Class 9 Work and Energy Notes – Complete NCERT Revision

Are you looking for Class 9 Work and Energy Notes that explain every important concept in simple language? These comprehensive CBSE Class 9 Science Work and Energy revision notes will help you understand the scientific meaning of work, types of work, kinetic energy, potential energy, the law of conservation of energy, power and the commercial unit of electrical energy.

The chapter introduces the relationship between force, displacement, work and energy. Students learn how moving objects possess kinetic energy, how objects at a height store gravitational potential energy and how energy transforms from one form into another.

These NCERT Class 9 Work and Energy notes with formulas and diagrams include the derivation of kinetic energy, gravitational potential energy, the work–energy theorem, solved numericals and important conceptual questions.

Whether you are preparing for a Class 9 Science unit test, school examination, periodic assessment or annual examination, these notes will help you revise important definitions, SI units, equations and numerical concepts effectively.

Learning Objective: By the end of these notes, you should be able to calculate work, kinetic energy, gravitational potential energy and power, explain energy transformations, and solve application-based questions.

CBSE CLASS 9 SCIENCE • NCERT REVISION

Class 9 Work and Energy Notes

Complete Chapter Notes with Formulas, Diagrams, Derivations, Solved Numericals and Important Questions

Understand every important concept of Work and Energy with clear explanations, visual diagrams, quick revision points and exam-focused practice.

1. What Is Work in Physics?

In everyday life, we use the word work for activities that require effort. However, the scientific meaning of work is different.

Definition: Work is said to be done when a force acting on an object produces displacement with a component in the direction of the force.

Conditions Necessary for Work

A force must act on the object.
The object must undergo displacement.
The force must have a component along the displacement.

Figure 1: The wall does not move, so work done on the wall is zero.

Important Concept: A person pushing a stationary wall may feel tired because muscles consume energy. However, if the wall does not move, the mechanical work done on the wall is zero.

Examples of Work

Lifting a book from the floor to a table.
Pulling a trolley so that it moves.
Pushing a box across the floor.
Gravity doing work on a falling object.

2. Work Done by a Force

When a constant force acts in the direction of displacement, work done is equal to the product of force and displacement.

W = F × s W = work done, F = force, s = displacement

SI unit of work: Joule (J).

1 J = 1 N × 1 m = 1 N m

One joule of work is done when a force of one newton produces a displacement of one metre in its direction.

Work is a scalar quantity.

Work Done When Force Acts at an Angle

If the force makes an angle θ with displacement, the general equation for constant force is:

W = Fs cos θ

For θ = 0°, the force and displacement are in the same direction, so W = Fs.

For θ = 90°, work done is zero.

Remember: Work depends on force, displacement and the angle between them.

3. Positive, Negative and Zero Work

Positive Work

Force and displacement act in the same direction.

Example: A person pushing a trolley forward.

W > 0

Negative Work

Force and displacement act in opposite directions.

Example: Friction acting on a sliding box.

W < 0

Zero Work

Displacement is zero, or force is perpendicular to displacement.

Example: Holding a stationary bag.

W = 0

Figure 2: Direction of force and displacement determines the sign of work.

Quick Memory Trick:
Same direction → Positive work
Opposite direction → Negative work
Perpendicular force → Zero work

4. What Is Energy?

Definition: Energy is the capacity to do work.

The SI unit of energy is the joule (J).

Energy exists in different forms, including mechanical, thermal, chemical, electrical, light and sound energy.

Mechanical Energy

Mechanical energy is associated with the motion and position or configuration of an object.

Mechanical Energy = Kinetic Energy + Potential Energy

Kinetic Energy

Energy due to motion.

Examples: moving car, rolling ball, flowing water.

Potential Energy

Energy due to position or configuration.

Examples: raised stone, stretched spring, water stored at a height.

5. Kinetic Energy – Formula and Derivation

The energy possessed by an object because of its motion is called kinetic energy.

K.E. = ½mv² m = mass in kg; v = speed in m/s

Derivation of Kinetic Energy

Consider an object of mass m moving with initial velocity u. A constant net force F accelerates it to velocity v through displacement s.

According to Newton's second law:

F = ma

Work done by the net force:

W_net = Fs = mas

Using the equation of motion:

v² − u² = 2as

Therefore:

as = (v² − u²)/2

Substituting:

W_net = ½m(v² − u²)

Hence:

W_net = ½mv² − ½mu²

If the object starts from rest, u = 0:

K.E. = ½mv²

Work–Energy Theorem: The net work done on an object equals the change in its kinetic energy.
W_net = ΔK.E.

