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EASY NOTES ON PHYSICS-IX

UNIT 3 ( DYNAMICS-I) by Farooq Iqbal


Q.1 Define Dynamics.
Ans. Dynamics: the branch of mechanics that deals with the study of motion of an object
and the cause of its motion is called Dynamics.
FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC
Q.2 Define the terms Force. SCHOOL RWP (0334-5044822)

Ans. Force: A force moves or tends to move, stops or tends to stop the motion of a body.
The force can also change the direction of motion of the body.
Q. Differentiate with examples between contact and Non contact forces, also, explain
fundamental forces and the role of Dr.Abdus Salam from Pakistan in unifying two
fundamental forces.

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Types of forces: Forces are classified as contact and non contact forces.
Contact forces: The force acting between two objects that are in physical contact are
termed as contact forces. e.g cricketer hitting a ball , friction force b/w two surfaces.
Contact forces are Normal forces, frictional forces, Tension, elastic forces,etc.
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Non-contact forces: The force acting at a distance without physical contact are called non
contact forces. e.g magnetic force, gravitational force, Electrostatic force.
Fundamental Forces in Nature: There are four fundamental forces in nature.
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A. Strong Nuclear Force: The force which keeps the Protons tightly packed in the
nucleus of an atom is called strong nuclear force.it has very short range nearly equal
to the diameter of a proton(10-15m). It is stronges of all the fundamental forces.
B. Electromagnetic Force: it acts between electric charges due to electric and magnetic
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effects. This force is mediated by a massless particle known as photon. Its range is
infinite.
C. Gravitational force: it is the force of attraction b/w all objects in the universe. It is
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weakest force.. this force is theorized to be an exchange force with a massless


mediating particle graviton. This graviton is not detected yet. Its range is infinite.
D. Weak Nuclear Force: it is very weak force, 104 times weaker than strong nuclear
force. It is an exchange force mediated by three particles called vector bosons. It is
responsible for radioactive decay.
Role of Dr.Abdus Salam: Electroweak Theory: This theory was given in 1960 by
Pakistani Scientist Abdus Salam and was awarded physics nobel prize in 1979.This theory
shows the unification of weak nuclear forces and the electromagnetic forces, one into
another. That is transmitted by four carrier particles, the so called gauge bosons. One is
photon of electromagnetism, while other three are electrically charged W+, W- and the
neutral Z0 .
Q. How many types of force Diagrams have? FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC
SCHOOL RWP (0334-5044822)
Ans. Force Diagram have two types. System Diagrams (SD) and Free Body diagram (FBD)
System Diagrams: it is visual expression of all the objects required.
Free Body Diagrams: it is a schematic representation in which only the object being
analyzed is drawn, with arrows showing all the forces acting on the object.
Q. Represent the forces acting on a body using free body diagram.
Ans. See fig 3.5 in book page 66 FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC
Q. What is meant by net force/Resultant force? SCHOOL RWP (0334-5044822)

Ans. Net Force: A resultant/ net force is a single force that has the same effect as the
combined effect of all the forces to be added.

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Q. Define inertia. Why is it important to have knowledge of inertia in our daily life?

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Elaborate your answer with examples.
Ans. Inertia: inertia of a body is its property due to which it resists any change in its state
of rest or motion. ineria depends upon mass of the body. greater the mass of an object the
greater its inertia and vice versa.
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Let us perform an experiment to understand inertia.
Experiment. Take a glass and cover it with a piece of cardboard. Place a coin on the
cardboard. Now kick the card horizontally with a jerk of your finger. The coin falls into the
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glass due to inertia.
Importance of inertia: knowledge of inertia helps us to control the effects of motion in our
daily activities, enhancing safety, efficiency and performance in various tasks. Some
examples are given below.
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1. Driving and Road Safety: Braking: when driver apply brakes, inertia causes the vehicle
to continue moving forward until the brakes exert enough force to stop it. Knowing
this helps drivers maintain safe distances from other vehicles. Turning: During sharp
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turn, driver must slow down to avoid skidding.


