E ExamMaster

CBSE Class 9 · Science

Work, Energy, and Simple Machines

Official NCERT chapter from Science (Grade 9, 2026 edition) (book code iesc1). ExamMaster notes are original teaching at CBSE Class 9 depth.

This lesson follows the official chapter “Work, Energy, and Simple Machines” in Science (Grade 9, 2026 edition). The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 9
  • Medium level
  • 8 concepts

1The Work-Energy Theorem

The work–energy theorem at this class: a net force that does work on a body changes its kinetic energy, as taught. Work along the motion is F × s when F is along s (school form). 6 N through 3 m is 18 J. A force that does not move the point of application does no work in this story.

Holding a bag still is tiredness, not this work.

Figure. Work on the block is F times s when they share a line: 15 N times 4 m is 60 J. That 60 J is the rise in kinetic energy, 20 J to 80 J.

How it works

  1. Name F along the displacement, and sThe given.
  2. Write W = F s (when they line up)The work.
  3. Say that work changed the motion-energy as the theorem framed itThe link.

6 N through 3 m

A net force of 6 N acts along a 3 m displacement. Find the work.

  • W = F s6 × 3
  • W18 J
  • ReadAlong; not a still hold

Pro tip. Lined-up force and displacement. Tiredness is not the joule.

2Forms of Energy

Forms of energy are named drawers: kinetic (motion), potential (a stored height or a taught spring-store), and other school forms if the lesson named them (heat, sound, chemical). A form is a drawer, not a new physics. One body can move from one drawer to another.

A slogan “energy is life” is not a form.

Figure. Energy keeps one unit and many forms. Kinetic, potential, heat, and light are four names for the same joule, not four substances.

How it works

  1. Name the drawerMotion-store or height-store…
  2. Name the body and the situationThis ball, this shelf.
  3. Refuse a form you cannot point toHonest drawers.

3Mechanical Energy

Mechanical energy here is kinetic plus the taught potential, taken together. A ball thrown up trades one drawer for the other if the lesson ignored air. Mechanical is that pair, not every form on Earth.

Adding a chemical-store into “mechanical” without the lesson is a steal.

Figure. Mechanical energy is PE plus KE. At the top, 8 J is all potential. At the ground, 2 J potential plus 6 J kinetic still totals 8 J.

How it works

  1. Write KE and the taught PEThe pair.
  2. Add them if the heading asks mechanicalThe sum.
  3. Watch a trade: up means more PE, less KE, as taughtThe pair still adds.

4Power

Power is how fast work is done: P = W/t. 18 J in 2 s is 9 W. A large work in a long time can be a small power. Power is not “strength” as a poster word.

Calling a slow crane weak without W and t is a miss.

Figure. Power is work over time. The same 60 J in 3 s is 20 W; in 6 s it is 10 W. Faster work is more power.

How it works

  1. Name W and tThe given.
  2. DivideP = W/t.
  3. Keep the watt as joule per secondThe rate.

5Simple Machines

A simple machine is a taught tool that trades force and distance (or changes direction): lever, pulley, inclined plane as named. It does not create energy. A long gentle ramp can mean a smaller force through a longer s — work-story still honest.

A “force multiplier” slogan that forgets the longer path is a lying machine.

Figure. A lever is a beam on a fulcrum. Load and effort both push down. The longer effort arm is how a small push balances a larger load.

How it works

  1. Name the machineLever, pulley, ramp — as taught.
  2. Name the trade: smaller force, longer path, or a turn of directionThe job.
  3. Refuse “free energy”Machines trade, they do not mint joules.

6A definition is a test you can run

Work / energy / power is a test: a force through a displacement, a named drawer, or W/t. If you only say “energy”, you have a heading.

A lightning photo is not the test.

Figure. Work is a test: force must have a part along the displacement. Sideways force on a path to the right does no work.

How it works

  1. Name W, a drawer, or PThe object.
  2. Give the school formula or the tradeThe test.
  3. Then the word has contentThe definition ran.

7Name the given before the unknown

The given is F, s, t or the height. The unknown is W, PE, or P. Copy “along” or “still” before you multiply.

Using 6 N × 3 m for a 6 N hold that never moved is a silent given.

Figure. Name the given before the unknown. Here F is 12 N and s is 5 m; W is the ask, not a third given.

How it works

  1. Copy F, s, tThe given.
  2. Name W or PThe unknown.
  3. Then multiply or divideGiven first.

8One worked case is enough at this class

One W = F s or one machine-trade is enough. A page of the same 18 J clone does not add a new pulley.

Ten twins of 9 W are still one idea.

Figure. One worked case at this class: 12 N times 5 m is 60 J. Do not pile extra cases that teach the same product.

How it works

  1. Run one work or one tradeThe case.
  2. A second only as a checkOptional.
  3. StopThis class.
Holding a 6 N bag still for a minute does
  1. No work in this story — no displacement
  2. 18 J
  3. Infinite power

Tiredness is not the joule.

Notes

  • Mapped to the official NCERT chapter “Work, Energy, and Simple Machines”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • W = F s (F along s)
  • P = W/t
  • mechanical ≈ KE + PE (as taught)

Recap

Hold these pegs from the official chapter “Work, Energy, and Simple Machines”. The wording is ExamMaster’s teaching, not a textbook recap.

The Work-Energy Theorem
The work–energy theorem at this class: a net force that does work on a body changes its kinetic energy, as taught.
Forms of Energy
Forms of energy are named drawers: kinetic (motion), potential (a stored height or a taught spring-store), and other school forms if the lesson named them (heat, sound, chemical).
Mechanical Energy
Mechanical energy here is kinetic plus the taught potential, taken together.
Power
Power is how fast work is done: P = W/t. 18 J in 2 s is 9 W. A large work in a long time can be a small power. Power is not “strength” as a poster word.
Simple Machines
A simple machine is a taught tool that trades force and distance (or changes direction): lever, pulley, inclined plane as named.
A definition is a test you can run
Work / energy / power is a test: a force through a displacement, a named drawer, or W/t.

Practise Work, Energy, and Simple Machines

Reading is free and needs no account. Practice, mocks and progress live in the app.

  • A 4-question practice set that ends the chapter
  • 1 quick check with worked explanations
  • Timed mocks scored with the real marking scheme
  • Readiness tracked per topic, kept on your device
Continue with Google — freeNo card, no trial. Works offline once installed.