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# 9th Class Chapter 4 Physics Short Questions

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##### Question: 1
Unit of torque.
1-123
It unit newton metre.
##### Question: 2
Define centre of mass
2-123
Centre of mass of a system is a point where an applied force causes the system to move without rotation.
##### Question: 3
Differentiate between like and unlike forces.
3-123
Like parallel forces: such a parallel forces whoe direction is same are called like parallel force. Example: the weights of apples in a sack are parallel to each other and also in the same direction.
##### Question: 4
What is 2nd condition for equilibrium?
4-123
A body is said to be satisfy 2nd condition of equilibrium if the resultant torque acting on a body is zero.
##### Question: 5
Differentiate between torque and couple.
5-123
Torque is produced by only only force one moment arm is there to produce the torque
##### Question: 6
What is centre of gravity.
6-123
A point where the whole weight of the body appears to act vertically downward is called centre of gravity of a body e.g. centre of gravity of uniform rod is centre of the rod.
##### Question: 7
Define parallel forces.
7-123
When a number of forces acting on a body in such a way that their point of action s are different but line of action are parallel to each other ,are called parallel forces.
##### Question: 8
When a body is said to be in equilibrium?
8-123
A body is said to be in equilibrium if no net force acts on it.
##### Question: 9
Give an example of a moving body which is in equilibrium.
9-123
WA paratrooper coming down with the constant speed is in equilibrium as well as in motion.
##### Question: 10
Explain 1st condition of equilibrium.
10-123
A body said to satisfy first condition of equilibrium if the resultant of all these forces acting on it is zero.
##### Question: 11
Define unlike parallel forces.
11-123
Such parallel forces which are opposite in direction are called unlike parallel forces.
##### Question: 12
Define Resultant vector.
12-123
A resultant vector is a single vector that has the same effect as the combined effect of all vectors to be added.
##### Question: 13
Define Trigonometry.
13-123
The branch of geometry which explains the properties of right angle triangle is called trigonometry.
##### Question: 14
Write the types of parallel force.
14-123
i) Like parallel forces. ii) Unlike parallel forces.
##### Question: 15
Define equilibrium. and give one example.
15-123
A body is said to be in equilibrium, if no net force acts on it. Example: i) A book lying on a table is in equilibrium because the weight of book and force of reaction of table are in opposite direction and hence cancel their effect.
##### Question: 16
Define unstable equilibrium. Give one example
16-123
Unstable equilibrium: If a body does not return to its previous position when set free after a slightest is said to be in unstable equilibrium. Example: A pencil just balanced at its tip with centre of gravity at the highest position is the example of unstable equilibrium=brium
##### Question: 17
What is meant by resolution of forces?
17-123
The decomposition or division of a vector into itscomponent is called resolution of a vector or force
##### Question: 18
What is line of action of force.
18-123
A line along which a force acts is called line of action of the force.
##### Question: 19
Define Trigonometric ratio.
19-123
The ratios between any two sides of a right angle triangle are called trigonometric ratio.
##### Question: 20
What is meant by principle of moments?
20-123
A body is balanced if the sum of clockwise moments acting on the body is equal to the sum of anticlockwise moments acting on its.
##### Question: 21
Give an example of a moving body which is in equilibrium?
21-123
A paratroopercoming down with terminal velocity is in equilibrium this type of equilibrium.
##### Question: 22
Define couple arm.
22-123
The perpendicular distance between two forces of couple is called couple arm.
##### Question: 23
Difference between like and unlike forces?
23-123
Like forces:
like forces are the forces that are parallel to each other and have the same direction
##### Question: 24
Define moment arm.
24-123
The perpendicular distance between the axis of rotation and line of action of force is called moment arm.
##### Question: 25
Define parallel forces and types?
25-123
In a plane, if number of forcesact on a body such that their points of action are different, but lines of action are parallel to each other, then these forces are called parallel forces.

There are two types of parallel forces
1- like parallel forces, unlike parallel forces
##### Question: 26
How head to tail rule helps to find the resultant of forces
26-123
When two forces are added with head to tail rule it gives the resultant of two forces, It gives the magnitude and direction both of the resultant force.
