Chapter 5 — Work, Energy and Power

Chapter 5 of the Class 11 Physics NCERT textbook, "Work, Energy and Power", introduces these fundamental quantities where work represents force applied over displacement, kinetic energy is the energy of motion, and power is the rate of work done—all related through the work-energy theorem stating that work done equals change in kinetic energy.

• Work, energy, and power defined — The chapter connects three linked ideas: work as force acting through a displacement, kinetic energy as energy of motion, and power as the rate at which work is done. The work-energy theorem ties work directly to changes in kinetic energy.
• Conservative forces and energy conservation — Forces like gravity and springs are conservative, depending only on start and end positions, which lets total mechanical energy stay constant. Non-conservative forces such as friction depend on the path and dissipate energy.
• Energy in collisions — Collisions always conserve momentum, but kinetic energy is preserved only in elastic collisions. Inelastic collisions lose kinetic energy to deformation, illustrating how energy transforms rather than simply persisting.

1. 01Work is done by a force only when there is displacement in the direction of the force; W = (F cos θ)d = F·d
2. 02Kinetic energy K = (1/2)mv² is always positive and represents energy due to motion
3. 03Conservative forces (gravity, spring forces) depend only on initial and final positions; non-conservative forces (friction) depend on the path
4. 04Mechanical energy is conserved for conservative forces: E = K + V(x) = constant, where V is potential energy
5. 05Power is the time rate of doing work: P = dW/dt = F·v, measured in watts (1 W = 1 J/s)
6. 06In collisions, linear momentum is always conserved; kinetic energy conserves only in elastic collisions, not in inelastic ones