Work and Energy Class 9 Science Chapter 10 Revision Notes 2026-27

CBSE Notes Class 9 Science Chapter 10 Work and Energy helps students understand some of the most important concepts of Physics used in daily life. Whenever we walk, lift a bag, ride a bicycle, or play sports, the ideas of work and energy are involved. This chapter explains how work is done, what energy means, different forms of energy, and the relationship between work, energy, and power in a simple and practical way.

These work energy and power class 9 notes are prepared according to the latest CBSE syllabus and NCERT guidelines. Students can use them for quick revision before exams and to strengthen their basic concepts. Many students also look for work energy and power class 9 notes pdf Download options to study anytime and anywhere.

In this chapter, you will learn about mechanical energy, kinetic energy, potential energy, law of conservation of energy, and power. The chapter also includes important numerical problems and real-life examples that make learning easier. Along with theory, students can practice work and energy class 9 questions and answers to improve their understanding and exam performance.

These notes work as a useful study resource for CBSE Class 9 students and can be used together with work and energy class 9 science NCERT Solution materials for better preparation. As part of your Class 9 Notes, this chapter builds a strong foundation for higher classes and future science studies.

Work

Work is said to be done when a force acts on a body and the body is displaced in the direction of the force (or in the direction of a component of force).

Two conditions for work to be done:

1. A force must act on the body.
2. The body must be displaced from one position to another.

In daily life, reading a book, cooking, walking with a box on the head, or pushing a stationary wall none of these involve "work" in the physics sense, because either no force is applied in the direction of motion, or no displacement occurs.

Measurement of Work

W = F⋅d cos ⁡θ

where F = applied force, d = displacement, θ = angle between force and displacement.

Work is a scalar quantity.

Special Cases

Positive and Negative Work

• Positive work When angle between force and displacement is acute (θ < 90°). E.g., lifting a body upward by an applied force.
• Negative work When angle is obtuse (θ > 90°). E.g., work done by gravity when a body is lifted up: W = −mgh.
• Zero work When F = 0, or d = 0, or θ = 90°.

Units of Work

Gravitational Units of Work

• CGS: 1 g-wt-cm = 981 erg
• SI: 1 kg-wt-m = 9.81 J

Solved Examples on Work

Example 1 Lifting Luggage

A porter lifts 15 kg luggage to a height of 1.5 m. Find the work done. (g = 10 m/s²)

Solution: W = mgd = 15 × 10 × 1.5 = 225 J

Example 2 Work at an Angle

A force of 10 N displaces a body by 5 m at 60° to the force. Find the work done.

Solution: W = Fd cos θ = 10 × 5 × cos 60° = 10 × 5 × ½ = 25 J

Energy

The capacity of a body to do work is called its energy.

A body with more energy can do more work.

Units of Energy

• SI: Joule (J), CGS: Erg
• Commercial unit: Kilowatt-hour (kWh) → 1 kWh = 3.6 × 10⁶ J
• Atomic scale: Electron-volt (eV) → 1 eV = 1.6 × 10⁻¹⁹ J

Different Forms of Energy

Kinetic Energy (KE)

The energy possessed by a body due to its motion is called kinetic energy.

Formula

K.E. = 1/2 mv²

Derivation

For a body of mass m, accelerated from rest (u = 0) to velocity v under force F over distance s:

Work done, W = Fs = (ma) × s = m × as

Using v² = u² + 2as → as = (v² − u²)/2

W = 1/2 mv² − 1/2 mu²

Since u = 0, this work done equals the kinetic energy gained:

K.E. = 1/2 mv²

Solved Example

What is the work done to increase the velocity of a 1500 kg car from 30 km/h to 60 km/h?

Solution: u = 30 km/h ≈ 8.33 m/s ; v = 60 km/h ≈ 16.67 m/s

W = 1/2 × 1500 ×[(16.67)² − (8.33)²] = 750 × (277.9 − 69.4) ≈ 1.56 ×10⁵ J

Potential Energy (PE)

The energy possessed by a body by virtue of its position, shape, or configuration is called potential energy.

Examples

• Water stored in a dam (due to position)
• A stone at the top of a hill (due to position)
• A stretched or compressed spring (due to shape)
• A wound watch spring
• A drawn bow with arrow

Types

Expression for Gravitational PE

For a body of mass m lifted to height h above the ground:

U = mgh

Important Properties of Gravitational PE

1. PE at the Earth's surface (h = 0) = 0.
2. PE increases with height; decreases as the body moves down.
3. PE depends only on initial and final positions, not on the path taken.

Interconversion of Potential and Kinetic Energy

For a freely falling body:

• At the top (height h): KE = 0, PE = mgh.
• At the ground: PE = 0, KE = ½mv² = ½m(2gh) = mgh.

→ Initial PE = Final KE

For an upward-projected body:

• At ground: KE = ½mu², PE = 0.
• At highest point (v = 0): KE = 0, PE = mgh = ½mu².

→ Initial KE = Final PE

Law of Conservation of Energy

Energy can neither be created nor destroyed; it can only be transformed from one form to another. The total energy of an isolated system always remains constant.

Law of Conservation of Mechanical Energy

When only conservative forces (like gravity) act:

K.E. + P.E. = constant

Any decrease in PE is matched by an equal increase in KE, and vice versa.

Proof for a Freely Falling Body

Consider a body of mass m at height h falling freely.

Total mechanical energy stays constant = mgh at every point during free fall.

Solved Example

A 10 kg body is held 10 m above the ground. After it is released, at some instant its KE is 450 J. Find its PE at that instant.

Solution: Total mechanical energy at the start = mgh = 10 × 10 × 10 = 1000 J

By conservation: 1000 = 450 + U → U = 550 J

More Examples of Energy Conservation

Simple Pendulum

• At extreme positions (B and C): KE = 0, PE = maximum.
• At mean position (A): PE = 0, KE = maximum.
• The bob continuously converts PE ↔ KE while swinging.

Ball on a Concave Watch Glass

• Released from edge B (height h): all energy is PE.
• At centre A (lowest): all energy is KE (maximum velocity).
• Climbs to opposite edge C: KE → PE again.

In real situations, some energy is lost to air resistance and friction as heat, but the total energy (including heat) is still conserved.

Power

Definition

Power is the rate of doing work i.e., work done per unit time.

P = Wt = F⋅s/t = F⋅ v 

Power is a scalar quantity.

Units of Power

Definition of 1 Watt

1 watt is the power of a machine that does work at the rate of 1 joule per second.

Energy vs Power

Example: An old man working 8 hours and producing 24 items has more energy but less power. A young man working 2 hours and producing 16 items has less energy but more power.

Solved Example

A boy of mass 50 kg runs up a staircase of 45 steps (each 15 cm) in 9 seconds. Find his power. (g = 10 m/s²)

Solution: h = 45 × 0.15 = 6.75 m

P = mgh/t = 50 × 10 × 6.75/9 = 375W 

Energy from the Sun

The Sun is the ultimate source of all forms of energy on Earth.

Wind Energy

Sun heats the Earth's surface → hot air rises, cool air rushes in → wind. Wind has kinetic energy and is converted into electrical energy using wind mills in wind farms.

Solar energy + air → Wind energy

Food and Muscular Energy

Green plants use sunlight to make food through photosynthesis (using chlorophyll):

CO2 + Water (sunlight/chlorophyll) → Sugar + Oxygen

Food gives us chemical energy → muscular energy → mechanical energy (work).

Work and Energy Class 9 science Formulas