##### Question: 27
Which factor torque depends.
27-123
Torque = F X L So, Torque depends upon force and moment arm.
##### Question: 28
What is centre of gravity
28-123
Centre of Gravity: A point where whole weight of the body appears to act vertically downward is called centre of gravity of a body e.g centre of gravity of uniform rod is centre of the rod
##### Question: 29
Define Resolution of vectors.
29-123
Resolution of Vectors: to split up a force into its perpendicular components is called resolution of forces.
##### Question: 30
Define first condition of equilibrium?
30-123
A body will be in equilibrium if the resultant of all the forces acting on it is zero this is first condition of equilibrium.
##### Question: 31
Define like parallel forces.
31-123
Such parallel forces which have same direction are called like parallel forces.
##### Question: 32
When a body is said to be in equilibrium
32-123
A body is said to be in equilibrium if no net force acts on it.
##### Question: 33
Define axis of rotation.
33-123
The line about which a body rotates is called axis of rotation.
##### Question: 34
Difference between Center of mass and center gravity?
34-123
Center of mass.
Center of mass of a system is such a point where an applied force cause the system to move without rotation
##### Question: 35
Differentiate between like and unlike forces
35-123
Like Parallel forces: Such a parallel forces whose direction is same called are called like parallel forces
Example: The weights of apples in a sack are parallet to each other and also in the same direction.
##### Question: 36
Define couple with the help of an example.
36-123
Couple: A couple is formed by two unlike parallel forces same magnitude but not along the same line.
##### Question: 37
Why the height of a vehicle is kept as low as possible
37-123
To lower the centre of mass of vehicles their height is kept low. Low centre of mass make them more stable as compare to the high centre of mass Due to this vehicles are made heavy at the bottom and their height is kept minimum
##### Question: 38
What is meant by additionof forces?
38-123
Force is a vector quantity. It has magnitude as well direction; therefore forces are not added by ordinaryarithmetic rules. They are added by a method known as head to tail rule.
##### Question: 39
What is stat of neutral equilibrium.
39-123
Neutral Equilibrium; If a body remains in its new position when disturbed from its previous position, then body is said to be in neutral equilibrium.
##### Question: 40
Two children are sitting on the sea-saw such that they cannot swing
40-123
In this situation the net torque is zero
##### Question: 41
Why the height of vehicles is kept as low as possible?
41-123
We Know that smaller the height of center of gravity of a body , greater will be its stability . The height of vehicles is kept low to lower their center of gravity and as a result their stability increases.
##### Question: 42
Define stable equilibrium with example.
42-123
Stable equilibrium: A body is said to be in stable equilibrium if after a slightest it returns to its previous position. Example: If a book returns to its previous position after a slightest tilt then book is said be in stable equilibrium.
##### Question: 43
What is 2nd condition for equilibrium
43-123
A body is said to be satisfy 2nd condition of equilibrium if the resultant torque acting on a body is zero

##### Question: 44
Define plumb line?
44-123
A plum be line is a piece of string with a weight attached to the end that is used to check that something such as wall is vertical
##### Question: 45
In a right angled triangle length of base is 4cm and its perpendicular is 3cm.
45-123
length of its hypotenuse, cos , sin, tan,
##### Question: 46
Define rigid body and give an example.
46-123
Rigid body is composed of large number of particles. If the distance between all pairs of particles of the body do not change by applying a force, then it is called a rigid body. Example: Adoor , a cylinder.
##### Question: 47
Can force be added by ordinary addition of number
47-123
No, forces cannot be added by ordinary addition of number They can be added by head to tail rule because force is vector quantity
##### Question: 48
Differentiate between stable and neutral equilibrium.
48-123
Stable equilibrium: A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position
In stable equilibrium centre of gravity of the body is at its lowest position and after the slight tilt it return to its previous position.
##### Question: 49
Define Resultant vector?
49-123
A resultant vector is a single force that has the same effect as the combined effect of all the forces to be added.
##### Question: 50
Define trigonometry
50-123
The branch of geometry which explains the properties of right angle triangle is called trigonometry.
##### Question: 51
Define couple and couple arm?
