⚡ CBSE · Class 11 · Physics · Chapter 6

Work, Energy
and Power

Complete chapter resources for CBSE Class 11 Physics — topic breakdown, key formulas, work-energy theorem, conservation of energy, sample questions, and instant AI question paper generation.

4Topics
8–10Board marks
8Sample questions
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Key Formulas — Chapter 6
  • Work (constant force): W = F · d · cos θ
  • Kinetic energy: KE = ½mv²
  • Work-energy theorem: W_net = ΔKE = ½mv² − ½mu²
  • Potential energy (gravity): PE = mgh
  • Conservation of ME: KE + PE = constant
  • Power: P = W/t = F · v
Chapter Overview

What this chapter covers

Work, Energy and Power (Chapter 6, NCERT Class 11 Physics) establishes the energy framework that underpins all of classical mechanics. Work is defined as W = F · d · cos θ — the product of force, displacement, and the cosine of the angle between them — making it a scalar quantity measured in joules. Zero work is done when force is perpendicular to displacement (e.g., a centripetal force on a circular orbit), a distinction frequently tested in CBSE board exams.

The chapter's central theorem is the work-energy theorem: the net work done on a body equals its change in kinetic energy (W_net = ½mv² − ½mu²). This links dynamics directly to energy and is the standard 5-mark derivation question. The chapter then distinguishes conservative forces (gravity, spring) from non-conservative ones (friction), introduces gravitational and elastic potential energy, and proves the law of conservation of mechanical energy for a freely-falling body — another flagship board derivation.

The final section covers collisions (elastic and inelastic in one and two dimensions) and power (P = W/t = F·v). Numerical problems on power, efficiency, and collision kinematics — especially the elastic collision velocity exchange formulas — round out the chapter's board weightage and are high-priority for the last-minute revision checklist.

Topics

What's inside Chapter 6

As per NCERT Class 11 Physics (CBSE syllabus)

Topic 1
Work Done by a Force
Definition W = F·d·cos θ. Work done by a variable force using integration. Positive, negative, and zero work — with key examples (gravity, tension, centripetal force).
Topic 2
Kinetic and Potential Energy
KE = ½mv². Gravitational PE = mgh. Elastic PE = ½kx² (spring). Concept of conservative and non-conservative forces. Derivation of the work-energy theorem.
Topic 3
Conservation of Mechanical Energy
Proof that KE + PE = constant for conservative forces. Application to a freely-falling body. Energy lost to friction in non-conservative systems.
Topic 4
Power and Collisions
Power P = W/t = F·v; units and efficiency. Elastic collisions (KE conserved) and inelastic collisions (KE not conserved). Velocity equations for 1-D elastic collision.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 4 · Laws of Motion
Newton's second law (F = ma) used in the work-energy theorem derivation
Ch 3 · Motion in a Plane
Vector dot product required for W = F · d · cos θ
Chapter 6 Work, Energy
& Power
Leads to
Ch 7 · System of Particles & Rotational Motion
Rotational KE, moment of inertia, angular momentum
Class 12 · Electrostatics
Electric potential energy and work done against electric force mirror this framework
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 11 Physics board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion-Reason 1 1–2 Zero work condition, nature of collision, or unit of power
Very Short Answer 2 1 Define work/power/energy, state work-energy theorem, or a quick numerical
Short Answer 3 1 Numerical on KE/PE, power calculation, or conservative force distinction
Long Answer / Derivation 5 1 Work-energy theorem, conservation of ME, or elastic collision velocity formulas
Total (approximate) 8–10 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 11 Physics Chapter 6. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
A force of 10 N acts on a body at an angle of 90° to the direction of displacement. The work done by the force is: (a) 10 J (b) −10 J (c) 0 J (d) 100 J
Q2 Easy 2 marks Short Answer
State the work-energy theorem. A car of mass 800 kg increases its speed from 20 m/s to 30 m/s. Calculate the work done by the net force acting on it.
Q3 Medium 2 marks Short Answer
Distinguish between conservative and non-conservative forces with one example each. Why is potential energy defined only for conservative forces?
Q4 Medium 3 marks Numerical
A body of mass 2 kg is projected vertically upward with a velocity of 20 m/s. Using conservation of mechanical energy, find (i) the maximum height reached and (ii) the velocity of the body when it is at a height of 15 m. (g = 10 m/s²)
Q5 Medium 3 marks Word Problem
A pump lifts 200 kg of water per minute from a well of depth 10 m and ejects it with a velocity of 4 m/s. Calculate the power of the pump. (g = 10 m/s²)
Q6 Hard 4 marks Derivation
Derive the work-energy theorem for a constant net force. A block of mass 5 kg starts from rest and is acted on by a net force of 20 N. Using the theorem, find the speed of the block after it has moved 10 m.
Q7 Hard 5 marks Derivation
Prove that the total mechanical energy (KE + PE) of a body falling freely under gravity is conserved at every point during its fall. Illustrate with a diagram showing energy variation at the starting point, midpoint, and just before hitting the ground.
Q8 Hard 5 marks Word Problem
A ball of mass 0.4 kg moving with a velocity of 10 m/s collides elastically with another ball of mass 0.6 kg initially at rest. (i) Derive or state the expressions for the final velocities after a 1-D elastic collision. (ii) Calculate the final velocity of each ball. (iii) Verify that kinetic energy is conserved in this collision.
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 11 Physics board papers — Work, Energy and Power chapter.

