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Download 9th Class Physics Notes (Unit # 3) for Federal Board (FBISE) Islamabad

Download 9th Class Physics Notes (Unit # 3) for Federal Board (FBISE) Islamabad

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Download 9th Class Physics Notes (Unit # 3) for Federal Board (FBISE) Islamabad


Question: Define followings:
Inertia of a body is its property due to which it resists any change in its state of rest or motion
Momentum of a body is the quantity of motion it possesses due to its mass and velocity. The momentum P of a body is given by the product of its mass m and velocity v. Thus
P      =      mv
It is vector quantity. Its SI unit is kgms-1.
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 a body.
F       =      ma
SI unit for force is Newton.
Force of friction:
The force that opposes the motion of moving objects is called friction
Centripetal Force:
Centripetal force is a force that keeps a body to move in a circle. In other words, the center seeking force is called the centripetal force. It always acts perpendicular to the motion of the body.
Fc      =      ----

Question: Differentiate between followings:
Mass and Weight:
·        The quantity of matter contained in a body is called its mass.
·        It remains constant everywhere.
·        It is scalar quantity.
·        Its unit is kilogram.
·        It can never be zero.
·        Weight is the force with which earth attracts a body towards its center.
·        It changes its value everywhere.
·        It is vector quantity
·        Its unit is Newton.
·        It can be zero.
Action and Reaction:
Let a body A exerts a force on another body B, the body B reacts against this force and exerts a force on body A. The force exerted by body A on B is the action force whereas the force exerted by body B on A is called the reaction force.
Sliding Friction and Rolling Friction:
·        A force between the sliding objects which opposes the relative motion between them is called sliding friction.
·        Rolling friction is a force of friction between a rolling body and a surface over which it rolls. It is lesser than sliding friction.
>> Question: Why is it dangerous to travel on the roof of a bus?
Because in this case the friction force due to air is acting on the upper part of body and lower portion remain at rest w.r.t the roof due to inertia. Friction force try to turn over which is dangerous for passenger.

>> Question: Why does a passenger move outward when a bus takes a turn?
·        An inward net force is required to make a turn in a circle. This required force is known as a centripetal force. In the absence of any net force, an object in motion (such as the passenger) continues in motion in a straight line at constant speed.
·        So, due to the absence of necessary centripetal force a passenger moves outward when a bus takes a turn.

>> Question: What will be the tension in a rope that is pulled from its end by two opposite forces 100 N each?
Tension in the rope and its force pulls equally at both the ends. If no forces are acting on the rope except its ends, and the rope itself being in equilibrium, then the tension is the same throughout the rope.

>> Question: Action and reaction are always equal and opposite. Then how does a body moves?
·        Actions and reactions are equal and opposite when the object is at equilibrium.
·        When we apply external force to pull, push and twist, the equilibrium is disturbed means, now the magnitude of action (force provided by you) and reaction (force provided by the objects) is not equal.
·        That is why it is possible to twist, pull, move and push the object in the direction of applied force.

>> Question: A horse pushes the cart. If the action and reaction are equal and opposite then how does the cart move?
  • First of all when the horse pulls on the cart, the cart exerts and equal but opposite reaction on the horse. If this was the only force in action then the horse and cart would indeed remain stationary.
  • However there is another force between the horse and the ground, the horse’s hooves press down on the ground and the ground pushes back on the horse.
  • If the reaction force of the ground is greater than the reaction force of the cart on the horse, then the horse will move forward.
  • The cart will move forward when the force exerted on it by the hose is greater than the frictional force between the cart and the ground.

>> Question: Define the law of conservation of momentum. What is its importance?
This law states that the momentum of an isolated system of two or more than two interacting bodies remains constant. This Law of conservation of momentum is applicable on all objects in the universe. A rocket and jet engine taking off, the recoil of a gun, and a bank-shot in a pool are examples which demonstrate the importance of law of conservation of momentum.
>> Question: Why is the law of conservation of momentum important?
Law of conservation of momentum is applicable on all objects in the universe. A rocket and jet engine taking off, the recoil of a gun and a bank shot in a pool are examples which demonstrate the importance of law of conservation of momentum.

>> Question: When a gun is fired, it recoils. Why?
Before firing the gun, both the gun and bullet are at rest, so the total momentum of the system is zero. As the gun is fired, bullet shoots out of the gun and acquires momentum. To conserve momentum of the system, the gun recoils.
>> Question: Describe two situations in which force of friction is needed.
i)            Friction is needed to walk on the ground.
ii)           To stop a bicycle, rubbers pads provide friction against the rims which stops the bicycle.