51-123
A couple is formed by two unlike parallel forces of the same magnitude, but not along the same line
##### Question: 52
A ladder leaning at a wall as shown in figure is in equilibrium how
52-123
The weight of ladder produces anticlock wise torque. The wall pushes the ladder at its tope end thus produce a clockwise torque. Both torques are equal and opposite and cancel their effect. So, the ladder is in state of equilibrium.
##### Question: 53
Define Resultant Vector
53-123
A resultant vector is a single vector that has the same effect as the combined effect of all vector to be added
##### Question: 54
Head to tale rule explain?
54-123
Draw the representative lines of all the vector to be added in such a way that head of first vector coincides with the tail of second vector, head of second vectorcoincides with the tail of third vector and on so
##### Question: 55
Explain 1st condition for equilibrium.
55-123
A body is said satisfy first condition of equilibrium if the resultant of all these forces acting on it is zero.
F=0
##### Question: 56
Give an example of a moving body which is in equilibrium
56-123
A paratrooper coming down with the constant speed is in equilibrium as well as in motion
##### Question: 57
Does the fan satisfy second condition for equilibrium when rotating with uniform speed
57-123
Yes, It satisfy because moment arm is zero than
##### Question: 58
What is line of action force
58-123
A line along which a force acts is called line of action of the force
##### Question: 59
What is head to tail rule
59-123
When two or more than two vectors are added such that the head of the first vector is joined with tail of second vector and finally the tail of first vector is joined with the head of the second vector. This method of addition is called head to tail rule. The vector joining the tail of first vector to the head of last vector is called resultant vector a Graphical method used to find the resultant of two or more forces is called head to tail rule.
##### Question: 60
How a force can be resolved into its prependicular components
60-123
To resolve a force into its perpendicular components we neend head to tail rule. It the given forces is considered as resultant force then the two components can be drawn In the figure force F is making an angle 0 with horizontal surface and it is resolved into Fx and Fy which are horizontal and vertical components respectively.
##### Question: 61
Define torque
61-123
Turning effect of a force is called torque or moment of force
##### Question: 62
Why a body regain its previous state in stable equilibrium
62-123
Because in stable equilibrium its centre of gravity is at the lowest position
##### Question: 63
Why door is easy to open or close by pushing or pulling at handle?
63-123
We open or close a door by pushing 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: 64
Why there is need for 2nd condition of equilibrium if body satisfy 1st condition of equilibrium
64-123
If a body satisfied 1st condition of equilibrium then it is not completely in equilibrium. Because there is a chance of producing torque in that body
If two same force are acting on a body in opposite direction in a same line then no rotation is produced but if those two forces act in different lines then the torque is produced so we need another condition for equilibrium to eliminate the torques.
##### Question: 65
What is base
65-123
The smaller side of right angle triangle with angle is called base e,g the side AB in the figure is base.
##### Question: 66
The weight of the ladder in fig produces an anticlock wise toruqe
66-123
Yes, the ladder is in state of equilibrium.
##### Question: 67
Define parallel forces
67-123
When a number of forces acting on a body in such a way that their point of actions are different but lines of action are parallel to each other are called parallel forces.
##### Question: 68
Can a small child play with a fat child on the sea-saw Explain how
68-123
Yes a small child can play with a fat child if the fat child sists near the axis of rotation of the sea-saw and the small child sists away the axist of rotation.
##### Question: 69
Think of a body which is at test but not in equilibrium
69-123
When we throw a body in up ward direction at its highest position the body is at rest for a moment but it is not in equilibrium because no other force is present to cancel the force of weight.
##### Question: 70
Define equilibrium and give its example
70-123
Equilibrium: A body is said toi be in equilibrium if no net force acts on it
##### Question: 71
Define resolution of vectors
71-123
To split up a force into its perpendicular components is called resolution of forces.
##### Question: 72
Define couple arm
72-123
The perpendicular distance between two force of couple is called couple arm
##### Question: 73
Does the speed of a ceiling fan go on increasing all the time.
73-123
No, speed of a ceiling fan is not increasing all the time but it is constant.
