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SCIENCE
FORCE AND LAWS OF MOTION




A ball on the ground, when given a small hit, does not move forever
...

 This remained the belief until Galileo Galilei and Isaac Newton developed an entirely
different approach to understand motion
...
The concept of force is based on this
push, hit or pull
...
However,
we always see or feel the effect of a force
...

 Pushing, hitting and pulling of objects are all ways of bringing objects in motion
...
The SI unit of force is kg m s2
...

 We also know that a force can change the shape and size of objects
...
 In Balanced Force, a body continues to be in its
position
...
 Unbalanced forces are not equal and opposite
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INERTIAL :- a property of matter by which it continues in its existing state of rest or uniform
motion in a straight line, unless that state is changed by an external force
...


 2ND LAW OF MOTION
The second law states that the acceleration of an object is dependent upon two
variables - the net force acting upon the object and the mass of the object
...
 Your bicycle is
the mass
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 3RD LAW OF MOTION
 Third law states that for every action (force) in nature there is an equal and opposite

reaction
...
For a collision
occurring between object 1 and object 2 in an isolated system, the total momentum of the two
objects before the collision is equal to the total momentum of the two objects after the collision
...
Which of the following has more inertia:
(a) a rubber ball and a stone of the same size?
(b) a bicycle and a train?
(c) a five-rupees coin and a one-rupee coin?
Answer:
(a) A stone of the same size
(b) a train
(c) a five-rupees coin
As the mass of an object is a measure of its inertia, objects with more mass have more inertia
...
In the following example, try to identify the number of times the velocity of the ball
changes
...

Also identify the agent supplying the force in each case
...
Explain why some of the leaves may get detached from a tree if we vigorously
shake its branch
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 Due to the inertia of rest, the leaves tend to remain in its position and hence detaches from
the tree to fall down
...
Why do you fall in the forward direction when a moving bus brakes to a stop and fall
backwards when it accelerates from rest?
Answer: When a moving bus brakes-to a stop: When the bus is moving, our body is also in motion, but due
to sudden brakes, the lower part of our body comes to rest as soon as the bus stops
...

When the bus accelerates from rest we fall backwards: When the bus’ is stationary our body is at rest but
when the bus accelerates, the lower part of our body being in contact with the floor of the bus comes in
motion, but the upper part of our body remains at rest due to inertia of rest
...


NCERT Textbook – Page 126-127
Question 1
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In this case the horse exerts a force on the ground with its feet while walking, the ground exerts an
equal and opposite force on the feet of the horse, which enables the horse to move forward and
the cart is pulled by the horse
...
Explain, why is it difficult for a fireman to hold a hose, which ejects a large
amount of water at a high velocity
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 It becomes difficult for a fireman to hold a hose
which experiences this large momentum
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From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m/s
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Answer:

Question 4
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They collide and after the collision the first object moves at a velocity of 1
...
/s
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Answer:

Extra Questions from NCERT Textbook for Class 9 Science
Question 1
...
Is it possible for the object
to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the
magnitude and direction of the velocity
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Answer: When an object experiences a net zero external unbalanced force, in accordance with second law
of motion its acceleration is zero
...
 It means that the
object may be travelling with a non-zero velocity but the magnitude as well as direction of velocity must
remain unchanged or constant throughout
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When a carpet is beaten with a stick, dust comes out of it
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Answer: The carpet with dust is in state of rest
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 Due to inertia of rest the dust particles retain their position of rest and
falls down due to gravity
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Why is it advised to tie any luggage kept on the roof of a bus with a rope?
Answer: In moving vehicle like bus, the motion is not uniform, the speed of vehicle varies and it
may apply brake suddenly or takes sudden turn
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To avoid the fall of the luggage, it is tied with the rope
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A batsman hits a cricket ball which then rolls on a level ground
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The ball slows to a stop because
(a) the batsman did not hit the ball hard enough
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(c) there is a force on the ball opposing the motion
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Answer: (c) there is a force 6n the ball opposing the motion
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A truck starts from rest and rolls down a hill with a constant acceleration
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Find its acceleration
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Answer:

Question 6
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What is the force of friction between the stone
and the ice?
Answer:

Question 7
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If
the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate:

Answer:

Question 8
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What must be the force between the
vehicle and road if the vehicle is to be stopped with a negative acceleration of 1
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What is the momentum of an object of mass m, moving with a velocity v?
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Using a horizontal force of 200 N, we intend to move a wooden cabinet across
a floor at a constant velocity
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 Hence the frictional force that will be exerted on the cabinet will be less than 200 N
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Two objects each of mass 1
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The velocity of each object is 2
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What will be the velocity of the combined object after collision?
Answer:

Question 12
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If the object is a massive truck parked along the
roadside, it will probably not move
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Comment on this logic and explain why the truck does not move
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 The small force exerted on the
truck cannot move it and the truck remains at rest
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Question 13
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Calculate the change of momentum
occurred in the motion of the hockey ball by the force applied by the hockey stick
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A bullet of mass 10 p travelling horizontally with a velocity of 150 m-1 strikes a
stationary wooden block and comes to rest in 0
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Calculate the distance of penetration of the
bullet into the block
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Answer:

Question 15
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Then they both move off together in the

same straight line
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Also, calculate the velocity of the combined object
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An object of mass 100 kg is accelerated uniformly from a velocity of 5 ms-1 to 8 ms-1 in
6 s
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Also, find the magnitude of the force
exerted on the object
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Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high
velocity on an expressway when an insect hit the windshield and got stuck on the
windscreen
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Kiran suggested that
the insect suffered a greater change in momentum as compared to the change in
momentum of the motorcar (because the change in the velocity of insect was much more
than that of the motorcar)
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And as a result the insect died
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Comment on these suggestions
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 As per the law of conservation of
momentum
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Question 18
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Answer:

ADDITIONAL EXERCISES
Question 1
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 Applying the relation

Question 2
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The same motorcar can be pushed by three persons to produce an
acceleration of 0
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With what force does each person push the motorcar? (Assume
that all persons push the motorcar with the same muscular effort
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When two persons push the car, they just manage to move it at a uniform velocity
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When three persons push the car, they apply a total force 3F on the car
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A hammer of mass 500 g, moving at 50 ms-1, strikes a nail
...
01 s
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A motorcar of mass 1200 kg is moving along a straight line with a uniform
velocity of 90 km/h
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Calculate the acceleration and change in momentum
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Answer:

-ve sign of acceleration, charge in momentum and force suggests that the force is opposing the
motion of motor car
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A large truck and a car, both moving with a velocity of magnitude v, have a
head- on collision and both of them come to a halt after that
...
 Hence both
vehicles experience same (equal) force of impact
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But mass of truck is much more than that of car, hence change in momentum of truck is more than
change in momentum of car
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(d) Car suffers more damage than the truck, as acceleration of car is more, its velocity falls to zero
in a shorter time and consequently, its momentum changes in a shorter time
...
Define force
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 4
Question 2
...
I
...
I
...

Question 3
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Answer: A force of one Newton produces an acceleration of 1 m/s2 on an object of mass 1 kg
...
What is balanced force?
Answer: When forces acting on a body from the opposite direction do not change the state of rest
or of motion of an object, such forces are called balanced forces
...
What is frictional force?
Answer: The force that always opposes the motion of object is called force of friction
...
What is inertia?
Answer: The natural tendency of an object to resist a change in their state of rest or of uniform
motion is called inertia
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State Newton’s first law of motion
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Question 8
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Answer: The rate of change of momentum of an object is proportional to the applied unbalanced
force in the direction of the force
...
What is momentum?
Answer: The momentum of an object is the product of its mass and velocity and has the same
direction as that of the velocity
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 I
...
 (p = mv)
Question 10
...

Answer: To every action, there is an equal and opposite reaction and they act on two different
bodies
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Which will have more inertia a body of mass 10 kg or a body of mass 20 kg?
Answer: A body of mass 20 kg will have more inertia
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Name the factor on which the inertia of the body depends
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Question 13
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Answer: Two factors on which momentum of a body depend is mass and velocity
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Question 14
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Question 15
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Forces A, B,
C and D are acting on the truck
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Answer: The forces A, B, C and D acting on the truck are:

EXTRA QUESTIONS
Question 1
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Answer:

Question 2
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It can change the speed of a body
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It can change the direction of motion of a body,
3
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Question 3
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Explain why?
Answer: It is due to law of inertia
...
 But when an unbalanced force is applied by the engine to
change the direction of motion of the motorcar
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Question 4
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Answer: While moving in a bus our body is in motion
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 While the upper part of our body stays in motion and moves

forward due to inertia of motion and hence we can fall in forward direction
...

Question 5
...
 fast moving cricket ball, a fielder on the ground gradually pulls his
hands backwards with the moving ball
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 Thus, the acceleration of the ball is
decreased and therefore the impact of catching the fast moving ball is reduced
...
In a high jump athletic event, why are athletes made to fall either on a
cushioned bed or on a sand bed?
Answer: In a high jump athletic event, athletes are made to fall either on a cushioned bed or on a
sand bed so as to increase the time of the athlete’s fall to stop after making the jump
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Question 7
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Question 8
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In an isolated system, the total momentum remain conserved
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Why are roads on mountains inclined inwards at turns?
Answer: A vehicle moving on mountains is in the inertia of motion
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Question 10
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Question 11
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 But when driver applies sudden breaks or stops the car our body tends to continue in
the same state of motion because of its inertia
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 Hence, safety belt exerts a force on our body to make the forward motion
slower
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Explain how momentum gets conserved in collision of two bodies
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e
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 The two bodies collide and force is exerted by each body
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∴ The total momentum of the two balls remains unchanged or conserved provided no other
external force acts
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When you kick a football it flies away but when you kick a stone you get huh
why?
Answer: This is because stone is heavier than football and heavier objects offer larger inertia
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 When we kick (action) the stone it exerts an equal and opposite force (reaction) and hence
it hurts the foot
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If a person jumps from a height on a concrete surface he gets hurt
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Answer: When a person jumps from a height he is in state of inertia of motion
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Question 15
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Second law states/explains about the amount of force required to produce a given acceleration
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Question 16
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Answer: Three examples based on Newton’s third law are :
1
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action – water is pushed behind
reaction – water pushes the swimmer ahead
2
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action – gun exerts force on the bullet
reaction – bullet exerts an equal and opposite force on the gun
3
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A bullet of m
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What is the
recoil velocity of the pistol?

Answer:

Question 18
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Two bodies as shown in the figure collide with each other and join thereafter

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