>> Question: How does oiling the moving parts of a machine lowers friction?
The friction can be reduced by lubricating the sliding surfaces. The oil helps slick (polished) the two surfaces so that the molecular surfaces become easier to slide on with less friction.
>> Question: Describe ways to reduce friction?
  • Friction can be reduced by making the sliding surfaces smooth.
  • Friction can be reduced by making fast moving objects a streamline shape (fish shape) such as cars, aeroplanes etc. This minimize the air resistance.
  • Friction can be reduced by lubricating the sliding surfaces.
  • Friction can be reduced by using ball bearings or roller bearings because rolling friction is much less than sliding friction.
>> Question: Why rolling friction is less than sliding friction?
  • Rolling friction is the force of friction between a rolling body and a surface over which it rolls. Rolling friction is lesser than the sliding friction.
  • But when a body moves over surface of another body, there is relative motion between the two surfaces, thus friction has some maximum value.
>> Question: What do you know about the followings?
Tension in a string: -
The force exerted by a string when it is subjected to pull is called tension in the string. In SI system, its unit is Newton.
Limiting force of friction:
The maximum value of friction is known as the force of limiting friction (Fs). It depends on the normal reaction (pressing force) between the two surfaces in contact.
Braking Force:
Friction between a rotating component (drum or disk) and a stationary component (the brake shoe or pad) causes the drum or disc to slow down such a force is called braking force.
There are four main aspects which determine the amount of braking force: -
  • The diameter of disc
  • The friction material
  • The size of the pad friction face
  • The force used to clamp the pads onto the disc.
Skidding of vehicles:
Skidding occurs when:
  • Clutch is suddenly engaged
  • Brakes are applied too hard
  • Vehicle accelerates too quickly
  • Steering wheel is turned too sharply
If the brakes are applied too hard, the wheels of the car will lock up and the car will skid due to its large momentum.
Without seatbelts, in case of accidents, a person will continue moving until stopped suddenly by something. Seatbelts are useful in two ways:
  • They provide external force to person.
  • The additional time required for stretching seat belts. This prolongs the stopping time for momentum to change is and reduces the effect of collusion.
Banking of Roads:
  • The curvature of the road must be inclined so as to control the centrifugal force of the vehicle.
  • It is helpful because if the velocity of car is more or there is less friction between the tyres and road which reduce the danger of car to move out of circular track.
Cream Separator:
  • It is high speed spinner. It acts on the principle of centrifuge machines.
  • The bowl spins at very high speed causing the heavier contents of milk to move outward in the bowl pushing the lighter contents inward towards the spinning axis.

>> Question: What would happen if all friction suddenly disappears?
If there was no friction then we could not walk, we would keep slipping nothing would be steady on the ground, many things would be just sliding and sliding.
>> Question: Why the spinner of a washing machine is made to spin at a very high speed?
  • They dryer of a washing machine is basket spinners.
  • They have a perforated wall having large numbers of fine holes in the cylindrical rotor.
  • The lid of the cylindrical container is closed after putting wet clothes in it.
  • When it spins at high speed, the water from wet clothes is forced out through these holes due to lack of centripetal force.

>> Question: Define dynamics.
The branch of mechanics that deals with the study of motion of an object and the cause of its motion is called dynamics.
>> Question: Put one rupee coin over a piece of card paper placed on an empty glass. Push the card with a sudden stroke of finger. Card will move ahead while the coin falls in the glass. Why it does so?
Due to inertia card will continue its motion in the forward direction while coin will remain at rest and will fall in the glass.
>> Question: A bullet has a very small inertia due to its small mass. But why does its impact is so strong when it is fired from the gun?
According to law of conservation of momentum mass of bullet is much smaller than the gun therefore the recoil is much greater than the velocity of gun. Therefore the impact of bullet is very strong.

>> Question: Why the impact of loaded truck on a body coming its way is very large even if the truck is moving slowly.
Greater is the mass of truck greater will be its momentum. Therefore loaded truck has large impact.

>> Question: Explain Newton’s first law of motion by practical examples of daily life?
A body continues its state of rest of uniform motion in a straight line provided no net force acts on it. For example
·        Objects at rest does not move at their own unless they are moved by some external force.
·        Moving bodies cannot stop themselves unless they are stopped by some opposing or frictional force.

