Notes unit 3(1)
Notes unit 3(1)
Notes unit 3(1)
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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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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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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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
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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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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
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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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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.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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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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