Best notes of Work and Energy

Class 9 Science – Work and Energy
(Detailed Notes | NCERT Based)
These notes are strictly based on NCERT Class 9 Science, Chapter: Work and Energy.
They are written in simple language, exam-oriented, and include definitions, laws,
formulas, explanations, examples, and important points.




1. WORK
What is Work?

In science, work is said to be done when a force acting on an object causes displacement
in the direction of the force.

Conditions for Work to be Done

Work is done only when:

1. A force is applied on an object
2. The object moves (displacement occurs)
3. Displacement is in the direction of force

If any one condition is missing, no work is done.

Examples

 Pushing a wall → No work (no displacement)
 Lifting a book → Work is done
 A porter carrying a load on his head → No work (force and displacement are
perpendicular)



Mathematical Expression of Work

Work (W) = Force (F) × Displacement (S)

W=F×S

SI Unit of Work

 SI unit: Joule (J)
 1 Joule: Work done when a force of 1 N moves an object by 1 m

, 2. ENERGY
What is Energy?

Energy is the capacity to do work.

An object having energy can do work.

SI Unit of Energy

 SI unit: Joule (J)




3. FORMS OF ENERGY
Energy exists in many forms:

 Mechanical energy
 Heat energy
 Light energy
 Electrical energy
 Chemical energy
 Sound energy

Energy can be converted from one form to another.




4. MECHANICAL ENERGY
Mechanical energy is of two types:

1. Kinetic Energy
2. Potential Energy




5. KINETIC ENERGY
Definition

The energy possessed by an object due to its motion is called kinetic energy.

Examples

 Moving car