2. Household activities: when moving furniture, understanding that heavier objects have
more inertia helps to planning the effort needed to move them and prevents
accidents.
3. Sports and Exercise: when running, your body’s inertia wants to keep you moving at a
constant speed. Changing direction or stopping suddenly requires additional force to
avoid falling.
Q.3. Define Momentum, what is its formula and unit? Is it a vector or scalar quantity?
Show that unit of momentum, Ns and kgm/s are equal.
Ans. Momentum: it is the quantity of motion it possesses of its mass and velocity. Or The
product of mass and velocity is called momentum. It is denoted by P so P = mv unit:
Kgms-1 which is equal to Ns. Momentum is a vector quantity .

FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC


SCHOOL RWP (0334-5044822)
Proof: Ns = Kgms-1
As we know that N = kgms-2 multiplying both sides by s FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC
Then Ns = Kgms-2 s so Ns = kgms-1 SCHOOL RWP (0334-5044822)

Q.4. State first law of motion. Explain with the help of examples.
Ans. First law of motion: Statement: A body continues its state of rest or of uniform motion
in a straight line provided no net force acts on it.
Explanation: According to first law a body at rest remains at rest provided no net force acts
on it. This part is true in our observation that objects do not move by themselves. For
example a book lying on the table remains at rest . Similarly a moving objects does not stop
moving by itself if no net force acts on them. We see that a ball rolled on a ground stops

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after some time. It is due to friction. If there would be no force to oppose the motion of a

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body then the moving body would never stop.
Q.5. Why first law of motion is also called law of inertia?
Ans. Newton’s first law of motion deals with the inertial property of matter , therefore it is
known as law of inertia.
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Q.6. Passengers standing in a bus fall forward when driver applies brakes, why?
Ans. It is due to inertia, because the upper part of the bodies tend to continue their motion,
while lower parts of their bodies in contact with the bus stop with it. Hence , they fall
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forward.
Q.7. When a bus takes a sharp turn, passengers fall in outward direction, why?
Ans. It is due to inertia they want to continue their motion in a straight line and thus fall
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outward. FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC


Q.7a Why is it dangerous to travel on the roof of a bus? SCHOOL RWP (0334-5044822)

Ans. Because when driver takes a sharp turn there are chances for passenger on the roof of
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bus to fall from roof due to inertia. Similarly if driver applied sudden brake then there are
chances for passengers to fall forward due to inertia.
Q.8. State and prove Newton’s second law of motion. Deduce Newton’s Second law of
motion from its first law.
Ans. Second law of motion: Statement: when a net force acts on a body, it produeces
acceleration in the body in the direction of the net force. The magnitude of this acceleration
is directly proportional to the net force acting on the body and inversely proportional to its
mass. Mathematically F = ma
Prove: if a force produces an acceleration a in a body of mass m , then
a ∝ F and a ∝ 1/m combining these two relations a ∝ F/m or F = kma where k is
proportionality constant. Its value in SI units comes out to be 1. So F = ma
Deduction of 2nd law from first law of Motion: According to first law of motion, A body
continues its state of rest or of uniform motion in a straight line provided no net force acts
on it. Mathematiclllay Fnet = 0 , let suppose if Fnet = 0 then body will move with some
velocity. Then acceleration will produced. then net force has magnitude which is equela to
mass times acceleration. Fnet = ma as mass is constant and never changed with force . so
acceleration will be directly propotional to net force applied. a ∝ Fnet and also acceleration
1
is inversely proportional to mass of the body. a ∝ . which is second law of motion.
m
FAROOQ IQBAL SST, F.G.SIRSYED
PUBLIC SCHOOL RWP (0334-5044822)
Q.9. Define 1 Newton force.
Ans. 1 Newton: one newton is the force that produces an acceleration of 1 ms-2 in a body of
mass 1 kg. mathematically 1N = 1kgms-2

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Q.10. Differentiate b/w mass and weight? FAROOQ IQBAL SST, F.G.SIRSYED
PUBLIC SCHOOL RWP (0334-5044822)
Ans. Difference b/w mass and weight:

Mass Weight
1 mass of a body is the quantity of matter 1 weight of a body is the force with which
possessed by a body.
AIS earth attracts a body towards its center.
2 – mass is a scalar quantity. 2- weight is a vector quantity.
3- mass remains constant everywhere. 3- weight varies with altitude.
4- m = w/g 4- w = mg
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5- unit of mass is kg. 5- unit of weight is Newton.
Q. What are gravitational field and gravitational field strength?
Ans. Gravitational Field: The region around which gravitational force is operative is termed
as gravitational field.
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Gravitational field strength: it is the amount of force per unit mass acting on objects in the
gravitational field. Its value is equal to value of ‘g’ at any point. Its unit is N/kg . it can also be
represented by ‘g’. it is a vector quantity and its direction is towards the center of earth.
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Q. Justify and illustrate the use of electronic balances to measure mass.


Ans. Electronic balances are designed to provide highly accurate and precise measurement
of mass. This is crucial in science experiments where even small deviations can effect
experiment results. Moreover, these are easy to use and provide rapid measurement.
In school laboratory, students can learn about mass measurement using electronic balances.
This enhances their understanding of practical skills.
Q.11. State Newton’s Third law of motion. Explain with examples from daily life.
Ans. Newton’s Third law of motion: Statement: To every action there is always an equal but
opposite reaction.
When an object A exerts a force on a second object B written as FAB object B also exerts
equal force on object A written as FBA but in opposite direction. Mathematically FAB = - FBA
Examples: 1. consider a book lying on the table . The weight of the book is acting on the
table which is action while the reaction of the table acts on the book in upward direction.
2. Take an air filled balloon. When the balloon is set free, the air inside it rushes out which is
action and the balloon moves forward which is reaction.
Q.State the Limitations of Newton’s laws of motion.
Ans. Limitations:Newton’s laws are not readily applied on the small scale: as one goes to
extremely low energies on the atomic scale, position and acceleration are not well defined.
Q.11a Differentiate b/w action and reaction. FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC

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SCHOOL RWP (0334-5044822)

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Ans. Difference b/w action and reaction.

Action Reaction
Force acting by source on an object is called The force exerted by object on source is
action called reaction.
it is positive force It is negative force
It is real force having origin.
Action is the cause
AIS It is imaginary force having no origin.
Reaction is the effect due to the cause.
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Q.12. How can you relate a force with the change of momentum of a body? or Prove
Newton’s second law of motion in terms of momentum.
Ans. Relation b/w Force and Change of Momentum: “The rate of change of momentum of a
body is equal to the net force acting on the body”.
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Suppose a force F produces acceleration a in a body of mass m . By Newton’s Second law of


motion it is written as F = ma ____(i). as a result of acceleration, velocity of the body
changes from initial velocity vi to final velocity vf during time interval ∆t . Then by definition
Vf −Vi
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of acceleration a = ______(ii) put this equation in (i)


∆t
Vf −Vi Pf −Pi ∆P
We get F = m( ) Hence F = or F = proved.
∆t ∆t ∆t
Q.13 What will be the tension in a rope that is pulled from its ends by two opposite forces
100N each?
Ans. The tension in the string will be 100N.
Q.14. Action and reaction are always equal and opposite in direction.These forces always
act in pair. Do they balance each other? Can bodies move under action-reaction pair?
Ans. As action and reaction are acting on two different bodies that’s why as a result of
reaction body moves.
Q.15 Define Isolated system. State law of conservation of linear momentum. Explain with
example.
Ans. isolated system: it is a collection of interacting bodies on which no external force is
acting.
Law of Conservation of Momentum: Statement: The Momentum of an isolated system
remains constant.
Mathematically m1v1 + m2v2 = m1u1 + m2u2
Explanation: Consider an isolated system of two spheres of masses m1 and m2 as shown in
fig. They are moving in a straight line with initial velocities u1 and u2 respectively. Such that
u1 is greater than u2 . After sometime m1 hits m2 . let Their velocities becomes v1 and v2 .

