Work and Energy — Science Class 9 Notes (CBSE & HBSE)
Free NCERT Science notes for Work and Energy (Class 9) on Siksha Sarovar, aligned to CBSE and Haryana Board (HBSE). This chapter is broken into 3 topics with clear explanations, formulas, solved examples and board-pattern practice — free to read, no sign-up required.
Board exam focus — Work and Energy (CBSE & HBSE)
CBSE emphasises the scientific definition of work, derivation of kinetic energy, and the law of conservation of energy with numerical applications; HBSE focuses on direct formula-based numericals on work, power, kWh and short conceptual definitions of positive/negative/zero work.
Work and Its Scientific Concept
What is Work?
In science, work is done only when a force acts on a body and the body moves (is displaced) along the direction of the force. Two conditions must be satisfied:
- A force should act on the object.
- The object must be displaced.
Formula for Work
When a constant force F acts on a body and displaces it by s, the work done is:
W = F × s (when force and displacement are in the same direction)
If the force acts at an angle θ to the displacement:
W = F s cos θ
Unit of work: joule (J). 1 joule = 1 newton × 1 metre (1 J = 1 N m). Work is a scalar quantity.
Positive, Negative and Zero Work
| Type of Work | Condition (angle θ) | Example |
|---|---|---|
| Positive | Force and displacement in same direction (θ = 0°, cos 0° = 1) | Pushing a body and it moves forward |
| Negative | Force opposite to displacement (θ = 180°, cos 180° = -1) | Friction acting on a sliding body |
| Zero | No displacement, or force ⟂ displacement (θ = 90°, cos 90° = 0) | Pushing a wall; coolie carrying load on head walking horizontally |
CBSE/HBSE Trap: A coolie walking on a level road with a load on his head does zero work against gravity, because the force (upward) and displacement (horizontal) are perpendicular. The Moon revolving around the Earth also does zero work (centripetal force ⟂ motion).
Sign Convention
- Work done by an agent is positive if it increases the body's energy.
- Work done against a force (like gravity, while lifting) is positive for the lifter but the gravitational work is negative.
Energy: Kinetic and Potential
What is Energy?
Energy is the capacity of a body to do work. Its SI unit is the joule (J), the same as work. A larger unit is the kilojoule (kJ); 1 kJ = 1000 J. Energy is a scalar quantity.
Forms of Energy
Mechanical energy (kinetic + potential), heat, chemical, electrical, light, sound, and nuclear energy.
Kinetic Energy (KE)
The energy possessed by a body due to its motion is called kinetic energy.
KE = ½ m v²
where m = mass and v = velocity of the body.
Work–Energy Theorem: The work done by a net force on a body equals the change in its kinetic energy.
W = KE_final − KE_initial = ½ m v² − ½ m u²
Potential Energy (PE)
The energy possessed by a body due to its position or configuration is called potential energy. For a body of mass m raised to height h:
PE = m g h
where g = 9.8 m/s² (acceleration due to gravity).
Tip: Gravitational PE depends only on the vertical height h, not on the path taken to reach that height.
Comparison Table
| Property | Kinetic Energy | Potential Energy |
|---|---|---|
| Cause | Motion of body | Position/configuration |
| Formula | ½ m v² | m g h |
| Example | Flowing water, moving car | Water in a dam, stretched bow |
Transformation of Energy
Energy changes from one form to another. Example: in a hydroelectric plant, PE of stored water → KE of falling water → electrical energy.
Conservation of Energy, Power and kWh
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 before and after the transformation remains constant.
Free Fall Example
For a body of mass m falling freely from height H, at any point the total mechanical energy (KE + PE) stays constant:
KE + PE = ½ m v² + m g h = constant
| Position | PE | KE | Total |
|---|---|---|---|
| At top (h = H, v = 0) | mgH | 0 | mgH |
| Midway | mgH/2 | mgH/2 | mgH |
| Just before ground (h = 0) | 0 | ½mv² = mgH | mgH |
Power
Power is the rate of doing work (or rate of transfer of energy).
P = W / t
SI unit: watt (W). 1 watt = 1 joule/second. A bigger unit is the kilowatt: 1 kW = 1000 W. Average power = total energy used ÷ total time taken.
Commercial Unit of Energy — kilowatt-hour (kWh)
The joule is too small for commercial use, so electrical energy is measured in kilowatt-hour (kWh), also called 1 unit.
1 kWh = 1 kW × 1 h = 1000 W × 3600 s = 3.6 × 10⁶ J = 3.6 × 10⁶ J
CBSE/HBSE Trap: 1 kWh is a unit of energy, not power. Always convert watt to kilowatt and hours separately when finding the number of units consumed.
Frequently asked questions
Are these Work and Energy notes free?
Yes — the Work and Energy notes for Science (Class 9) on Siksha Sarovar are completely free to read, with no account required.
Do these notes follow CBSE and HBSE?
Yes. The Work and Energy notes are NCERT-aligned and include guidance for both CBSE and Haryana Board (HBSE), with important questions and MCQs for revision.
What does the Work and Energy chapter cover?
Concept explanations, key formulas and definitions, fully solved examples and board-pattern practice questions for Work and Energy.