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Download 9th Class Physics Notes (Unit # 4) for Federal Board (FBISE) Islamabad

Download 9th Class Physics Notes (Unit # 4) for Federal Board (FBISE) Islamabad

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Download 9th Class Physics Notes (Unit # 4) for Federal Board (FBISE) Islamabad

SHORT QUESTIONS
>> Question: Define the followings.
Resultant Vector:
A resultant vector is a single vector that has the same effect as the combined effect of all the vectors to be added.
Torque:
The turning effect of a force is called torque or moment of the force
Torque       =      t       =      F x L
Torque is vector quantity and its direction can be found by using right hand rule. Its unit is Nm. Torque depends upon two factors.
i)       Magnitude of the force (F)
Greater is a force; greater is the moment of the force.
T   α   F
ii)      Moment arm
Longer is the moment arm greater is the moment of the force.
T   α    L
Center of mass:
Center of mass of a system is such a point where an applied force causes the system to move without rotation.
Center of gravity:
A point where the whole of the body appears to act vertically  downward is called center of gravity of a body.
>> Question: Differentiate between followings
Like and unlike forces:
  • Like parallel forces are the forces that are parallel to each other and have the same direction.
  • Unlike parallel forces are the forces that are parallel but have directions opposite to each other.
Torque and couple:
  • The turning effect of a force is called torque or moment of the force.
  • When we apply two equal but opposite forces on a body, it forms a couple. The turning effect of a couple is the sum of moment of the two forces.
Stable and neutral equilibrium:
  • When the center of gravity of a body lies below the point of suspension or support, the body is said to be in stable equilibrium.
  • When the center of gravity of a body lies above the point of suspension or support, the body is said to be in unstable equilibrium.
>> Question: How head to tail rule helps to find the resultant of forces?
  • To add the vectors, draw the representative lines of these vectors in such a way that the head of the first vector coincides with the tail of the second.
  • The line joining the tail of the first vector with the head of the second vector represents the resultant vector.
  • The direction of the resultant vector is from the tail of the first vector towards the head to second.
  • This is called head to tail rule.
>> Question: How can a force be resolved into its rectangular components?
The process of splitting up vectors (forces) into their component forces is called resolution of vectors (forces).
>> Question: What are perpendicular components of a vector.
If a force is formed from two mutually perpendicular components then such components are called its perpendicular components.

>> Question: When a body is said to be in equilibrium?
A body is said to be in equilibrium if no net force acts on it. A body in equilibrium thus remains at rest or moves with uniform velocity. For example a car moving with uniform velocity on a leveled road and an aeroplane flying in the air with uniform velocity are the examples of bodies in equilibrium.

>> Question: Explain the first condition for equilibrium?
A body is said to satisfy first condition for equilibrium if the resultant of all the forces acting on it is zero.
Let n number of forces F1, F2, F3, ……….. Fn are acting on a body such that

F1 + F2 + F3 + …………………. Fn      =      0
OR
∑ F    =      0
OR
∑ Fx  =      0
∑ Fy  =      0

>> Question: Why there is a need of second condition for equilibrium if a body satisfies first condition for equilibrium?
Case-I:
·        First condition for equilibrium does not ensure that a body is in equilibrium.
·        Consider a body pulled by the forces F1 and F2.
·        The two forces are equal but opposite to each other.
·        Both are acting along the same line, hence their resultant will be zero.
·        According to the first condition, the body will be in equilibrium.
Case-II:
·        If a body has a tendency to rotate then this situation demands another condition for equilibrium in addition to the first condition for equilibrium. This is called second condition for equilibrium.
·        As per second condition of equilibrium, a body is said to be in equilibrium when the resultant torque acting on it is zero.
·        Mathematically
∑ t    =      0
>> Question: Define Second condition of equilibrium.
·        As per second condition of equilibrium, a body is said to be in equilibrium when the resultant torque acting on it is zero.
·        Mathematically
∑ t    =      0
>> Question: Give an example of a moving body which is in equilibrium.
·        A car is moving with uniform velocity on a leveled road and an aero plane flying in the air with uniform velocity are the examples of bodies in equilibrium.
·        A paratrooper coming down with terminal velocity (constant velocity) also satisfies first condition for equilibrium and is thus in equilibrium.

>> Question: Think of a body which is at rest but not in equilibrium.
·        Rest implies stationary; equilibrium implies a resultant force of zero.
·        Therefore a body in equilibrium could be moving, for example a sky diver at that a body can be in equilibrium and not at rest, but a body at rest must be in equilibrium, otherwise it would move.
·        So, it is impossible.

