an object that is not moving starts to
move
• an object that is moving changes speed
or direction 5
 example

• Resultant forces
• The size of the overall force acting on an object
is called the resultant force. If the forces are
balanced, this is zero. In the example above, the
resultant force is the difference between the two 6
LAWS OF
MOTION
7
NEWTON
Sir Isaac Newton PRS MP (25 December
1642 – 20 March 1727) was an
English physicist and mathematician who is
widely regarded as one of the most influential
scientists of all time and as a key figure in
the scientific revolution
. Newton also made seminal contributions
to optics and shares credit with Gottfried
Leibniz for the invention of the infinitesimal
calculus.
Newton's Principia formulated the laws of
motion and universal gravitation that
dominated scientists' view of the physical
universe for the next three centuries. It also
demonstrated that the motion of objects on the
8
Earth and that of celestial bodies could be
NewtON’s Laws Of MOtiON
1. 1st 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.
2. 2nd Law – Force equals
mass times acceleration.
3. 3rd Law – For every action 9
 First law of
motion
law of inertia
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 First law
the law of inertia
According to Newton's first law, an
object in motion continues in motion
with the same speed and in the same
direction unless acted upon by an
unbalanced force. It is the natural
tendency of objects to keep on doing
what they're doing. All objects resist
changes in their state of motion. In
the absence of an unbalanced force, 11
According
to
Newton's
first
law, the
marble on
that bottom
ramp 12

should just
 MATHEMATICALLY
FIRST LAW

The first law can be stated mathematically

as:-

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‹#›
 Factors which determine
the Moment of Inertia of a
body
 The mass of the body. Experiments show
that Inertia is directly proportional to the
mass.
 The distribution of mass in the body.

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 Affect of inertia on change
Of Object ‘s Mass

A toy car has a A real car has a large

small mass, so it mass ,so it has a
has a small inertia large mass , so it has
,and hence can be a large inertia, and
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moved easily by hence quite difficult to
Affect of inertia if object
is at rest
 Initially, both the coin
and card ,are in state
of rest. Now when we
hit the card with our
fingers , a force acts
on the card and
changes its state of
rest to that of motion.
The force of flicker
however, does not
acts on the coin and it17
falls into the tumbler.
 Affect of inertia if object is
moving

 This is what happens if passengers do not wear

seat belts while travelling in a car and the car
stops suddenly due to an accident. The large
force of inertia on the body of passengers can
throw passengers violently in forward direction
causing serious injuries.
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Second law of
motion
(F= m x a)
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 momentum
Momentum can be defined as "mass in motion." All objects
have mass; so if an object is moving, then it has
momentum - it has its mass in motion. The amount of
momentum that an object has is dependent upon two
variables: how much stuff is moving and how fast
the stuff is moving. Momentum depends upon the
variables mass and velocity. In terms of an equation, the
momentum of an object is equal to the mass of the object
times the velocity of the object.
Momentum = mass • velocity
In physics, the symbol for the quantity momentum is the
lower case "p". Thus, the above equation can be rewritten
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as
• The units for momentum would be mass
units times velocity units. The standard
metric unit of momentum is the kg•m/s.
While the kg•m/s is the standard metric
unit of momentum, there are a variety of
other units that are acceptable (though not
conventional) units of momentum.
Examples include kg•mi/hr, kg•km/hr, and
g•cm/s. In each of these examples, a
mass unit is multiplied by a velocity unit to21
 Momentum In Everyday Life

• A karate player
is able to break
so many tiles,
because he
strikes with his
hand very, very
fast, producing
a extremely
large
momentum.
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 Second law of motion
• According to the second law of
motion :- The rate of change of
momentum of a body is directly
proportional to the applied
force, and takes place in the
direction in which the force acts.

