Chapter 7 — Work, Energy and Simple Machines

Chapter 7 of the Class 9 Science NCERT textbook, "Work, Energy, and Simple Machines", defines work as force times displacement (W = F × s), explains kinetic and potential energy, the work-energy theorem, conservation of mechanical energy, power, and the pulley, inclined plane, and lever.

• Work and Energy — Work is done only when a force causes displacement in its direction (W = F × s, in joules). The work-energy theorem states work done equals the change in energy, whether kinetic (½mv²) or potential (mgh).
• Conservation of Energy — Mechanical energy, the sum of kinetic and potential energy, is conserved when only gravity acts on an object, as shown by a freely falling body and a swinging simple pendulum.
• Power as a Rate — Power measures how fast work is done, P = W/t, with SI unit the watt, where 1 W equals 1 joule per second, distinguishing quick work from slow work of the same total amount.
• Simple Machines — Pulleys, inclined planes, and levers make effort easier by changing the magnitude or direction of force without reducing total work; their mechanical advantage is load divided by effort.

1. 01Work is done on an object only when a force causes displacement in the direction of the force; W = F × s (SI unit: joule, J).
2. 02The work-energy theorem states that the work done on an object equals the change in its energy; kinetic energy K = ½mv² and gravitational potential energy U = mgh.
3. 03Mechanical energy (sum of kinetic and potential energy) is conserved when no external forces other than gravity act on an object, as demonstrated by a freely falling body and a simple pendulum.
4. 04Power is the rate at which work is done: P = W/t; the SI unit is watt (W), where 1 W = 1 J s⁻¹.
5. 05Simple machines (pulley, inclined plane, lever) reduce the effort needed by changing the magnitude or direction of force, but do not reduce the total work done; mechanical advantage = load ÷ effort.
6. 06A fixed pulley changes only the direction of force (mechanical advantage = 1); an inclined plane has mechanical advantage = L/h (length ÷ height); a lever's mechanical advantage = effort arm ÷ load arm.