9th Class Physics Chapter 4 Short Questions with Answers Solved

Question: 1

State and explain principle of moments with example

Answer: 1

1-19

Principal of moments: For an object to be in equilibrium the sum of the clockwise torque taken about the piot must be equal to the sum of anti-clock wise torque taken about the same pivot this principleis known as principle of moments.

i.e. sum of anti clock wise torque = sum of clock wise torque.

Second condition of equilibriu is also called principle of moments.

Example: In the given figure a od is balance abut pivot.Here torque produce is by " w1 " and "w2" is anti clock wise and torque produced by "w3" is clockwise.

Math

Question: 2

What is Couple? Explain with examples.

Answer: 2

2-19

Couple: Two equal and opposite parallel forces acting along different lines on a body is called a couple.

Explanation: Couple does not produce any translational motion bt only rotational motion.In other words the resultant force of a coupleis zero but the resultant of a coup;e is not zero, It is pure moment. The shortest distance between two couple forces is called couple arm.

Example:

1- Exrting force on bicycle pedals

2- Winding up the spring of a toy car

3- Opening and closing the cap of a bottle

4- Turning of a water tap etc.

Question: 3

Define Like and Unlike parallel forces.

Answer: 3

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Like Parallel force: Like parallel force are those forces which are parallel to each other and having the same direction They may have same or different magnitude.

Example: When we lift a box with double support we are applying like parallel force from each support. These forces may not equal but parallel and act in the same direction

Question: 4

Define Stable Equilibrium:

Answer: 4

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When a body in equilibrium is slightly disturbed, its CM moves up and afer removing external foce. the CM of a body comes to its original position and regain its stability This state of equilirbrium is called stable equilibriu.

Question: 5

What is centre of mass.

Answer: 5

5-19

Centre of Mass: The centre of mass of the body is the point about which ass is equaly distributed in all diretion. It is denoted by "CM"

The identification of this point is possible by applying a force at this point which will produce linear

Question: 6

Explain the addition of forces, in conection with head to tail rules.

Answer: 6

6-19

Addition of Forces: Addition of forces is a process of obtaining a single force which produces the same effect as produced by number of forces acting together.

Explanation: Forces are vector quantities and may be added geometrically by drawing them to common scale and placing them head to tail.

The addition of forces is simple for parallel force. In case of like parallel forces, add the magnitude of vectors and in case of unlike parallel forces,substract the magnitude of vectors.

Addition of Non-Parallel forces: When the forces are non -parallel that are acting at angle other than 0^oand 180 Degree than for addition of uch vectors , we apply a special method called Head to tail rul ein order to find their resultant force

^{Head to tail rule. According to head to tail rule, we will get a resultant force by drawing the representative lines of the givne forces in such a way that the tail of first force vector joins with the head of last forces vector.}

Question: 7

What is resolution of forces? Exampte how force can be resolved into rectangular components.

Answer: 7

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RESOLUTION OF FORCES: The process of splititng a force vector into two or more force vectors is called resoltion of force.

Question: 8

Define Eqilibrium . Explay its types and state the two conditions of equilibrium

Answer: 8

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Equilibrium: Defination:

The state of abody in which under the action of several forces acting together . There is no change in translatioal motion as well as rotational motionis called equilibrium.

Types of Equilibirum:

1. Static equilibrium

2- Dymamic equilirbium

1- Static Equilibrium: When a body s at rest under the action of seveal forces acting together and several torques acting, the body is said be in static equilibrium.

Example: For example, a book is resting on the table and two forces are acting on it i.e. weight of book and reaction force of table. Both forces are equal to magnitude but opposite indirection, So, the net force is zero and the book is said to be in state of static equilirbrium.

2- Dynamic Equilirbirum:

When a body is moving at uniform velocity under action of servaral forces acting together , the body is said to be in dynamic eequilibrim.

Dynamic equilibium is further divided into two types.

1- Dynamic translational equilibrium

2- Dynamic Rotational Equilibrium.

1- Dynamic Translational Equilibrium:

When a body is moving with uniorm linear velociyt., theobdy is said to be in dynamic translational equilibrium.

Example: For example, a paratrooper falling, down with constantvelocity is instate of dynamic translational equilibiurm.