>> Question: Why Newton’s first law of motion is also known as law of inertia?
Since Newton’s first law of motion deals with the inertial property of matter, therefore it is known as law of inertia.

>> Question: Why the passengers standing in a bus fall forward when its driver applies brakes suddenly?
It is because the upper parts of their bodies tend to continue their motion while lower parts of their bodies in contact with the bus stop with it. Hence they fall forward.

>> Question: Define net force.
Net force is the resultant of all the forces acting on a body.

>> Question: State and prove the Newton’s second law of motion OR show that F=ma.
When a net force acts on a body, it produces acceleration in the bod in the direction of net force. The magnitude of this acceleration is directly proportional to the net force acting on the body and inversely proportional to its mass.

If a force produces acceleration ‘a’ in a body of mass ‘m’ then we can state mathematically that

a      α      F
a      α      1/m
a      α      F/m
F       =      ma
>> Question: Define Newton (Unit of Force).
One Newton is the force that produces an acceleration of 1 ms-2 in a body of mass of 1 kg.
>> Question: Define Newton’s third law of motion.
To every reaction, there is always an equal but opposite reaction.

>> Question: What is Atwood machine?
An Atwood machine is an arrangement of two objects of unequal masses. Both the objects are attached to the ends of a string. The string passes over a frictionless pulley. This arrangement is sometime used to find the acceleration due to gravity.

>> Question: Why the fragile objects are packed with suitable materials?
Fragile objects such as glass wares etc. are packed with suitable materials such as Styrofoam rings, balls, polythene sheets with air sacks etc.
Air enclosed in the cavities of these materials makes them flexible and soft. During any mishap, they increase the impact time on fragile objects. An increase in impact time lowers the rate of change of momentum and hence lessens the impact of force. This lowers the possible damage due to an accident.

>> Question: Vehicles have rigid cages for passengers with crumple zones at their front and rear ends? Why?
It is for safety purpose. During an accident, crumple zones collapse. This increases the impact time by providing extra time for crumpling. The impact of force is highly reduced and saves the passengers from severe injuries.

>> Question: Define system and isolated system.
  • A system is a group of bodies within certain boundaries.
  • An isolated system is a group of interacting bodies on which no external force is acting. The momentum of an isolated system is always conserved. This is the law of conservation of momentum.

>> Question: Why a moving ball stops?
A moving ball stops due to friction.
>> Question: Why a bicycle stops when the cyclist stop peddling?
Bicycle stops due to force of friction.
>> Question: What is microscopic concept of friction?
No surface is perfectly smooth. A surface that appears smooth has pits and bumps that can be seen under a microscope. A magnified view of two smooth surfaces in contact shows the gaps and contacts between them. Thus greater is the pressing force greater will be the friction between the sliding surfaces.

>> Question: What is coefficient of friction?
The ratio between the force of limiting friction Fs and the normal reaction R is constant. This constant is called coefficient of friction and is represented by µ.
µ      =      Fs/R
Fs      =      µR
>> Question: The first thing about a wheel is that it rolls as it moves rather than to slide. This greatly reduces friction. Why?
When the axle of a wheel is pushed, the force of friction between the wheel and the ground at the point of contact provides the reaction force. The reaction force acts at the contact points of the wheel in a direction opposite to the applied force. The wheel rolls without rupturing the cold welds. That is why the rolling friction is extremely small than sliding friction.

>> Question: Why ball bearing or roller bearings are used to reduce friction?
The fact that rolling friction is less than sliding friction is applied in ball bearings or roller bearings to reduce losses due to friction.

>> Question: Why it is dangerous to drive on a wet road?
It is dangerous to drive on a wet road because the friction between road and tyres is very small.

>> Question: Describe the advantages of friction?
  • It enables animals to walk or crawl without slipping.
  • It stops cars, trains, bicycles, MRT trains etc.
  • It enables us to hold things firmly with our hands.
  • It prevents objects from sliding down a slope.
  • It allows nails to hold thing.

>> Question: Describe the disadvantages of friction?
  • It causes energy lost and reduces the efficiency of machines.
  • It causes rapid wear and tear of the moving parts of machines.




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Download Class Notes: Download 9th Class Physics Notes (Unit # 3) for Federal Board (FBISE) Islamabad
Download 9th Class Physics Notes (Unit # 3) for Federal Board (FBISE) Islamabad
Download 9th Class Physics Notes (Unit # 3) for Federal Board (FBISE) Islamabad
Download Class Notes
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