##### Question: 74
What is state of neutral equilibrium
74-123
Neutral Equilibrium: If a body remains in its new position when disturbed from its previous position then body is said to be in neutral equilibrium
##### Question: 75
Why a body cannot be in equilibrium due to single force acting on it
75-123
When a single force acts on a body acceleration and torque is produced due to this reason a body is not equilibrium.
##### Question: 76
On which factor torque depends
76-123
So, torque depends upon force and moment arm
##### Question: 77
Discuss centre of gravity of some symmetrical object
77-123
Uniform rod, Square plate and rectangular plate, Uniform circular disc, Uniform solid sphere, Triangular plate, Uniform solid or hollow cylinder,
##### Question: 78
Define like parallel forces and moment arm
78-123
Like parallel forces: Like parallel force are the forces that are parallel to each other and have the same direction
##### Question: 79
What is perpendicular
79-123
The line facing the angle in a right angle triangle is called perpendicular e.g the side BC in the figure is perpendicular
The line have 90 angle with any plane is called perpendicular
##### Question: 80
Why the handle of a door is placed at the outer edge of the door
80-123
We can open or close door more easily by applying a force at the outer edge of a door rather than near the hinge (axist ) Because by this a long moment arm is obtained which produce more torque which helps to open the door more easily.
##### Question: 81
Write the types of parallel forces
81-123
There are two types of parallel forces.
Like parallel force
Unlike parallel force
##### Question: 82
What is meant by perpendicular components
82-123
Perpendicular Component: Those components which makes an angle of 90 with each other are called perpendicular components. The perpendicular components of force is fx and fy
##### Question: 83
Why a body cannot be in equilibrium due to a single force acting on it
83-123
A body cannot be in equilibrium due to a single force because a single force cannot be considered zero as a net force and the torque produced by it will also not be zero
##### Question: 84
What is directional graph
84-123
To draw a vector on a paper we need direction Graphically two mutually perpendicular lines are used to indicate the direction. These two line are used to direct a vector on paper or plane.
##### Question: 85
Differentiate between Axis of rotation and Mometn
85-123
Axis of rotation The straight line passing through a body about which the body can rotate is called axis of rotation
##### Question: 86
Define axis of rotation
86-123
The line about which a body rotates is called axis of rotation
##### Question: 87
What is hypotenuse
87-123
The facing the angle of 90 is called hypotenuse e.g the side AC in the figure is hypotenuse
In right angle triangle the side opposite to right angle is called hypotenuse
##### Question: 88
Define like parallel forces
88-123
Such parallel forces which have same direction are called like parallel forces. e.g in the above figure f1 and f2 are like parallel to each other and the weight of apples closed in a sack are also in downward direction. These forces are also parallel forces.

##### Question: 89
Define equilibrium
89-123
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 velcity
##### Question: 90
Define like and unlike parallel forces
90-123
Like parallel forces: Like parallel forces are the forces that are parallel to each other and have the same direction
##### Question: 91
What is right angle triangle
91-123
A triangle having an angle of 90 in it is called Right angle triangle.
##### Question: 92
What is meant by principle of moments
92-123
A body is balance if the sum of clockwise moments acting on the body is equal to the sum of anticlock wise moments acting on its
##### Question: 93
A force of 150 N is making an angle of 40 with horizontal surface what is its vertical components.
93-123
Given data: Give data F= 15N
0=40

##### Question: 94
Write the principle of moments
94-123
State the principle of moments: A body initially at rest does not rotate if the sum of all the clockwise moments acting on it balanced by the sum of all the anti clockwise moment acting on it This principle is known as the principle of moments. Mathematically clockwise moments=Anti-clockwise moments
##### Question: 95
Force of 150N can loosen a nut when applied at end of a spanner 10cm long
95-123
Given data: F=150N
L=10cm=01.m
##### Question: 96
How head to tail rule helps us to find resultant of vectors
96-123
To determine resultant vector by head to tail rule we have to draw the resultant vector from head of last to tail of 1st vector It given us resultant graphically
##### Question: 97
Define unstable equilibrium
97-123
Unstable Equilibrium: If a body does not return to its previous position when set free after a slightest tilt is said to be in unstable equilibrium
##### Question: 98
Why the height of vehicles is kept as low as possible
98-123
The height of vehicles is kept as low as possible to make them more and more stable because stability increased as height of vehicle decrease.