2- Dynamic Rotational Equilibrium:

When a body is moving with uniform rotation, the body is said to be dynamic rotational equilibrium.

Example: For example, a Compact disk rotationg in CD player with constant angular velocity is in state of dynamic rotational equilibriu.

Question: 9

Write first condtion of Equilibrium

Answer: 9

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Wnen the sum of all the forces acting on the body in Zero, then first condition of equilibrium is satisfied.

Mathematically: Fnet = sig F = 0

Where Sigma represents the sigma or summation.

Question: 10

Define moment of a force. Give its mathematical descriptio and elaborate the factors on which it depends?

Answer: 10

10-19

Torque or Moment of force: The turning effect produced in a body about a fixed point due to applied force is called torque or moment of force.

Explanation: Torque is the cause of changes in rotational motion andis similar to force, which casues changes in translatioal motion. For example, opening a door or tightening a nut with spanner etc. Torque may rotate an objectin clock wise or anticlock wise direction.

Mathematical form: Torque is equal to the product of applied forces "F" and the moment arm "d" which is the perpendicular distance from the axis of rotation to the line of action of rotaton Mathemcially , it can be written by.

Torque = Force x moment are

Quanity and Unit: Torque is a vector quantity and its. S.I unit is " Newton meter (Nm)

Question: 11

Write second condition of equilibrium

Answer: 11

11-19

When the sum of all the torque actig on the body is zero then the second condition of equlibrium is satisfied.

Mathematically:

Torque net = Sigma torque = 0

First condition is valid up to translational motion while the second condition is up to rotaional motion. Thus, for complete equilibriun bothe the first and second condition of equilibrium must be satisfied by a body.

Question: 12

Differene between CG and CM.

Answer: 12

12-19

The CG is based on weight of a body where as the CM is based on mass of a body. Also , CG depends on the gravitational field wheresas. CM does not depend upon the gravitational field.

Therefore CM and CG are present at the same point inside or outside a body. However , hen gravitationa field is non-uniform, the CM and CG does not lie at same point is an object.The CG will move closer to regions of the object in a stronger gravitational field. Where as CM is unmoved.

Question: 13

Define Anti clokwise torque.

Answer: 13

13-19

If the object rotates in anticlockwise direction, the torque is known as anticlockwise torque. The anticlokwise torque is always taken positive.

Question: 14

What is Unstable Equilibrium.

Answer: 14

14-19

When a body in equilibrium is slightly distrurbed and its CM moves down and cannot come back to its original posion after removing exernal force. This stat eof equilibrium is called unstable equilibrium.

Question: 15

Define Neutral Equilibrium.

Answer: 15

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When a body is slightly disturbed and its CM does not change from its original position. This state of equilibriu is called neutral equilibiurm.

Question: 16

Define Clocke wise torque

Answer: 16

16-19

If the object rotates in clockwise direction, the torque is known as clockwise torque. The clockwise torque is always taken negaive.

Question: 17

Why it is better to use a long spanner rather than a short one to loosen rusty nut?

Answer: 17

17-19

As we know that torque depends upon fore applied and moment arm. As torque is directly proportional to the moment arm which means greater is the moment arm, greater will be torque produced So, it is easier to loosen a rusty nut by using a long spanner rather than a short spanner because more turning effect is produced on the nut by increasing the moment arm with less effort.

Question: 18

Can a single force applied to body change both tis translational and rotational motion. Explain.

Answer: 18

18-19

Yes , a single force applied to a body can change both its translational and rotational motion, As we know that a force applied at the centre of mass will cause translational motion and a torque will cause rotational motion. So, if the force is applied at the centre of mass of a body. it will perform translational motion only But , if the force is applied at a point other than centre of mass , the body will also rotate along with translational otion, For example, if a foot ball is kick off, it will perorm both tranlational and rotational otion because foot ball will rotate as it oves forward.

Question: 19

Define rotational motion.

Answer: 19

19-19

Rotational Motion: Motion where all the points of an object moves about a single fixed axis is called rotational motion.

Example: The motion of a top, the wheel of a bicycle and car, the hands of clock and the blades of fan are the examples of rotational motion.

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9th Class Chapter 4 Physics Short Questions

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