Work, Energy, and Simple Machines – Class 9 Science Chapter 7 Notes & NCERT PDF

Class 9ScienceChapter 7NCERT book: Exploration - Science Class 9

Notes and a simple summary of Chapter 7 of the NCERT Class 9 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 7: Work, Energy, and Simple Machines

Think It Over

What will be the speed of a child at the bottom of a slide?

Will two children of different masses reach the bottom of the same slide with the same speed?

Which slide gives the largest speed at the bottom?

By the end of this chapter you will be able to answer all three — using energy instead of long force calculations.

Introduction

In Chapters 4 and 6 you used kinematic equations and Newton's laws to study motion. But when forces change with time or act in complicated ways, those tools become hard to use. This chapter introduces a simpler and more powerful idea: work, energy and power. You will also study simple machines — the pulley, the inclined plane and the lever — which make tasks easier. The book's opening picture reminds us that energy comes from many sources: food lets us walk, electricity turns a fan, fuel moves a car.

Key Concepts

7.1 Work done by a constant force

Lifting 3 wheat bags one after another, lifting all 3 together, or lifting 1 bag three times as high — each needs 3 times the work of lifting 1 bag by 1 m. So work grows with both force and distance.

Work done = force × displacement in the direction of the force, i.e. W = F × s

SI unit: joule (J). 1 J = 1 N × 1 m = 1 kg m² s⁻².

Work is the area under a force–displacement graph (useful even when force is not constant). Example from the book: 10 N × 1 m = 10 J.

Work has no direction, but it can be positive or negative.

When is work zero?

No force (F = 0), or

No displacement (s = 0) — pushing a rigid wall does no work on the wall, even though you feel tired (your muscles use internal energy), or

Force perpendicular to displacement — a girl carrying a box while walking does no work on it with her upward force.

Positive and negative work

Force along displacement → positive work (pushing a wheelchair).

Force opposite to displacement → negative work (a goalkeeper stopping a ball; the ball does positive work on her).

7.2 The work-energy theorem

An object that can do work has energy. A thrown cricket ball knocks down the wickets; a raised flowerpot can damage something below.

Work done on an object = change in its energy (work-energy theorem)

It holds for systems of objects and for changing forces too. Energy can also be transferred as heat, by radiation (Sun to Earth), in electric circuits, by sound and in nuclear reactions. The unit joule honours James Prescott Joule.

7.3 Forms of energy

Mechanical, thermal, light, sound, electrical, chemical (stored in bonds of fuels and food) and nuclear energy. Energy changes form: a bulb turns electrical energy into light, a heater into thermal energy, a ringing bell turns mechanical energy into sound.

7.4 Mechanical energy

Kinetic energy (K) — energy due to motion. Using v² = u² + 2as and F = ma, work W = ½m(v² − u²). Starting from rest:

K = ½ m v²

Doubling speed makes kinetic energy 4 times larger.

Potential energy (U) — energy stored by deformation (stretched slingshot band, bent bow, compressed spring) or by relative positions in a system (separated magnets, separated charges, Earth–ball system). For an object lifted slowly to height h near Earth's surface:

U = m g h

Activity 7.1: a heavy ball dropped on sand from 1 m, 2 m and higher makes a deeper dent from greater height — more height, more potential energy.

Conservation of mechanical energy — For a freely falling object, potential energy lost = kinetic energy gained, and K + U stays equal to mgh if no other external force acts. Activity 7.2: a pendulum released from point P swings to almost the same height R on the other side; at the lowest point Q its energy is all kinetic. It slowly stops because of friction at the support and air resistance.

7.5 Power

Running up the stairs in 1 minute vs walking up in 5 minutes needs the same work but different power.

Power = work ÷ time, P = W / t — SI unit watt (W), 1 W = 1 J s⁻¹. 1 horsepower = 746 W. The unit honours James Watt.

7.6 Simple machines

Simple machines do not reduce total work; they change the size or direction of the force. Effort = force we apply; load = force to overcome.

Mechanical advantage (MA) = load ÷ effort

Fixed pulley — only changes direction; MA = 1. Movable pulleys or pulley systems can give MA > 1 (cranes, lifts).

Inclined plane — pushing a load along length L to height h: F′ × L = mgh, so MA = L / h (>1). A longer, gentler slope needs less effort (Activity 7.3 with a cart and spring balance).

Lever — rigid bar turning about a fulcrum. Balance condition: effort × effort arm = load × load arm, and MA = effort arm ÷ load arm. Three classes: Class I (fulcrum in between — scissors, crowbar, seesaw), Class II (load in between — wheelbarrow, bottle opener, lemon squeezer), Class III (effort in between — tweezers, broom, hammer, oar).

Worked Numericals

Goalkeeper (Example 7.2): force 200 N, hand moves back 0.15 m opposite to the force → W = 200 × (−0.15) = −30 J.

