⚡ CBSE · Class 9 · Science · Chapter 11

Work and
Energy

Complete chapter resources for CBSE Class 9 Science — topic breakdown, key formulas, sample questions, previous year board questions, and instant AI question paper generation.

4Topics
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Key Formulas — Chapter 11
  • Work done: W = F × d × cos θ (SI unit: joule, J)
  • Kinetic energy: KE = ½ mv²
  • Gravitational PE: PE = mgh
  • Work–energy theorem: W = ΔKE = ½mv² − ½mu²
  • Power: P = W / t (SI unit: watt, W)
  • 1 kWh = 3.6 × 10⁶ J
Chapter Overview

What this chapter covers

Chapter 11 of NCERT Class 9 Science introduces the scientific meaning of work, which differs from everyday usage. Work is done on an object only when a force causes displacement in the direction of the applied force: W = F × d × cos θ. The SI unit of work is the joule (J). Special cases — zero work when force and displacement are perpendicular, and negative work when force opposes displacement — are important conceptual points tested in exams.

The chapter then develops the concept of energy as the capacity to do work. The two principal mechanical forms are kinetic energy (KE = ½mv², the energy of motion) and gravitational potential energy (PE = mgh, the energy stored due to height). The work–energy theorem links these: the net work done on a body equals the change in its kinetic energy. The law of conservation of mechanical energy — demonstrated classically through free fall — states that the total of kinetic and potential energy remains constant in the absence of friction.

Power is the rate of doing work (P = W/t), measured in watts (W). The chapter also introduces the commercial unit of energy, the kilowatt-hour (kWh) equal to 3.6 × 10⁶ J, which connects physics directly to household electricity bills — a popular application context in board questions.

Topics

What's inside Chapter 11

As per NCERT Class 9 Science (CBSE syllabus)

Topic 1
Work
Scientific definition of work: W = Fd cos θ. Conditions for zero, positive, and negative work. SI unit — joule. Concept of 1 joule as the work done by 1 N force over 1 m displacement.
Topic 2
Kinetic Energy and the Work–Energy Theorem
KE = ½mv². Derivation using equations of motion. Work–energy theorem: net work = change in KE. Applications — stopping distance of a vehicle, comparing KE of objects with different masses and speeds.
Topic 3
Potential Energy and Conservation of Energy
Gravitational PE = mgh. Law of conservation of mechanical energy — total mechanical energy (KE + PE) is constant during free fall. Proof by substituting equations of motion at any height h.
Topic 4
Power and Commercial Unit of Energy
Power P = W/t, unit watt (W) and kilowatt (kW). Average power vs instantaneous power. Commercial unit: 1 kWh = 3.6 × 10⁶ J. Calculation of electricity consumption and cost — standard board numericals.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 8 · Motion
Equations of motion (v² = u² + 2as) used to derive KE and the work–energy theorem
Ch 9 · Force and Laws of Motion
Newton's second law and net force concept — prerequisite for work done by a net force
Chapter 11 Work and
Energy
Leads to
Ch 12 · Sound
Sound as a mechanical wave that carries energy — energy transmission concepts extend here
Class 10 · Electricity
Electrical energy (P = VI), power dissipation in resistors, and kWh billing build directly on this chapter
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 9 Science board-pattern papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Objective 1 1–2 Definition of work, unit identification, or condition for zero work
Very Short Answer 2 1 Numerical on KE or PE; state and explain the work–energy theorem
Short Answer 3 1 Power calculation or commercial-unit conversion problem
Long Answer / Derivation 5 1 Derivation of KE formula or conservation of energy during free fall
Total (approximate) 5–7 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 9 Science Chapter 11. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
A coolie lifts a load of 20 kg and walks horizontally. The work done by the coolie on the load is: (a) 20 × g joules (b) Positive (c) Zero (d) Negative
Q2 Easy 2 marks Short Answer
A force of 50 N acts on an object and displaces it by 4 m in the direction of the force. Calculate the work done by the force. State the SI unit of work.
Q3 Medium 2 marks Numerical
An object of mass 5 kg is moving with a velocity of 10 m/s. Calculate its kinetic energy. If its velocity is doubled, by what factor does the kinetic energy change?
Q4 Medium 3 marks Numerical
A pump lifts 200 kg of water per minute to a height of 5 m. Calculate the power of the pump. (Take g = 10 m/s²)
Q5 Medium 3 marks Short Answer
Define the commercial unit of energy. Convert 1 kWh into joules. A household uses a 100 W electric bulb for 10 hours daily. Calculate the energy consumed in 30 days in kWh.
Q6 Hard 4 marks Derivation
Derive the expression for the kinetic energy of an object of mass m moving with velocity v using the work–energy theorem. Start from Newton's second law and the equation of motion v² = u² + 2as.
Q7 Hard 5 marks Long Answer
State the law of conservation of energy. Show that the total mechanical energy (kinetic + potential) of a freely falling body remains constant throughout its fall. Draw the energy diagram at the highest point, midpoint, and just before hitting the ground.
Q8 Hard 5 marks Word Problem
A body of mass 2 kg is dropped from a height of 20 m. (i) What is its potential energy at the top? (ii) What is its kinetic energy just before hitting the ground? (iii) What will be its velocity just before impact? (iv) Verify the law of conservation of energy at any intermediate height of 10 m. (Take g = 10 m/s²)
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 9 Science board-pattern papers — Work and Energy chapter.

Board 20223 marks
A body of mass 4 kg is taken to the top of a building 25 m high. Calculate its potential energy at the top. If it falls freely from this height, what will be its kinetic energy just before reaching the ground? (g = 10 m/s²) (CBSE 2022)
Board 20232 marks
Under what conditions is the work done by a force (i) positive, (ii) zero, and (iii) negative? Give one example in each case. (CBSE 2023)
Board 20205 marks
State the work–energy theorem. Derive an expression for the kinetic energy of a moving body. A car of mass 1200 kg is moving with a velocity of 20 m/s. Calculate the work required to bring it to rest. (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Work and Energy carry in the CBSE Class 9 Science exam? +
Work and Energy typically carries 5–7 marks in the CBSE Class 9 Science annual examination. Questions appear as 1-mark MCQs or definitions, 2–3 mark short-answer numericals on kinetic/potential energy or power, and an occasional 5-mark long-answer on the law of conservation of energy. The chapter has featured in every major board-pattern paper for this class.
What is the scientific definition of work done and why is it different from everyday usage? +
In science, work is done on an object only when a force is applied AND the object moves in the direction of (or at least partially in the direction of) that force. W = F × d × cos θ. If a person pushes a wall and it does not move, no work is done scientifically — even though the person feels tired. This contrast between scientific and everyday meaning is a common 1–2 mark question in CBSE exams.
What is the law of conservation of energy and how is it tested in board exams? +
The law states that energy can neither be created nor destroyed; it only changes from one form to another. The total energy of an isolated system remains constant. Board questions typically ask students to show that the sum of kinetic energy and potential energy at any point during free fall is equal to the total mechanical energy at the start — this is a standard 5-mark derivation.
What is the difference between work, energy, and power — and what units should students write? +
Work is the energy transferred when a force displaces an object (W = Fd cos θ); unit is joule (J). Energy is the capacity to do work; it is measured in joules (J) as well. Power is the rate of doing work (P = W/t); unit is watt (W) or joule per second. One commercial unit of electrical energy is 1 kWh = 3.6 × 10⁶ J. Writing the correct SI unit is worth 0.5–1 mark in every numerical.
How do I generate a custom question paper for Work and Energy using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 9 Science, select Chapter 11 (Work and Energy), set your preferred question-type mix and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.