NCERT Notes Class 11 Physics Chapter 5 Work, Energy and Power

These Class 11 Physics Notes Chapter 5 Work Energy and Power pull together every definition, formula, derivation and collision rule that the Boards, JEE Main, JEE Advanced, NEET, CUET and NDA papers actually test in 2026-27. Use them to revise the whole chapter fast, with work, energy, power and collisions summed up in one place.

This chapter builds the idea of energy from the work done by a force, then uses it to explain motion, springs and collisions across the rest of mechanics.

• CBSE Weightage: 6 to 8 marks, usually one derivation plus a numerical on the work-energy theorem or a collision.
• Topics covered: work by constant and variable force, work-energy theorem, kinetic and potential energy, conservation of mechanical energy, power, and collisions.
• Key formulas: work integral, kinetic energy, gravitational and spring potential energy, power, and the elastic-collision velocity equations.

These Class 11 Physics Notes Chapter 5 Work Energy and Power are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board, JEE Main and NEET papers.

Topic-by-Topic Summary of Work, Energy and Power

The chapter turns force into energy. It starts with the work a force does, links that work to a change in speed, and then uses energy to explain springs, gravity and collisions. Here is the quick map of what each topic gives you.

• Work: the energy a force transfers when it moves an object, for both a constant force and a variable force.
• Kinetic energy: the energy of motion, tied to work by the work-energy theorem.
• Potential energy: stored energy, covering gravitational potential energy and the spring potential energy of a stretched or compressed spring.
• Conservation of mechanical energy: for a conservative force, kinetic plus potential energy stays constant.
• Power: the rate of doing work, as both an average and an instantaneous value.
• Collisions: elastic and inelastic collisions in one and two dimensions.

Revise the topics in this order, because each one uses the one before it. Get work and the work-energy theorem right first, and collisions become far easier. These Class 11 Physics Notes Chapter 5 Work Energy and Power follow the same sequence as the NCERT textbook.

Work Done by a Constant Force and a Variable Force

Work is done when a force moves its point of application through a distance. For a constant force, work is the product of the force, the displacement, and the cosine of the angle between them. Only the part of the force along the displacement does work.

• Constant force: W = F d cosθ, where θ is the angle between force and displacement.
• Positive work: the force has a component along the motion, so θ < 90°.
• Negative work: the force opposes the motion, as friction does, so θ > 90°.
• Zero work: the force is perpendicular to the motion, as with circular motion, so θ = 90°.

When the force changes with position, you cannot just multiply. A variable force needs the work written as an integral, W = ∫ F dx. The work done by a variable force equals the area under the force-displacement graph. This graph reading is a favourite in objective papers, so practise it until it is automatic.

Work-Energy Theorem and Kinetic Energy

Kinetic energy is the energy an object has because it is moving. For an object of mass m at speed v, it is K = ½ m v². The work-energy theorem then ties work directly to this energy.

• Statement: the net work done on an object equals the change in its kinetic energy, W_net = ΔK.
• Expanded form: W_net = ½ m v² − ½ m u², the final kinetic energy minus the initial value.
• Use: it finds a final speed without needing the time, which makes many numericals shorter than using the equations of motion.

The theorem works for both constant and variable forces, which is why it is so useful. Add up the work done by every force first, then set the total equal to the change in kinetic energy. A common exam question gives a graph of force against distance and asks for the speed gained, which is a direct use of this theorem.

Potential Energy and Conservation of Mechanical Energy

Potential energy is stored energy that depends on position. The chapter uses two forms most often, and both convert into kinetic energy when the object is released.

• Gravitational potential energy: U = m g h for a mass raised a height h near the Earth's surface.
• Spring potential energy: U = ½ k x² for a spring of stiffness k stretched or compressed by x.
• Conservative force: a force, like gravity or a spring force, whose work does not depend on the path taken.

For a conservative force, the total mechanical energy stays fixed. Kinetic energy plus potential energy is constant, so K + U = constant. This is the law of conservation of mechanical energy, and it turns a hard force problem into a simple energy bookkeeping. When friction is present, the lost energy appears as heat, so mechanical energy alone is no longer conserved.

