# Work, Energy and Power Notes, Class 11 Physics | SwaVid Work, Energy and Power, NCERT Class 11 Physics: notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers. Canonical: https://www.swavid.com/cbse/class-11/physics/chapter/work-energy-and-power Source: https://www.swavid.com/cbse/class-11/physics/chapter/work-energy-and-power # Work, Energy and Power ## Formulas and definitions ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 11 Physics on SwaVid ### Scalar product and work done ### Work-energy theorem ### Work done by variable force ### Conservative forces and potential energy ### Conservation of mechanical energy ### Potential energy of a spring ### Power and collisions ### Next chapter ### Related chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 11 › Physics › Work, Energy and Power Notes written by SwaVid from the NCERT textbook. Class 11 Physics, Chapter 5. Key ideas, formulas and common mistakes. CBSE prints 17 marks for Units IV-VI together, not for this chapter alone. Short answer. This chapter defines work, kinetic energy, potential energy, and power, and explains how forces transfer and transform energy. It shows that the total mechanical energy of a system remains constant when only conservative forces do work. Source: NCERT Physics Part I (Reprint 2026-27) , Chapter 5, Work, Energy and Power The scalar or dot product of two vectors A and B is A · B = A B cos θ. Work done by a constant force F causing displacement d is W = F · d = F d cos θ, where θ is the angle between F and d. The change in kinetic energy of a particle equals the total work done on it by all forces. For an initial speed vᵢ and final speed v f , K f - Kᵢ = W. For a one-dimensional variable force F(x), the work done is the integral of F(x) dx from the initial to the final position. This work equals the area under the force-displacement curve. A force is conservative if the work done by it depends only on the end points and is zero over any closed path. For such a force, potential energy V(x) is defined such that F(x) = - dV/dx. The total mechanical energy E = K + V remains constant if the forces doing work on the system are conservative. Non-conservative forces like friction dissipate mechanical energy into heat or sound. A spring exerts a restoring force F = - k x, where k is the spring constant and x is the displacement from equilibrium. The potential energy stored in the stretched or compressed spring is V(x) = (1/2) k x². Power is the time rate at which work is done, given by P = dW/dt = F · v. Collisions conserve total linear momentum; elastic collisions conserve total kinetic energy as well, whereas inelastic collisions do not. Try each question first. Then tap it to see the answer. W = F d cos θ W = 20 × 4 × cos 60° = 20 × 4 × 0.5 = 40 J 40 J W = K f - Kᵢ = (1/2) m (v f ² - vᵢ²) W = (1/2) × 1000 × (20² - 10²) = 500 × (400 - 100) = 500 × 300 = 150000 J 1.5 × 10⁵ J P = F · v = F v cos 0° P = 10000 × 2 × 1 = 20000 W 20000 W System of Particles and Rotational Motion . Laws of Motion . Gravitation . The 14 chapters of Class 11 Physics . CBSE does not give marks to one chapter. It prints 17 marks for Units IV-VI together (Work, Energy and Power; System of Particles and Rigid Body; Gravitation). Work, Energy and Power is part of that group. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. System of Particles and Rotational Motion. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 5 NCERT book - 17 marks, Units IV-VI together - 3 quick checks - Treating work as a vector quantity because force and displacement are vectors . Work is the scalar product of force and displacement. It has magnitude and sign, but no direction. - Forgetting the negative sign in F(x) = - dV/dx . A conservative force always points in the direction of decreasing potential energy. Keep the minus sign when finding force from potential energy. - Assuming total energy is conserved only in elastic collisions . Total energy and momentum are conserved in all collisions. Only kinetic energy is conserved specifically in elastic collisions. - Calculating work as force times distance when force is not parallel to displacement . 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