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Schooling · Class 9 · Science · Work, Energy, and Simple Machines · Chapter on a page

CBSE Class 9 · Science · NCERT Exploration

Work, Energy, and Simple Machines: chapter on a page

A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).

About the notes: Written by Shishya's AI on 26 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.

This note's current text has no applied fact-check on record.

Mind map

Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.

Mind map of Work, Energy, and Simple Machines: Key ideas, Formulas and facts, Watch out forKey ideasWork is done when a force acts on an…Work equals force multiplied by…Energy is the capacity to do work.…Kinetic energy is the energy an…Potential energy is stored energy…The law of conservation of energy…Power is the rate of doing workFormulas and factsW = F × sKE = (1/2) × m × v²PE = m × g × hP = W / tMechanical advantage (MA) = Load /…1 joule of work is done when a force…When a freely falling object loses…Watch out forThinking work is done whenever force…Confusing mass and weight in energy…Using speed without squaring it in…Believing machines create extra energyMixing up joule and wattWork, Energy, andSimple Machines
The mind map as a list
  • Key ideas
    • Work is done when a force acts on an…
    • Work equals force multiplied by…
    • Energy is the capacity to do work.…
    • Kinetic energy is the energy an…
    • Potential energy is stored energy…
    • The law of conservation of energy…
    • Power is the rate of doing work
  • Formulas and facts
    • W = F × s
    • KE = (1/2) × m × v²
    • PE = m × g × h
    • P = W / t
    • Mechanical advantage (MA) = Load /…
    • 1 joule of work is done when a force…
    • When a freely falling object loses…
  • Watch out for
    • Thinking work is done whenever force…
    • Confusing mass and weight in energy…
    • Using speed without squaring it in…
    • Believing machines create extra energy
    • Mixing up joule and watt

What this chapter is about

This chapter introduces you to the scientific meaning of work, which is different from everyday usage. In science, work is done only when a force moves an object in the direction of that force. You will learn how to calculate work and understand its unit, the joule.

Key ideas

  • Work is done when a force acts on an object and the object moves in the direction of that force. If there is no movement, or movement is perpendicular to the force, no work is done.
  • Work equals force multiplied by displacement in the direction of the force: W = F × s. The SI unit of work is the joule (J), where 1 joule = 1 newton × 1 metre.
  • Energy is the capacity to do work. An object that can do work possesses energy.
  • Kinetic energy is the energy an object has because of its motion: KE = (1/2) × m × v².
  • Potential energy is stored energy due to position or configuration. Gravitational potential energy near Earth's surface is PE = m × g × h.
  • The law of conservation of energy states that the total energy of an isolated system remains constant; energy only transforms from one type to another.
  • Power is the rate of doing work: P = W / t. Its SI unit is the watt (W), where 1 watt = 1 joule per second.
  • Simple machines (lever, pulley, inclined plane, wheel and axle, wedge, screw) help us do work by changing the magnitude or direction of the applied force.

Formulas and facts to remember

  • Work: W = F × s (force times displacement in the direction of force). Unit: joule (J).
  • Kinetic energy: KE = (1/2) × m × v². Unit: joule.
  • Gravitational potential energy: PE = m × g × h. Unit: joule.
  • Power: P = W / t. Unit: watt (W). 1 kilowatt = 1000 watts.
  • Mechanical advantage (MA) = Load / Effort. It tells how many times a machine multiplies your force.
  • 1 joule of work is done when a force of 1 newton moves an object by 1 metre in its direction.
  • When a freely falling object loses potential energy, it gains an equal amount of kinetic energy (ignoring air resistance).
  • Commercial unit of energy: 1 kilowatt-hour (kWh) = 3.6 × 10⁶ J.

Common mistakes

  • Thinking work is done whenever force is applied → Work requires displacement in the direction of the force; holding a heavy bag while standing still does zero work scientifically.
  • Confusing mass and weight in energy formulas → Mass (kg) is used in KE and PE formulas; weight is the force (N) due to gravity.
  • Using speed without squaring it in kinetic energy → Remember KE depends on v², so doubling speed quadruples kinetic energy.
  • Believing machines create extra energy → Machines only transfer or transform energy; output work can never exceed input work.
  • Mixing up joule and watt → Joule measures energy or work; watt measures power (rate of doing work).

Quick revision

  • Work = Force × Displacement (in the same direction). Unit: joule.
  • Kinetic energy = (1/2) × mass × (speed)².
  • Potential energy = mass × g × height.
  • Total mechanical energy stays constant when only gravity acts (conservation of energy).
  • Power = Work / Time. Unit: watt.
  • Simple machines make work easier, not less; they trade force for distance.