All Formulas for Work, Energy and Power

Every formula you need for the chapter sits in one table below, with its meaning and its SI unit. Learn the work-energy theorem and the collision rows first, since those carry the most marks in both Boards and entrance papers.

Carry the SI unit on every line of your working. Work, energy and heat all share the joule, and power uses the watt, so mixing them is an easy slip. Keep this table open while you solve the back-exercise numericals.

Key Definitions and Derivations for Work, Energy and Power

Boards short-answer questions often ask for a clean definition in one or two lines. Learn these word-for-word, because a vague definition loses easy marks. Each one also sets up a derivation you can be asked to show.

A common derivation asks you to prove the work-energy theorem for a variable force. Start from W = ∫ F dx, replace F with m v (dv/dx), and integrate to reach W = ½ m v² − ½ m u². This clean derivation is a repeat favourite in the Boards.

Power: Average Power and Instantaneous Power

Power tells you how fast work is done. Two machines can do the same work, but the faster one has more power. The chapter splits power into an average value and an instantaneous value.

• Average power: the total work divided by the total time, P_avg = W/t.
• Instantaneous power: the power at a single moment, P = F · v, the dot product of force and velocity.
• Unit: the SI unit is the watt, and a common practical unit is the kilowatt-hour, a unit of energy not power.

Instantaneous power is the one students forget can be written as force times velocity. When a vehicle moves at constant speed, the engine power equals the resisting force times the speed. Numericals on lifting a load, pumping water, or a car climbing a slope all use these two power formulas, so keep both ready in the exam.

Collisions: Elastic and Inelastic in One and Two Dimensions

Collisions are where the chapter is tested most in entrance exams. In every collision, momentum is conserved. What changes is whether kinetic energy is also conserved, and that splits collisions into two types.

In a one-dimensional elastic collision, solving the momentum and energy equations together gives the two final velocities. When two equal masses collide elastically and one is at rest, they simply swap velocities. A two-dimensional collision, such as a glancing hit between two balls, needs momentum conserved separately along two perpendicular axes. Write the x and y momentum equations on their own lines to avoid mixing components.

Common Mistakes Students Make in Work, Energy and Power

These slips happen while writing or calculating, not because the concept is unclear. Each one costs 1 to 3 marks in the paper, so watch for them at the exact step.

Work, Energy and Power Weightage in CBSE Boards, JEE and NEET

This chapter is a heavy scorer. It nearly always carries a derivation or a numerical in the Boards, and it is one of the most tested mechanics chapters in JEE and NEET. Here is how the marks split across the main exams for 2026-27.

Collisions and the work-energy theorem are the two most tested ideas from this chapter across all four exams. Master them first, then power, then spring potential energy, in that order of return on effort.

How to Revise Work, Energy and Power Quickly

Use these Class 11 Physics Notes Chapter 5 Work Energy and Power for a fast, ordered recap the night before a test. The checklist below takes about 30 minutes and hits every marks-heavy idea.

• First 10 minutes: write the work formula, the kinetic and potential energy formulas, and both power formulas from memory.
• Next 10 minutes: redo one work-energy theorem numerical and one conservation of mechanical energy problem.
• Last 10 minutes: solve one 1D elastic collision and check that both momentum and kinetic energy balance.

Close the loop by proving the work-energy theorem once without notes. If you can do all three blocks and the derivation without help, the chapter is exam-ready. Keep the All Formulas table beside you for the first pass only, then try it closed-book.

Student Feedback on the Work, Energy and Power Notes

Other Work, Energy and Power Class 11 Physics Resources

Pair these notes with the solved answers, the handwritten notes, the formula sheet, and the textbook PDF for the same chapter.

NCERT Notes for Class 11 Physics: All Chapters

Jump to the revision notes for any other Class 11 Physics chapter below.

FAQs on Work, Energy and Power Class 11 Physics Notes