Work and Energy Class 9 Notes – Easy Explanation with Formulas

Looking for Work and Energy Class 9 Notes that are simple, clear, and exam-ready? In this post, we cover everything you need to master the chapter “Work and Energy” from Class 9 Science. Understand the scientific meaning of work, learn about different forms of energy, important formulas, and the law of conservation of energy — all explained in an easy-to-understand way. Perfect for quick revision and scoring high marks!

In everyday language, “work” could mean anything that makes you tired. But in science, work has a very specific meaning!

🔵 What is Work? (Scientific Definition)

Work is said to be done when a force is applied on an object and the object displaces in the direction of the force.

Formula for Work:Work (W)=Force (F)×Displacement (s)Work (W)=Force (F)×Displacement (s)

SI Unit: Joule (J)
Positive Work: Force and displacement in the same direction.
Negative Work: Force and displacement in opposite directions.
Zero Work: No displacement occurs.

🔵 Energy – The Ability to do Work

Energy is the capacity to do work.
It also has the same unit as work: Joule (J).

Different Forms of Energy:

Mechanical Energy: (Kinetic + Potential)
Heat Energy
Light Energy
Chemical Energy
Electrical Energy

🔵 Kinetic Energy (Energy of Motion)

Energy possessed by an object due to its motion.

Formula:Kinetic Energy (KE)=12mv2Kinetic Energy (KE)=21mv2

where mm = mass, vv = velocity.

Examples: Moving car, flying ball, rolling stone.

🔵 Potential Energy (Stored Energy)

Energy possessed due to the position or configuration.

Formula:Potential Energy (PE)=mghPotential Energy (PE)=mgh

where mm = mass, gg = acceleration due to gravity, hh = height.

Examples: Water stored in a dam, stretched rubber band, compressed spring.

🔵 Law of Conservation of Energy

One of the most important laws in physics!

Energy can neither be created nor destroyed.
It can only be transformed from one form to another.
Total energy of an isolated system always remains constant.

Example:
When an object falls freely, its potential energy decreases while its kinetic energy increases — but their sum remains constant!

🔵 Power – Rate of Doing Work

Power is the rate at which work is done.

Formula:Power (P)=Work Done (W)Time Taken (t)Power (P)=Time Taken (t)Work Done (W)

SI Unit: Watt (W)
1 Watt = 1 Joule/second
Bigger unit: 1 kilowatt (kW) = 1000 Watts

Average Power:Average Power=Total Energy ConsumedTotal Time TakenAverage Power=Total Time TakenTotal Energy Consumed

📝 Quick Recap from Work and Energy Class 9 Notes

Work is done only when displacement occurs.
Energy is the ability to do work.
Kinetic energy is due to motion; Potential energy is due to position.
Energy can change forms but total energy remains constant.
Power tells how fast work is done.

What You’ll Learn in Work and Energy Class 9 Notes

These Work and Energy Class 9 Notes will help you understand the chapter deeply and score full marks in exams. Keep practicing and keep learning!

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