Class 9 Science Chapter 7, Work, Energy, and Simple Machines, offers a simpler route than Newton’s laws when forces change with time or act in complicated ways: the ideas of work, energy and power. It ends with simple machines (the pulley, inclined plane and lever) that make work easier.
What Class 9 Science Chapter 7 Covers
7.1 Work Done by a Constant Force explains when work done is zero and when it is positive or negative.
7.2 to 7.4 Energy state the work-energy theorem, list forms of energy, and cover kinetic energy, potential energy including gravitational potential energy, and the conservation of mechanical energy.
7.5 Power defines power as the rate of doing work.
7.6 Simple Machines covers the pulley, the inclined plane and the lever, followed by a Ready to Go Beyond section.
Worked examples: the SI unit of work and energy, the joule, is named after James Prescott Joule. The chapter calculates the kinetic energy of a 0.2 kg cricket ball bowled at about 154.8 km/h, the speed of a 15,000 kg jet landing on an aircraft carrier deck, and the minimum length of a ramp needed to stop a runaway truck. Power is measured in watts (1 W = 1 J/s); lifting 75 kg through 2 m in 5 s needs 300 W, and horsepower is still used for car engines and water pumps.
Exercises in Chapter 7
- Revise, Reflect, Refine numericals on work, kinetic and potential energy, and power.
- The Journey Beyond: an energy experiment with a pen refill, building simple machines, and computer simulations of energy changes.
Read Class 9 Science Chapter 7 Online
More Chapters
- Previous: Chapter 6: How Forces Affect Motion
- Next: Chapter 8: Journey Inside the Atom
- Hindi medium: Class 9 Science Chapter 7 PDF in Hindi (Anveshan)
- All Science chapters and the other Class 9 books: Class 9 NCERT books 2026-27
FAQ
What is the work-energy theorem?
The work done on an object or a system equals the change in its kinetic energy.
Which simple machines are in Class 9 Science Chapter 7?
The pulley, the inclined plane and the lever.
What is the SI unit of power?
The watt (W): 1 watt is 1 joule of work done per second.