Board 20225 marks
State and prove the work-energy theorem. A bullet of mass 20 g moving with a velocity of 500 m/s strikes a wooden block of mass 1 kg and gets embedded in it. Find the velocity of the block after impact and the loss in kinetic energy. (All India 2022)
Board 20233 marks
A body of mass 5 kg is taken to a height of 10 m and then allowed to fall freely. Using conservation of energy, calculate (i) kinetic energy at half height, and (ii) velocity just before reaching the ground. (g = 10 m/s²) (Delhi 2023)
Board 20202 marks
Define the coefficient of restitution. Under what condition is it equal to 1? What does e = 0 signify? (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Work, Energy and Power carry in the CBSE Class 11 Physics board exam? +
Work, Energy and Power typically carries 8–10 marks in the CBSE Class 11 Physics board exam, spread across 3–4 questions — one 1-mark MCQ or assertion-reason, one 2-mark short answer on formulas or concepts, and one 4–5 mark derivation or numerical. The work-energy theorem and conservation of mechanical energy are the highest-probability derivation topics.
What is the work-energy theorem and how is it derived for board exams? +
The work-energy theorem states that the net work done on an object equals its change in kinetic energy: W_net = ΔKE = ½mv² − ½mu². For CBSE boards, the standard derivation uses Newton's second law (F = ma) combined with the kinematic equation v² = u² + 2as to eliminate acceleration and arrive at the result. Students must present this as a step-by-step proof.
What is the difference between conservative and non-conservative forces? +
A conservative force (e.g., gravity, spring force) does work that depends only on the initial and final positions, not the path taken — and mechanical energy is conserved in such systems. A non-conservative force (e.g., friction, air drag) does path-dependent work and converts mechanical energy into heat or other forms. CBSE board questions test this distinction through short answers and often link it to potential energy definitions.
How are elastic and inelastic collisions different, and which is more important for boards? +
In an elastic collision both momentum and kinetic energy are conserved (e.g., billiard balls). In an inelastic collision only momentum is conserved; kinetic energy is lost (e.g., a lump of clay sticking together after impact). CBSE boards test both, but the elastic collision in one dimension — with expressions for final velocities (v₁ = (m₁−m₂)u₁/(m₁+m₂) and v₂ = 2m₁u₁/(m₁+m₂)) — is the most frequently asked 5-mark derivation in this chapter.
How do I generate a custom question paper for Work, Energy and Power using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 11 Physics, select Chapter 6 (Work, Energy and Power), set your preferred question-type mix (MCQ, short answer, word problem) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.