##### Question: 99
What is line of action of force
99-123
Line of action of force and moment arm always makes angle 90 with each other
##### Question: 100
What is Plumb line
100-123
Plumb line: A plumb line consists of a small metal bob lead or brass supported by a string. When the bob is suspended freely by the string, it rests the vertical direction due to its weight acting vertically down ward.
##### Question: 101
At which point an applied force does not produce rotation
101-123
A force applied at the centre of mass of a body does not produce any rotation rather the body moves in the direction of applied force
##### Question: 102
How can we increase the stability of a vehicle
102-123
Stability of the vehicle can be increased by lowering the centre of mass of vehicle and by making them heavy at the bottom.
##### Question: 103
Think of a body which is at rest but not in equilibrium
103-123
A body thrown vertically upward at maximum height is at rest for a while but at that moment gravitational force
##### Question: 104
Why a body does not regain its previous state in unstable equilibrium
104-123
Because in this state centre of gravity of body is at highest position.
##### Question: 105
Define Resolution of forces
105-123
Forces into their component forces is called resolution of forces
##### Question: 106
Explain couple with the help of an example
106-123
Couple A Couple is formed act on a steering wheel and produce torque to torque to turn the vehicle
##### Question: 107
Define resultant of forces and explain by diagram
107-123
Resultant of forces: A resultant force is a single force that has the same effect as the combined effect of all the force to be added
##### Question: 108
Where are the centre of gravity of uniform square and uniform triangular sheets.
108-123
Centre of gravity of a square of a rectangular Square: The centre of a gravity of a uniform square or a rectangular sheet is the point of intersection of its diagonals
##### Question: 109
Define center of gravity
109-123
Resultant vector: A resultant vector is a single vector that the same effect as the combined effect of all the vectors be added
##### Question: 110
Define rigid body and give an example
110-123
Rigid Body: A body is composed of large number of particles. It the distance between all pairs of particles of the body do not change by applying a force, then it is called a rigid body
##### Question: 111
Differentiate between clockwise and anti-clockwise moment
111-123
Clockwise Moment: The force that trrns a spanner in the clockwise direction is known is clockwise force. The torque or moment of force produced to this force is called clockwise moment
The moment produced during tighten a nut
##### Question: 112
Define parallel forces. Write n names of its types
112-123
Parallel forces: Such forces which are parallel to each other are called parallel forces.
##### Question: 113
Define Centre of mass of a body
113-123
Centre of mass: The point where when the force is applied, the system move without rotation is called centre of mass.
##### Question: 114
Define stable equilibrium with example
114-123
Stable Equilibrium: A body is said to be in stable equilibrium if after a slightest tilt it returns to its previous postion.
##### Question: 115
What is meant by graph
115-123
A graph is a diagram showing the relationship between variable quantities typically of two variables, each measured along a pair of axis at right angle.
##### Question: 116
What is difference between stable and unstable
116-123
Stable: equilibrium: A body is said to be in stable equilibrium if after a slight tilt it return to its previous position.
##### Question: 117
Define torque and Center of mass
117-123
Torque: The turning effect of a force is called torque or moment of the force
##### Question: 118
On doubling the moment arm, what will be its effect on the value of torque
118-123
On doubling the moment arm the value of torque will also be double
##### Question: 119
What is meant by Rigid body
119-123
Rigid Body: A body is composed of large number of small particles. If the distance between all pairs of particles of the body do not change by applying of force the, it is called rigid body A rigid body is the one that is not deformed by force or forces acting on it
##### Question: 120
What is meant by Couple of forces
120-123
Couple: A Couple is formed by two unlike parallel forces of the same magnitude but not along the same line.
##### Question: 121
Differentiate between torque and couple
121-123
Torque: Turing effect of force is called torque .
To produce a torque we need only one force at least
Torque or moment of force is equal to product of force F and moment arm L
##### Question: 122
What is the second condition of equilibrium