>> Question: Why a body cannot be in equilibrium due to single force acting on it?
·        No, with only a single force present, the body would accelerate infinitely in the direction of that force.
·        Because the force which is alone applied will have some direction and the object will try to move in this direction under its influence.
·        However, if two opposite and equal forces take part it.
·        Gives rise to a null vector force. The body can be in rotational equilibrium under the impact of a single force.

>> Question: Why the height of vehicles is kept as low as possible?
·        As the whole weight of a body acts on center of gravity so in case of racing car center of gravity must be close to the earth so that there are less chances of overturning of the car.
·        If the car is high, it is easy to produce the torque in car due to large moment arm, and the car can takes the somersault (forward roll).

>> Question: Can the nut of the axle of a bike be loosened with hand why we use a spanner  for this purpose?
No, we cannot loosen the nut of the excel of a bike. Normally we use a spanner because a spanner increases the turning effect of the force which easily loosened the nut of excel of a bike.

>> Question: Women and children in the villages often carry pitchers with water  on their  heads how  this  is possible?
·        Woman and children keep itself upright when carry pitchers on their  heads. Pitcher has a heavy semi-spherical base.
·        When it is tilted, its centre of mass rises. It returns to its upright position at which its centre of mass is at the lowest.
·        That is why Women and children in the villages often carry pitchers with water on their heads.

>> Question: With  a little effort we can learn to balance a stick  vertically up on our finger tip how this is possible.
  • In order to balance something, the center of gravity of the object is either directly above or directly below the pivot point.
  • An example would be balancing the stick on the end of a finger with the stick pointing vertically up.
  • If you do this you will find that the stick wants to fall over, and you need to keep moving your finger around to keep this from happening.

>> Question: Many people push a bus to start it why all of them push it in the same direction?
Like parallel forces acting in the same direction increases the resultant force which moves the bus easily.

>> Question: What is meant by trigonometry?
Trigonometry is that branch of mathematics which deals with the properties of a right angled triangle.

>> Question: Why  it is  easy  to  open  and  close  the  door  by  pulling or pushing it at its handle?
We open or close a door by pushing or pulling it. Here push or pull turn the door about its hinge or axis of rotation. The door is opened or closed due to the turning effect of the force acting on it.

>> Question: What do you mean by a rigid body?
  • A body is composed of large number of small particles. If the distances between all pairs of particles of the body do not change by applying a force then it is called a rigid body.
  • In other words, a rigid body is the one that is not deformed by force or forces acting on it.                               .

>> Question: What do you mean by axis of rotation?
Consider a rigid body rotating about a line. The particles of the body move in circles with their centers all lying on this line. This line is called the axis of rotation of the body.

>> Question: Why the handle of a door is fixed near the outer edge of a door? OR Why door handles  usually  on the  opposite  edge of the  door  from the hinge?
We can open or close a door more easily by applying a force at the outer edge of a door rather than near the hinge.
The moment produced  by a force using a greater  moment  arm is greater than  the  torque  produced  by  the  same  force  using  by  shorter  moment  arm. Therefore the handle of a door is fixed near the outer edge of a door. ( T   α    L)

>> Question: Why it is easy to tighten a nut using a spanner of longer arm than a spanner of shorter arm?
A spanner having long arm helps to loosen or tighten a nut or a bolt with greater ease than the one having short arm. It is because the turning effect (torque) of the force increases. (t α  L)

>> Question: What do you mean by line of action of a force?
The line along which a force acts is called the line of action of the force.

>> Question: Define moment arm.
The perpendicular distance between the axis of rotation and the line of action of the force is called the moment arm of the force. It represented by the distance L.

>> Question: What do you mean by newton-metre (Nm)?
A torque of 1 N m is caused by a force of 1 N acting perpendicular to the moment arm 1 m long.

>> Question: Describe principle of moment?
A body is balanced if the sum clockwise moments acting on the body is equal to the sum of anticlockwise moments acting on it
Explanation:
Clockwise moment:
A force that turns a spanner in the clockwise direction is generally used to tighten a nut. The torque or moment of the force so produced is called clockwise moment.
Anticlockwise moment:
To loosen a nut, the force is applied such that it turns the nut in the anticlockwise direction. The torque or moment of the force so produced is called anticlockwise moment.