Fast delivery (Example 7.5): 0.2 kg ball at 154.8 km h⁻¹ = 43 m s⁻¹ → K = ½ × 0.2 × 43² = 184.9 J.

Jet on a carrier (Example 7.6): ½ × 15000 × v² = 367500 N × 100 m → v² = 4900, v = 70 m s⁻¹ = 252 km h⁻¹.

Thrown ball (Example 7.7): U = 0.2 kg × 10 m s⁻² × 10 m = 20 J.

Escape ramp (Example 7.9): truck 10000 kg at 72 km h⁻¹ (20 m s⁻¹) has K = 2,000,000 J. On a 30° ramp (rises 1 m per 2 m) with sand force 50000 N: 2,000,000 J = (50000 + 50000) N × d → d = 20 m.

Weightlifter (Example 7.10): 75 kg × 10 × 2 m = 1500 J in 5 s → 300 W.

Car (Example 7.11): 1000 kg from rest to 20 m s⁻¹ in 10 s → W = 200000 J, P = 20000 W.

Ramp (Example 7.12): height 30 cm, base 40 cm → ramp 50 cm, MA = 50/30 ≈ 1.67.

Seesaw (Example 7.13): 15 kg × 2 m = 30 kg × L → L = 1 m (seat D).

Activities in the Chapter

Activity 7.1: drop a heavy ball on sand from different heights — higher drop, deeper dent.

Activity 7.2: pendulum against a line on the wall — the bob returns almost to the same height.

Activity 7.3: pull a cart up planks of different slopes with a spring balance — less steep, less force, longer distance.

Activity 7.4: a scale on a pencil lifts a stapler with erasers — a lever.

Activity 7.5: a scale beam with paper-cup pans and coins shows n₁ × L₁ = n₂ × L₂.

Key Terms

Work (कार्य) — force × displacement along the force

Joule (जूल) — SI unit of work and energy

Energy (ऊर्जा) — capacity to do work

Work-energy theorem (कार्य-ऊर्जा प्रमेय)

Kinetic energy (गतिज ऊर्जा) — ½mv²

Potential energy (स्थितिज ऊर्जा) — mgh near Earth

Mechanical energy (यांत्रिक ऊर्जा) — K + U

Conservation of energy (ऊर्जा संरक्षण)

Power (शक्ति) — rate of doing work

Watt (वाट)

Effort / Load (प्रयास / भार)

Mechanical advantage (यांत्रिक लाभ)

Pulley (घिरनी) · Inclined plane (आनत तल) · Lever / Fulcrum (उत्तोलक / आलंब)

Real-life Connections

Hill roads wind in gentle slopes — a longer inclined plane needs less effort.

An inclined ladder is easier to climb than a vertical one.

Opening a can lid with a spoon uses the spoon as a lever.

Gharat / panchakki water mills in the Himalayas turn the potential energy of falling water into kinetic energy to grind grain; dams do the same to make electricity.

Common Misconceptions

"Holding a heavy bag still is work." — No displacement means zero work on the bag.

"Machines reduce the work needed." — They reduce effort, not total work.

"A heavier child slides down faster." — Ignoring friction, v = √(2gh): speed depends only on height.

"Doubling speed doubles kinetic energy." — It quadruples it.

"A perpetual motion machine is possible." — Friction always turns some energy into heat, so real machines stop.

Exercise Hints

For true/false on work, check both force and displacement along the force.

For energy-transformation questions (truck uphill, watch spring, photosynthesis, dam, matchstick, cracker, microphone, bulb, solar panel), name the starting and final forms.

Potential energy gained depends only on height, not on the path (lift vs stairs).

For the crane question, energy doubles when height doubles; if time also doubles, power stays the same.

💡 Key Learning Points

✓Define work as force × displacement along the force and identify positive, negative and zero work
✓Use the work-energy theorem to relate work to changes in energy
✓Derive and use K = ½mv² and U = mgh, and apply conservation of mechanical energy
✓Calculate power as work per unit time and use the watt
✓Explain how pulleys, inclined planes and levers give mechanical advantage without reducing total work

👨‍🏫 Teaching Tips

→Start with the Think It Over slides; return to them after deriving v = √(2gh)
→Do Activity 7.1 with a sand tray and let groups measure dent depth for different heights
→Use a pendulum against a chalk line (Activity 7.2) to show energy conservation and its limits
→Let students pull a cart up planks of different slopes with a spring balance (Activity 7.3)
→Build the coin-and-cup beam balance (Activity 7.5) and record n × L values in a table
→Take the class to a playground seesaw or school ramp to measure lengths and heights
→Discuss the gharat/panchakki and hydroelectric dams as real energy conversions

📋 Assessment Questions

Can students decide when work is positive, negative or zero in everyday situations?
Can they solve numericals on K, U and power with correct SI units?
Can they explain why the speed at the bottom of a slide is independent of mass?
Can they calculate MA for an inclined plane and a lever and classify levers?

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