Matric Part 1/9th Class Physics Chapter 4 Long Question With Answer for Unit 4 Turning Effect of Forces

Question # 1

Define center of mass and explain it with the help of example

Ans 1: Centre of mas: center of mass of a system is a point where an applied force causes the system to move without rotationExplanation: it is observed that the center of mass of system moves as if its entire mass is confined at that points, the force acting on this point will not produce torque.

Question # 2

Write and explain the states of equilibrium

Ans 1: States of equilibrium: there are three states of equilibriumStable equilibrium ,Unstable equilibrium, Neutral equilibrium
Ans 2: Stable equilibrium: a body is said to be in stable equilibrium if after a slight tilt it returns to its previous positionExplanation: when a body is in stable equilibrium its center of gravity is at the lowest position. When it is tilted, its center of gravity rises. It returns to its state of sable equilibrium by lowering its center of gravity. Note that a body remains in state o stable equilibrium as long as the center of gravity acts through the base of the bodyExample: consider a block. when the block is tilt its entre of gravity G rises passes through its base in the tilt position. the block returns to its previous position. If the vertical line through G gets out of its base . the block does not return to its previous position. That is why a vehicle is made heavy at its bottom to keep its center of gravity as low as possible. A low center of gravity keep it stable. Moreover , the base of a vehicle is made wide so that the vertical line passing through its center of gravity should not get out of its base during a turn
Ans 3: Unstable equilibrium; if a body does not return to its previous position when et free after slightest till is said to b in unstable equilibriumExplanation: in state of unstable equilibrium C.G is at is highest position. Try to keep a pencil in vertical position on its top. whenever we leave it the pencil will topple over its top and hence fall down. Because of its center of gravity becomes lower and its weight acts out of its base. So the pencil is in the unstable equilibrium
Ans 4: Neutral equilibrium: if a body remains in its new position when disturbed from its previous position, it is said to be in state of neutral equilibriumExplanation: in neutral equilibrium C.G of the body remains at the same height . When a ball is placed on a horizontal surface and after displacing it the ball attains a new position and does not return but in both the positions its center of gravity remains at the same level . So a ball on a horizontal surface is in neutral equilibrium.

Question # 3

Q no:6(B) The steering of a has a radius 16 cm. Find the torque produced by a couple of 50 N.

Ans 1:
F = 50 N
r= 16 cm
d= AB = 32cm =0.32m
t=?
As we know that
t = Fx AB
t = 50 x 0.32
t = 16Nm

Question # 4

What is center of gravity. also explain it

Ans 1: Centre of gravity: a point where the whole weight of the body appears to act vertically downward is called center of gravityExplanation: a body is made of large number of particles. Earth attracts all particles vertically downward. Ass the weight is a vector quantity, the resultant of all these parallel forces is a single force equal to the weight of the whole body. A point where this resultant force acts vertically towards the center of earth is called center of gravity

Question # 5

Discuss torque or moment of force

Ans 1: Before discussing torque we need some terms to define first.Rigid body: a body is composed of large numbver of small particles. If the distance between all paisrs of particles of the body do not change by applting all pairs of particles of the body don not change by applying a force . then it is called a righid bodyAxis of rotation: when a rigid body rotate s its particles also rottate in specific circle. The line joininig the centres of therse circles is called axis of rotatioinORThe line passes through the body and body can rotate aroun it. This line is called axis of rotation
Ans 2: Torque: turning effect of a force is called torque or moment of force .Example: force that produces turning effects are very common. Turning a pencil in a sharpener, turning a door knob etcDependence of torque: torque depends on two thingshow much force is appliedwhere the force is applied( moment arm)
Ans 3: Force: if we apply more force on a body to turn it the torque produced will also be more . Less force produces less amount of torqueExample : when someone press the pedal of bicycle applying slighter force . it will move slowly. But on the other hand more force on the pedals will produce more torque and hence speed will be more .
Ans 4: Where the force is applied: the location of force is also important. force at the door knob can easily open the door but the same force near the hinge o the door cannot open the doorMoment arm: the perpendicular distance between the axis of rotation and line of action of the force is called moment arm.
Ans 5: Line of action of force: a line along which a force acts is called the line of action of forceExplanation; the product of force F and moment arm L is called torqueUnit of torque; unit of torque is newton meterQuantity: it is a vector quantity.Unit of torque: a toque of 1 Nm is produced by a force of 1 newton acting perpendicular to the moment arm of 1m long