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•So, newton's second law
of motion can be
expressed as :-
Force ∝ change in momentum / time taken

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MATHEMATICAL FORMULATION OF
SECOND LAW OF MOTION
Suppose an object of mass, m is moving along a straight
line with an initial velocity, u. It is uniformly accelerated to
velocity, ν in time, t by the application of a constant force, F
throughout the time, t. The initial and final momentum of
the object will be, p1 = mu and p2 = mν respectively.
The change in momentum
α p2 – p1
α mν – mu
α m (ν – u).
The rate of change of momentum α m (ν −u) / t
Or, the applied force, F α m (ν −u) / t
Or, the applied force, F = km (ν −u) / t (2)
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= kma (3)
Here a [ = (ν-u) / t] is the acceleration, which
is the rate of change of velocity. The
quantity, k is a constant of proportionality.
The SI units of mass and acceleration are kg
and m s-2 respectively. The unit of force is so
chosen that the value of the constant, k
becomes one. For this, one unit of force is
defined as the amount that produces an
acceleration of 1 m s-2 in an object of 1 kg
mass. That is,
1 unit of force = k (1 kg) (1 m s-2). 26
 Demonstration of second
law of motion

Since the acceleration produced is inversely proportional to the mass of

the object, it is easier to move (or accelerate) a small ball (having small
mass) than a big truck (having large mass ) by the force of our push. 27
• The SI unit of force is newton which
is denoted by N. A newton is that
force which when acting on a body
of mass 1 kg produces an
acceleration of 1m/s2 in it. We
have just seen that
F=ma
Putting m=1kg and a=1m/s2, F
becomes 1 newton.
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So 1 newton = 1kg 1m/s2.
Application of
second law of
motion

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In a cricket match a fielder moves his arms back while trying to catch
a cricket ball because if he tries to stop the fast moving ball suddenly
then the speed decreases to zero in a very short time. Therefore the
retardation of the ball will be very large. As a result the fielder has to
apply a larger force to stop the ball. Thus, if he tries to stop a fast
moving cricket ball the fielder may get hurt as the ball exerts a great
pressure on the hands but if he tries to stop it gradually by moving his
arms back then the velocity decreases gradually in a longer interval 30
of time and hence retardation decreases. Thus the force exerted by
A cushion like surface is made for a „high jump
athlete‟. This reduces the large momentum of
falling athlete more gently. Due to this, less
opposing force acts on the athlete's body and
injuries are prevented. 31
 NewtON’s 2nd 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 2nd Law we
know that they
don‟t hit the ground 32
third law
of motion
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 NewtON’s third Law Of
motion
According to Newton‟s Third Law of Motion
:-

To Every Action There is an

Equal and Opposite Reaction

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Newton‟s third law of motion says :
Whenever one body exerts a force on
another body , the second body exerts an
equal and opposite force on the first body.

The force exerted by the first body is known

as “action” and the force exerted by the
second body on the first body is known as
“reaction”.

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 Examples to
illustrate
third law of motion

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 How Do We Walk

• When we walk on ground , then our foot

pushes the ground backward. The forward
reaction exerted by the ground on our foot
makes us move forward. 37
 Recoiling of gun

• When a bullet is fired from a gun, the force sending

the bullet forward is equal is equal to the force
sending the gun backward. But due to the high
mass of the gun, it moves only a little distance
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backward and gives a backward jerk or kick to
 Flying of jet aeroplanes
and rockets

Modern jet aeroplanes and rockets work on the principle of

action and reaction. In aeroplanes engines exert a
backward force on the exhaust gases; the backward
rushing exhaust gases exert a forward force on the plane
which makes it move forward. 39
 The case of a boat and the
ship

• Diagram shows “action” and • The men push the water

“reaction” when a man steps out backwards with the
of a boat. oars. The backward
going water exerts an
equal and opposite push
on the boat, which 40
makes the boat move
Conservation of
momentum
Momentum is never
created or
destroyed.
When two(or more) bodies act
upon one another, their total
momentum remains constant(or 41
conserved) provided no external
 A Newton's
cradle demonstrates
conservation of momentum.

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• According to law of
conservation of mass
total momentum before
collision=total 43
 Application of law of
conservation of momentum

A rocket works on A jet aeroplane also works on

the principle of the principle of conservation of
conservation of momentum
momentum
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