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So according to law of conservation of momentum.

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Total momentum of the system before collision = Total momentum of the system after
collision
m1u1 + m2u2 = m1v1 + m2v2
Example: Rockets and jet engines also work on the same principle. In these machines, hot
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gases produced by burning of fuel rush out with large momentum. The machines gain an
equal and opposite momentum. This enables them to move with very high velocities.
Q.16. Why is the law of conservation of momentum important?
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Ans. This law is useful in determining forces that occur for a short time that is almost not
recognizable. Rocket and jet engines also work on the same principle.
Q.17 When a gun is fired, it recoils why?
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Ans. When a gun is fired, Due to conserve momentum it recoils. OR from third law of
motion when gun fired it recoils due to reaction of the bullet.
FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC
Short Response Questions/ conceptual Questions:
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Q.1 When motor cyclist hit a stationary car, he may fly off the motor cycle and driver in
the car may get neck injury. Explain
Ans. Due to inertia , the motorcyclist want to continue their motion and fly off from the
motorcycle. While the driver in the stationary car may get neck injury due to reaction force
acting on his body. which is according to the third law of motion , for every action there is an
equal and opposite reaction.
Q.2 In autumn, when you shake a branch, the leaves are detached, why ?
Ans. When you shake a branch, leaves tend to remain in their original position due to inertia
which causes the leaves to detach.
Q.3 Why it is not safe to apply brakes only on the front wheel of a bicycle?
Ans. when apply only front brake, the momentum of the bicycle and rider shift forward
which can lead to a forward flip or rider being thrown over to conserve momentum. OR
When you apply brake only on front wheel the rear wheel want to continue its motion due
to inertia which causes the bicycle to Skid or rider thrown over the handlebars.
Q.4 Deduce Newton first law of motion from Newton’s second law of motion.
Ans. According to 2nd law of motion Fnet = ma when no net force acts on a body then ma = 0
as mass is constant and cannot be zero so a = 0 , meaning the object will maintain its current
state of motion which is the first law of motion.
FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC
SCHOOL RWP (0334-5044822)

Q.5 Action and reaction are always equal and opposite in direction.These forces always

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act in pair. Do they balance each other? Can bodies move under action-reaction pair?

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Ans. As action and reaction are acting on different bodies, so they do not cancel each other.
Yes bodies move under action-reaction pair.
Q.6 A man slips on the oily floor; he wants to move out of this area. He is alone. He throws
his bag to move out of this slippery area. Why is it so?
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Ans. on a slippery surface, there is little friction that,s why it is difficult to gain the necessary
friction to push off and move, he throws his bag which is his action and as a reaction force
he move out of this area.
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Q.7 How would you use Newton’s third law of motion and law of conservation of
momentum to explain motion of rocket.
Ans. According to third law, every action has an equal and opposite reaction. In a rocket the
action is the expulsion of exhaust of gases and the reaction is the force that propels the
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rocket upward. According to law of conservation of momentum, momentum of isolated


system remains constant. When gases exhaust out with momentum, to conserve
momentum rocket move upward.
FAROOQ IQBAL SST, F.G.SIRSYED PUBLIC
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Q.8 Why are cricket batter gloves padded with foam? SCHOOL RWP (0334-5044822)
Ans. cricket batter gloves are padded with foam to reduce the impact of force and
momentum of the ball. Which help to protect the hand from injury.
Q.9 Where will your weight be greater, near earth or moon? What about mass?
Ans. weight will be greater near the earth because the value of ‘g’ is greater near earth as
compared to moon. However mass remains constant.
Q.10 When Ronaldo kicks the ball, at the heighest point of ball both earth and ball attract
each other with the same magnitude of force. Why then the ball moves towards Earth and
not the Earth?
Ans. According to Law of gravitation, earth and ball exert equal and opposite gravitational
force on each other. As earth has larger mass that’s why ball move towards the earth.
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