Amaze Academy (Er.
Ankit Goyal)
Ch-8:Force & Laws Of Motion
Force: It is an influence that can change the motion of an object
Or ( Push or Pull that causes a change in an object )
Examples: kicking, catching etc..
EFFECTS OF FORCE A force applied on an object can produce the
following changes:
1. A force can change a state of rest. For Example – If we kick a football kept on
the ground, then the football starts moving; If we push a resting table, then the
table will start moving. In both the cases, the force applied has changed the state
of rest.
2. A force can stop a moving body. For Example – A football moving on ground
stops after sometime, because of the force of friction of ground.
3. A force can change the speed of an object. For Example – A speed of a moving
car can be increased by accelerating the car or can be decreased or stopped by
applying the brakes.
4. A force can change the direction of motion of an object. For Example – A
force exerted on the steering wheel of a car changes the direction of motion; A
force exerted by the cricket player’s bat changes the direction of moving cricket
ball.
5. A force can change the shape of an object. For Example – If we pull a spring
from both the ends, then the shape of spring changes.
BALANCED FORCES: If the resultant of all the forces acting on a body is zero,
the forces are called balanced forces. For Example - A game of tug of war with
equally matched opponents is an example of balanced force; When the man
pushes against the wall, the forces at play balance each other, and as a result, the
wall does not move.
UNBALANCED FORCES: If the resultant of all the forces acting on a
body is not zero, the forces are called unbalanced forces. For Example -
The ball moves from one place to another after it’s kicked.
Inertia : It is tendency of an object to resist changes in the state of
motion or rest.
TYPES OF INERTIA
a) Inertia of rest – The tendency of a body to remain in its state of rest is
called inertia of rest.
Example – A person standing in a bus falls backward when the bus
suddenly starts moving forward. When the bus moves, the lower part of
his body begins to move along with the bus while the upper part of his
body continues to remain at rest due to inertia. That is why, a person
falls backward when the bus starts.
b) Inertia of motion – The tendency of a body to remain in its state of
uniform motion in a straight line is called inertia of motion.
Example – When a moving bus suddenly stops, a person sitting in it falls
forward. As the bus stops, the lower part of the body comes to rest
along with the bus while the upper part of body continues to remain in
motion due to inertia and falls forward.
c) Inertia of direction – The inability of a body to change its direction of
motion is called inertia of direction.
Example – When a bus takes a sharp turn, a person sitting in the bus
experiences a force acting away from the centre of the curved path due
to his tendency to move in the original direction.
Note: Inertia depend on mass If a body has more mass, it has more inertia. And
if a body has less mass, it has less inertia.
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1. Newton’s First Law Of Motion: According to Newton’s first law of
motion, a body at rest will remain at rest, and a body in motion will
continue its uniform motion in a straight line until and unless some
external force is applied.
(A) Inertia of Rest
(i) Dust is removed from a hanging carpet by beating it with a stick. As
the carpet is beaten, it suddenly moves forward while the dust particles
tend to remain at rest due to inertia of rest and so fall off.
(ii) Coin falls into the tumbler when the card is given a sudden jerk.
Place a coin on a playing card covering a glass tumbler. Give a sudden
jerk to the card, the card flies off and the coin drops into the tumbler.
This is because the coin tends to remain at rest due to inertia.
(B) Inertia of Motion
(i) An athlete runs for a certain distance before taking a long jump. The
inertia of motion gained by him at the time of jumping adds to his
muscular effort and helps him in taking a lower jump.
(ii) A ball thrown upward in a moving train comes back into the
thrower’s hands. The ball acquires a horizontal velocity of the train and
maintains it inertia of motion during its upward and downward motion.
In this period the ball covers the same horizontal distance as the train,
so it comes back to thrower’s hands.
(C) Inertia of Direction
(i) During the sharpening of a knife, the sparks coming from the grind
stone fly off tangentially. This is due to inertia of direction.
(ii) When a stone is tied to one end of a string is whirled and if the string
or thread suddenly breaks then the stone flies off along the tangent to
the circle. This is due to inertia of direction.
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Momentum of a body is the quantity of motion possessed by the body.
It is equal to the product of mass and velocity of a body.
Momentum = Mass x Velocity
p = m x v, where, p = Momentum m = Mass of the body v = Velocity of body
Momentum is a vector quantity.
Its direction is same as the direction of the velocity of the body.
S.I unit of momentum is kg m/s.
Every moving body possesses momentum.
Change in momentum = Final momentum – Initial momentum = mv – mu
2. Newton’s Second Law Of Motion
According to Newton’s second law of motion, the rate of change of
momentum of a body is directly proportional to the external force
applied on the body and the change takes place in the direction of the
applied force.
Force ∝ 𝐂𝐡𝐚𝐧𝐠𝐞 𝐢𝐧 𝐦𝐨𝐦𝐞𝐧𝐭𝐮𝐦 𝐓𝐢𝐦𝐞 𝐓𝐚𝐤𝐞𝐧
Newton’s second law of motion gives the measurement of force.
Consider a body of mass ‘m’ having an initial velocity ‘u’. The initial
momentum of this body will be ‘mu’. Suppose a force ‘F’ acts on a body
for time ‘t’ and cause the final velocity to become ‘v’. The final
momentum of this body will be ‘mv’. Now, the change in momentum of
this body is ‘mv – mu’ and the time taken for this change is ‘t’. So,
according to Newton’s second law of motion:
F = 𝐦𝐯−𝐦𝐮 /𝐭 Or,
F = m(𝐯−𝐮) /𝐭 Or,
F=mxa
S.I unit of force is Newton(N) or kg m/s2 . @AmazeAcademy
Applications Of Newton ‘s Second Law Of Motion:-
(i) A cricket player moves his hands backward while catching a fast
cricket ball. A cricket player moves his hands backward while catching a fast
cricket ball so that the momentum of the ball reduces to zero as the momentum
of the fast-moving ball is large. By moving his hand, he is increasing the time
taken thereby reducing the momentum.
(ii) The use of seat belts in cars. The cars are provided with seat belts for
passengers to prevent injuries in case of an accident. In an accident, the fast
running car stops suddenly. Due to this the momentum of the car reduces to zero
in a very short time. The stretchable seat belts worn by the passengers increase
the time taken by the passengers to fall forward.
3. Newton’s Third Law Of Motion: According to Newton’s third law
of motion, To each and every action, there is an equal and opposite
reaction.
Forces in nature always occur between pair of bodies. Force on body A
by body B is equal and opposite to the force on the body B by A. i.e. FAB
= - FBA
Applications Of Newton ‘s Third Law Of Motion:-
(i) Walking of a person. A person is able to walk because of the
Newton’s Third Law of Motion. During walking, a person pushes the
ground in backward direction and in the reaction the ground also pushes
the person with equal magnitude of force but in opposite direction. This
enables him to move in forward direction against the push.
(ii) Recoil of gun: When bullet is fired from a gun, the bullet also pushes
the gun in opposite direction, with equal magnitude of force. This results
in gunman feeling a backward push from the butt of gun.
