Chapter 7 — Work, Energy and Simple Machines

Chapter 7 of the NCERT Class 9 Science textbook 'Exploration' (2026-27 edition) covers work, energy, power, and simple machines. It defines work as force multiplied by displacement in the direction of force (W = F × s), explains kinetic energy (K = ½mv²) and gravitational potential energy (U = mgh), the work-energy theorem, conservation of mechanical energy, power (P = W/t), and three simple machines: pulley, inclined plane, and lever.

NCERT Class 9 Science Chapter 7 'Work, Energy, and Simple Machines' from the 2026-27 Exploration edition introduces the scientific definition of work (W = F × s, unit: joule), and the work-energy theorem stating that work done equals the change in energy. Kinetic energy is given by K = ½mv² and gravitational potential energy by U = mgh. The chapter explains conservation of mechanical energy through freely falling objects and pendulums, defines power as the rate of doing work (P = W/t, unit: watt), and examines three simple machines — pulley, inclined plane, and lever — showing how each changes the magnitude or direction of force without reducing total work done.

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.