WORK AND ENERGY, CBSE CLASS IX, PHYSICS NOTES PART I

 

CBSE CLASS IX, WORK AND ENERGY, SCIENCE (PHYSICS) NOTES-(PART I)

                       WORK AND ENERGY

According to the CBSE Syllabus 2025-26

CBSE Class 9 Science Chapter 11 Work and Energy Notes

In Class 9 Science Chapter 11 Work and Energy.

Topics in the Chapter

1. Introduction

2. Work

a. Conditions for work to be done.     

b. Conditions when work is not done.    

c. Unit of Work.

3. Negative, positive, and Zero Work

4. Energy

5. Forms of Energy                   

a. Mechanical Energy.        

b. Kinetic Energy.

6. Potential Energy.

7. Transformation Energy.

8. Law of conservation of Energy.

9. Free Fall of a Body (Energy Conservation).

10. Power

a. Unit of Power

11. Commercial unit of Power

a. Solved Numerical

INTRODUCTION TO WORK AND ENERGY

Class 9 Chapter 11, ‘Work and Energy’, discusses the concept of work, energy, and power in detail. In day-to-day life, we consider any useful physical or mental labour as work, but work is defined differently in science. Work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force. Work has only magnitude and no direction. Similarly, we often use the term energy in our daily life; this also has a different definition in science. In physics, energy is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat and light.

WORK

Work done on an object is defined as the product of the magnitude of the force acting on the body and the displacement in the direction of the force. W = F.s. The SI unit of force is Newton.        
If a force acting on a body causes no displacement, the work done is 0. For example, pushing a wall.
The force component F cos θ gives the component of force along the direction in which the body is displaced. Cos θ is the angle between the force vector and the displacement vector. The direction of the Force must not be perpendicular to the displacement. Work is a scalar quantity.

MATHEMATICAL DEFINITION OF WORK

1. A constant force is applied in the direction of the displacement of an object.

Work = Force ✕ displacement

W = F ✕ s

 2. A Constant force is applied at a certain angle with the direction of the displacement of an object.

W = Fs Cos θ

UNIT OF WORK

> The unit of work is Newton metre or Joule.

> When a force of 1 Newton moves a body through a distance of 1 metre in its own direction, then the work done is 1 Joule.
1 Joule = 1 Newton × 1 metre
1 J = 1Nm                                                                            

In CGS Unit is Erg.

SOME IMPORTANT POINTS RELATED TO WORK

1. If θ = 0°

Then cos
 0° = 1  
W = F ✕ s

2. If θ = 90°                            

then W = 0

3. If θ = 180°
Then Cos 180° = -1  
W = - F ✕ s

NOT MUCH WORK IS DONE INSPITE OF WORKING HARD.

Reading, writing, drawing, thinking, and analysing are all energy-consuming. However, in a scientific manner, no work has been done in the above cases.

Example: A man is completely exhausted in trying to push a rock (wall), but work done is zero as wall is stationary.
A man standing still with a heavy suitcase may be tired soon, but he does no work in this situation as he is stationary.                                                                              When force is applied to the wall, the wall doesn't move. Therefore, no work is done here.

WORK IS SAID TO BE DONE WHEN                               

(i) A moving object comes to rest.                                                                           

(ii) An object at rest starts moving.

(iii) The velocity of an object changes.

(iv) The shape of an object changes.

WORK IS DONE WHEN                                                                           

(i)  A cyclist is pedaling the cycle.                                                                     

(ii) A man is lifting a load in the upward or downward direction.

WORK IS NOT DONE WHEN,

(i)  A coolie carrying some load on his head stands stationary.                       

(ii) A man is applying force to a big rock.

THE AMOUNT OF WORK DONE DEPENDS ON THE FOLLOWING FACTORS  

(i) Magnitude of force:  
The Greater the force, the greater is the amount of work & vice versa.

(ii) Displacement: 
The Greater the displacement, the greater is the amount of work & vice versa.

NEGATIVE, POSITIVE, AND ZERO WORK

→ Work done by a force can be positive, negative or zero.

(i) Work done is positive when a force acts in the direction of motion of the body.

Example: A child pulls a toy car with a string horizontally on the ground.

Here work done is positive.

W = F × S

 (ii) Work done is negative when a force acts opposite to the direction of motion of the body.

Example: When we kick a football lying on the ground, the force of our kick moves the football. Here direction of force applied & motion of football is same so work done is positive. But when football slows due to force of friction acting in a direction opposite to direction of motion of football, thus work done is negative.

(iii) Work done is zero when a force acts at right angles to the direction of motion.

Example: The moon moves around the earth in circular path. Here force of gravitation acts on the moon at right angles to the direction of motion of the moon. So work done is zero.

• -ve (negative) sign indicates that work is done against gravity.

Note: If work is done against the direction of motion (gravity), then it is taken as positive.

CONCEPTS OF POSITIVE AND NEGATIVE WORK

1. When the angle between the force and displacement is acute i.e., 0°< θ < 90°, the work done is positive.
2. When the angle between the force and displacement is obtuse i.e. 90°< θ < 180° The work done is negative.
3. Whenever force is in the direction of motion, velocity of the object increases, and the work done is positive.
4. Whenever force opposes motion, velocity of object decreases and the work done is negative.
5. Work is positive when force and displacement are parallel to each other and that are in the same direction and work is negative when force and displacement are antiparallel to each other.

CONDITION FOR ZERO WORK

Condition of Zero Work	Calculation of Work Done
The net force should be equal to zero	W = Fs Cos θ = 0 ✕ s ✕ Cosθ = 0
The net displacement should be equal to zero	W = Fs Cos θ = F ✕ 0 ✕ Cosθ = 0
The force and displacement should be perpendicular to each other	W = Fs Cos θ = F ✕ s ✕ Cos 90° = 0

ENERGY

Energy is defined as the ability to do work. Its unit is the same as that of work.

(i) The sun is the biggest source of energy.                                           

(ii) Most of the energy sources are derived from the sun.                       

(iii) Some energy is received from the nucleus of atoms, the interior of the Earth, and the tides.

→ The amount of energy a body possesses is equal to the amount of work it can do. 

→ The working body loses energy, and the body on which work is done gains energy.

→ Energy is a scalar quantity.

Unit: The SI unit of energy is Joule (J), and its bigger unit is kilojoule (kJ).
1 kJ = 1000 J

→ The energy required to do 1 Joule of work is called 1 Joule of energy.

FORMS OF ENERGY

 (i) Kinetic energy
(ii) Potential energy
(iii) Heat energy
(iv) Chemical energy
(v) Electrical energy
(vi) Light energy
(vii) Sound energy
(viii) Nuclear energy