Work and Energy

Why This Chapter Matters

Reading for three hours may feel like hard work, but physics uses the word work more narrowly. If an object does not move in the direction of force, mechanical work on that object may be zero. Once work is defined precisely, energy becomes the capacity for change and power tells how quickly energy is transferred. The chapter's deepest idea is conservation: energy changes form rather than disappearing.

Bridge from Earlier Learning

• Force
• Displacement
• Velocity
• Gravitation
• Basic algebra and squares

These ideas are reused and deepened in this chapter.

1. Scientific Meaning of Work

2. Energy

Energy is the capacity of a system to do work or bring about change. Mechanical energy appears mainly as kinetic energy of motion and potential energy due to position or configuration. Energy has the same SI unit as work: joule.

3. Kinetic Energy

A moving object can do work by transferring motion. Using Newton's second law and equations of motion, the work needed to accelerate a mass from rest to speed v is 1/2 mv². Because speed is squared, doubling speed quadruples kinetic energy if mass stays constant.

4. Gravitational Potential Energy

Lifting a mass against gravity requires work. Near Earth's surface, lifting mass m through height h increases gravitational potential energy by mgh. The zero level is chosen for convenience; physically meaningful predictions depend on changes in potential energy.

5. Conservation of Energy

A falling object starts with more gravitational potential energy and less kinetic energy. As it falls, potential energy decreases while kinetic energy increases. Ignoring air resistance, total mechanical energy remains constant. In real situations some mechanical energy becomes thermal or sound energy, but total energy is conserved when all forms are included.

6. Power

Two machines may do the same work in different times. The faster one has greater power. Power is work done or energy transferred per unit time. One watt is one joule per second. Power ratings therefore describe rate, not total energy used.

7. Kilowatt-Hour

The joule is small for household electrical energy. One kilowatt-hour is the energy transferred by a 1 kW device operating for one hour. It is an energy unit, not a power unit. It equals 3.6 × 10⁶ J.

8. Choosing the Right Energy Idea

Speed change suggests kinetic energy, height change suggests gravitational potential energy, and time rate suggests power. In conservation questions, define the system and compare energy forms before and after instead of stacking formulas without a physical story.

Visualise the Concept

Energy changes form; conservation means total energy does not vanish.

Formula / Relationship Sheet

Worked Examples & Scientific Reasoning

Observe Like a Scientist

Exam Thinking Strategy

Common Mistakes

Quick Revision

Focused Practice

Final Chapter Summary