Class 11 Physics Chapter 5 Work, Energy and Power – Revision Notes

Chapter 5 of Class 11 Physics tracks how forces transfer energy, from the work-energy theorem to the different forms of potential and kinetic energy. It also distinguishes elastic collisions, where kinetic energy is conserved, from inelastic ones, where only momentum survives.

Last Updated: September 23, 2026

Key Formulas

• Work: W=Fd cosθ.
• KE=½mv²; Gravitational PE=mgh; Spring PE=½kx².
• Work-energy theorem: W=ΔKE.

Conservation of Energy

• KE+PE=constant (no non-conservative forces).

Collisions

• Elastic: momentum + KE conserved.
• Inelastic: only momentum conserved.

Power

• P=W/t=F·v; SI unit watt; 1 HP=746 W.

One-Line Summary

Work, energy, and power describe how forces transfer energy to bodies, with mechanical energy conserved in ideal (frictionless) systems and different collision types conserving momentum with or without kinetic energy.

Quick visual: a worked diagram from the full Solutions page, for reference.

Frequently Asked Questions

Why is work considered zero if a force does not cause any displacement?
Work is defined as the product of force and displacement in the direction of the force. If there is no displacement, or if the force is perpendicular to the displacement, the work done is zero, even if the force itself is large.

What is the work-energy theorem, and why is it useful?
The work-energy theorem states that the net work done on an object equals the change in its kinetic energy, which allows problems involving forces and motion to be solved using energy methods without calculating acceleration or time directly.

Written by Satish

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