Factors Affecting Kinetic Energy

Kinetic energy is directly proportional to mass.
Kinetic energy is directly proportional to the square of speed.
If mass doubles, kinetic energy doubles at constant speed.
If speed doubles, kinetic energy becomes four times.
If speed triples, kinetic energy becomes nine times.

Exam Tip: Never forget to square the speed in K.E. = ½mv².

6. Gravitational Potential Energy

The energy possessed by an object due to its position in a gravitational field is called gravitational potential energy.

P.E. = mgh m = mass, g = gravitational acceleration, h = height

Derivation of Gravitational Potential Energy

Consider an object of mass m raised slowly through a vertical height h near Earth's surface.

The gravitational force acting downward is:

F = mg

The work done by the lifting force against gravity is:

W = F × h

Substituting F = mg:

W = mgh

This work increases the object's gravitational potential energy.

P.E. = mgh

Here, h is measured relative to the chosen reference level, where potential energy is taken as zero.

Figure 3: Raising an object increases its gravitational potential energy.

Remember: The formula mgh applies to gravitational potential energy changes near Earth's surface, where g is approximately constant.

7. Transformation and Conservation of Energy

Transformation of Energy

The conversion of energy from one form into another is called energy transformation.

Example	Energy Transformation
Falling stone	Potential → Kinetic
Electric fan	Electrical → Mechanical + Thermal + Sound
Electric bulb	Electrical → Light + Thermal
Hydroelectric station	Gravitational Potential → Kinetic → Electrical
Thermal power station	Chemical → Thermal → Mechanical → Electrical
Photosynthesis	Light → Chemical

Law of Conservation of Energy

Statement: Energy can neither be created nor destroyed. It can only be transformed from one form into another. The total energy of an isolated system remains constant.

Conservation of Energy During Free Fall

When an object falls freely under gravity, neglecting air resistance, its gravitational potential energy decreases and its kinetic energy increases.

Figure 4: Ideal free fall from rest, neglecting air resistance.

K.E. + P.E. = Constant When only gravity does work

Important: If air resistance is present, some mechanical energy is transformed into thermal energy and other forms. Total energy is still conserved, but mechanical energy need not remain constant.

8. Power – Definition, Formula and SI Unit

Power is the rate of doing work or the rate at which energy is transferred.

P = W/t P = average power, W = work done, t = time

SI unit: Watt (W).

1 W = 1 J/s

One watt is the power when one joule of work is done in one second.

Average Power

Average Power = Total Work Done / Total Time Taken

Other useful units:

1 kW = 1000 W
1 MW = 10⁶ W

Concept: Two students may do the same amount of work, but the student who completes it in less time develops greater average power.

9. Commercial Unit of Electrical Energy

The commercial unit of electrical energy is the kilowatt-hour (kWh).

One kilowatt-hour is the energy consumed by a 1 kW appliance operating for one hour.

1 kWh = 1000 W × 3600 s

1 kWh = 3.6 × 10⁶ J

Figure 5: Understanding one kilowatt-hour.

Important Difference:
kW = unit of power
kWh = unit of energy

10. Class 9 Work and Energy Solved Numericals

These solved Work and Energy numerical questions demonstrate the correct method of writing given values, formula, substitution, calculation and SI units.

Numerical 1: Work Done

Question: A force of 25 N moves a box through 4 m in the direction of force. Calculate the work done.

Given: F = 25 N, s = 4 m

Formula: W = Fs

Solution: W = 25 × 4

W = 100 J

Numerical 2: Kinetic Energy

Question: Calculate the kinetic energy of a 4 kg object moving at 3 m/s.

Given: m = 4 kg, v = 3 m/s

Formula: K.E. = ½mv²

Solution: ½ × 4 × 3²

K.E. = 18 J

Numerical 3: Potential Energy

Question: A 5 kg object is raised to a height of 4 m. Calculate its increase in potential energy. Take g = 10 m/s².

Given: m = 5 kg, h = 4 m, g = 10 m/s²

Formula: P.E. = mgh

Solution: 5 × 10 × 4

P.E. = 200 J

Numerical 4: Power

Question: A machine does 1200 J of work in 20 seconds. Calculate its average power.