Question # 6

Define couple and explain it mathematically

Ans 1: Couple: a couple is formed by two unlike parallel forces of same magnitude but not along the same lineExample: two unlike forces act on a steering wheel and produce torque to turn the vehicle. Similarly, a double arm spanner is used to open a nut of the tyre . Two equal and unlike forces on a water tap produce torque hence , they act as a couple.
Ans 2: Couple arm: the perpendicular distance between two forces of couple armMathematically: Torque of couple = Force * couple armThe torque of couple is given by the product of one of the two forces and perpendicular distance between them
Ans 3: Quantity: it is a vector quantityUnit: its unit is Nm or J

Question # 7

How we determine the center of gravity of an irregular shaped thin lamina

Ans 1: Center of gravity of an irregular shaped thin lamina: a simple method to find center of gravity of an object is by the use of a plumb linePlumb line: a plumb line consists of a small metal bob supported by a string. When the bob is suspended freely by the truing. Its weight acts vertically downward. In this state, center of gravity of the bob is exactly below its point of suspension
Ans 2: Experiment: take an irregular piece of card board. Make three holes on its edge. If a nail on the wall . Support the card board on the nail through hole A. So that it can swing freely about A. Vertical line from A can be located using a plumb line hung from the nail. Repeat it by supporting the cardboard from hole B and C. The line drawn from B and C and the line from A , all will pass through a single point G. Hence G is the center of gravity of card board

Question # 8

Q no: 5 (A) What is meant by torque or moment of a force ? Explain the factors on which it depend

Ans 1: The rotational effect of a force is measured by a quantity know as torque
Dependence of torque:
Rotation produced in a body depends on the following two factors
1-force
2 Moment Arm

Force:
Greater is the force; greater is the moment of the force
Line of action of force:
The line along which a force acts is called the line of action of force.
Moment Arm:
The perpendicular distance between the line of the force and the axis of rotation is as moment arm. Longer is the moment arm greater is the moment of force.
Mathematicle form:
Torque = Moment arm x Force
torque = F x l
Unit:
The unit of toque is Newton meter (Nm)

Question # 9

state and explain second condition for equilibrium.

Ans 1: Second condition for equilibrium: if the sum of all the torques acting on a body is zero then the body is in state of equilibrium
Ans 2: Explanation: 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 first condition the body will be in equilibrium
Ans 3: NoteClockwise rotation: clockwise rotation is taken as negativeAnticlockwise rotation: anticlockwise rotation is taken as positive

Question # 10

Explain like parallel and unlike parallel forces

Ans 1: Parallel forces: such forces which are parallel to each other are called parallel forces.ORthe forces which can never cross 3each other a re called parallel forces.Example: we often come across objects on which many forces are acting in many cases, we find some or all forces acting on the body in the same direction e.g. many people push a bus to start it, all the people apply the forces in the same direction so thy are called parallel forces.
Ans 2: Like parallel force: like parallel forces are the forces that are parallel to each other and have same directionExample: consider a bag with apples in it. The weight of the bag is due to the weight of all apples in it. Since the weight of every apple in the bag is the force of gravity acting on it vertically down words, therefore weight of all apples are parallel forces
Ans 3: Unlike parallel forces: unlike parallel forces are the forces that are parallel but have directions opposite to each otherExample: an apple is suspended by a string: The string is stretched due to its weight. The forces acting on it are weight of the apple acting down words, and tension in the string pulling it vertically upwards . The two forces are parallel but opposite to each other . These force are called unlike parallel forces