>> Question: Two  children are -sitting  on  the  see-saw,  such  that  they  cannot swing. What is the net torque in this situation?
Net torque in the situation is zero. Because clockwise torque will cancel the effect of anticlockwise torque.

>> Question: Explain how  center  of mass helps the system to  move as well as rotate?
Centre of mass of a system is such a point where an applied force causes the system to move without rotation.
Explanation:
It is observed that the centre of mass of a system moves as if its entire mass is confined at that point.  A force applied at such a point in the body does not produce any torque in it i.e. the body moves in the direction of net force F without rotation.

>> Question: Define centre of gravity?
A  point  where  the  whole  weight  of  the  body  appears  to  act  vertically downward is called centre of gravity of a body.

>> Question: How does a paratrooper come down?
  • A paratrooper comes down with terminal velocity is in equilibrium.
  • A paratrooper coming down with terminal velocity (constant velocity) also satisfies first condition for equilibrium and is thus in equilibrium.

>> Question: Define terminal velocity?
The maximum and constant velocity of an object falling vertically downward is called terminal velocity.

>> Question: Does the speed of ceiling fan go on increasing all the time?
No, the speed of a ceiling fan does not go' on increasing all the time. Fan will move with constant speed.

>> Question: Does the fan satisfy second condition for equilibrium when rotating with uniform speed?
Yes, a rotating ceiling fan satisfy second condition for equilibrium. Because ceiling fan rotating at constant speed is in equilibrium as net torque acting on it is zero.

>> Question: Why a vehicle is made heavy at its bottom?
  • A vehicle is made heavy at its bottom to keep its centre of gravity as low as possible. A lower centre of gravity keeps it stable.
  • Moreover, the base of a vehicle is made wide so that the vertical line passing through its centre of gravity should not get out of its base during a turn.



LONG QUESTIONS


>> Question: Explain the states/types of equilibrium:
There are three states of equilibrium.
i)      Stable equilibrium
ii)     Unstable equilibrium
iii)    Neutral Equilibrium.

Stable Equilibrium:
·        When the center of gravity of a body lies below the point of suspension or support, the body is said to be in stable equilibrium.
·        For example a body lying on a horizontal surface is an example of stable equilibrium. If the book is lifted from on edge and then allowed to fall, it will come back to its original position.
·        The reason for its stability is this, when the book is lifted its center of gravity is raised. The line of action of weight passes through the base of the book. A torque due to weight of the book brings it back to the original position.
Unstable Equilibrium:
·        When the center of gravity of a body lies above the point of suspension or support, the body is said to be in unstable equilibrium.
·        For example pencil standing on its point or a stick in vertically standing position.
·        The reason for instability is this when the pencil is slightly disturbed its center of gravity is lowered. The line of action of its weight lies outside the base of pencil.
Neutral Equilibrium:
·        When the center of gravity of a body lies at the point of suspension or support, the body is said to be in neutral equilibrium. For example rolling ball.
·        The reason for neutral equilibrium is that if the ball is rolled, its center of gravity is neither raised nor lowered.

>> Question: Explain  an  experiment  to  find  the   centre   of  gravity   of  a  an irregular shaped thin lamina?
A simple method to find the centre of gravity of a body is by the use of a plumb line.
Plumb line:
·        A plumb line consists of a small metal bob supported by a string.
·        When the bob is suspended freely by the string, it rests along the vertical direction due to its weight acting vertically downward.
Experiment:
·        Take an irregular piece of cardboard. Make holes A, B and C near its edge.
·        Fix a nail on a wall.
·        Support the cardboard on the nail through one of the holes (let it be A), so that the cardboard can swing freely about A.
·        The cardboard will come to rest with its centre of gravity just vertically below the nail.
·        Vertical line from A can be located using a plumb line hung from the nail
·        Mark the line on the cardboard behind the plumb line.
·        Repeat it by supporting the cardboard from hole B.
·        The line from B will intersect at a point G.
·        Similarly, draw another line from the hole C. Note that this line also passes through G.
·        It Will be found that all the vertical lines from holes A, B and C have a common point  G.
·        This common point G is the centre of gravity of the cardboard.

=*=*=*=*=


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Download Class Notes: Download 9th Class Physics Notes (Unit # 4) for Federal Board (FBISE) Islamabad
Download 9th Class Physics Notes (Unit # 4) for Federal Board (FBISE) Islamabad
Download 9th Class Physics Notes (Unit # 4) for Federal Board (FBISE) Islamabad
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