Given: W = 1200 J, t = 20 s

Formula: P = W/t

Solution: 1200/20

P = 60 W

Numerical 5: Electrical Energy

Question: A 2 kW heater runs for 3 hours. Find the electrical energy consumed.

Formula: Energy = Power × Time

Solution: 2 kW × 3 h

Energy = 6 kWh

In joules:

6 × 3.6 × 10⁶ = 2.16 × 10⁷ J

Numerical Strategy:
Given → Formula → Substitution → Calculation → Unit

11. Class 9 Work and Energy Important Formulas

Concept	Formula	Unit
Work	W = Fs (same direction)	J
Work at angle	W = Fs cos θ	J
Work against gravity	W = mgh	J
Kinetic energy	K.E. = ½mv²	J
Gravitational potential energy	P.E. = mgh	J
Mechanical energy	ME = KE + PE	J
Net work–energy theorem	W_net = ΔKE	J
Average power	P = W/t	W
Watt	1 W = 1 J/s	W
Kilowatt	1 kW = 1000 W	kW
Electrical energy	E = Pt	J or kWh
Kilowatt-hour	1 kWh = 3.6 × 10⁶ J	kWh

12. Common Mistakes in Work and Energy

Work is not always positive. It can be positive, negative or zero.
Force alone is not enough. Displacement in the appropriate direction is required.
Kinetic energy depends on v², not v.
mgh is gravitational potential energy, not the formula for every kind of potential energy.
Net work equals change in kinetic energy. Do not confuse net work with the work of one force.
kWh measures energy, not power.
Mechanical energy is not always conserved. Friction and air resistance can transform mechanical energy into other forms.
Always convert units before substituting into an SI-based formula.

13. Important Work and Energy Questions for Class 9

Test your understanding with these Class 9 Work and Energy important questions. Try answering before opening the solutions.

Q1. Define one joule of work.

One joule is the work done when a force of one newton produces a displacement of one metre in its direction.

Q2. Why is work done zero when a person pushes a stationary wall?

The wall undergoes no displacement. Therefore W = Fs = F × 0 = 0.

Q3. What happens to kinetic energy when speed doubles?

Kinetic energy becomes four times because KE is proportional to v².

Q4. State the law of conservation of energy.

Energy can neither be created nor destroyed. It can only change from one form to another.

Q5. What is the SI unit of power?

Watt (W). One watt equals one joule per second.

Q6. Calculate work done by a force of 10 N through 6 m in its direction.

W = Fs = 10 × 6 = 60 J.

Q7. Why does a falling stone gain kinetic energy?

Gravity does positive work on the stone, increasing its speed and kinetic energy.

Q8. What is the difference between kW and kWh?

kW is a unit of power, whereas kWh is a unit of energy.

14. Frequently Asked Questions – Class 9 Work and Energy

What are the main topics in Class 9 Work and Energy?

Important topics include work, positive and negative work, kinetic energy, potential energy, conservation of energy, power and commercial energy units.

What is the formula for work in Class 9?

When force and displacement are in the same direction, W = F × s. For a constant force acting at an angle, W = Fs cos θ.

What is the formula for kinetic energy?

Kinetic energy is calculated using KE = ½mv², where m is mass and v is speed.

What is the formula for potential energy?

Gravitational potential energy near Earth's surface is PE = mgh.

How can I prepare Work and Energy numericals?

Learn the formulas, understand the meaning of each quantity, convert values into appropriate units and practise questions involving work, KE, PE and power.

Why is 1 kWh equal to 3.6 × 10⁶ joules?

Because 1 kW = 1000 W and 1 hour = 3600 seconds, so 1000 × 3600 = 3,600,000 J.

15. One-Minute Work and Energy Revision

Work: Force × displacement in the same direction.

Kinetic Energy: Energy due to motion.

Potential Energy: Energy due to position or configuration.

Conservation of Energy: Total energy remains constant in an isolated system.

Power: Rate of doing work.

Commercial Energy Unit: kWh.

To score well in Class 9 Science, understand the concepts instead of memorising formulas alone. Practise derivations, numerical questions, diagrams and application-based problems regularly.

Practise Class 9 Work and Energy Questions

Finished revising the chapter? Test your preparation with our Class 9 Work and Energy Worksheet containing chapter-based practice questions.

Get Work and Energy Worksheet →