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Chapter 7: Work, Energy, and Simple Machines

Grade 9 Science | Chapter 7

Work, Energy, and Simple Machines

What does it mean to do work, and where does energy go? This chapter connects force and motion to work, energy and power, and shows how simple machines make tasks easier.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: Work in Science

Energy and Its Forms

Kinetic and Potential Energy

Conservation of Energy

Power

Simple Machines

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: Work in Science

In science, work is done when a force moves an object in the direction of the force. Work equals force multiplied by the distance moved in that direction, W = F × d, and it is measured in joules (J). If there is no movement, or the movement is at right angles to the force, no work is done, even if you feel tired holding something still.

Doing work transfers energy. Energy is the capacity to do work, also measured in joules. This chapter follows energy as it changes from one form to another, introduces power as the rate of doing work, and shows how simple machines let a small effort move a large load.

Core idea

Work is done only when a force moves an object along its own direction: W = F × d. Doing work transfers energy, and both are measured in joules.

2. Energy and Its Forms

Energy is the ability to do work. It comes in many forms, including kinetic, potential, heat, light, sound, chemical and electrical energy. Energy can change from one form to another but is never created or destroyed. A torch, for example, changes chemical energy in its cell into electrical energy and then into light and heat.

3. Kinetic and Potential Energy

Kinetic energy is the energy of a moving object, given by KE = ½ m v², so it grows quickly as speed increases. Potential energy is stored energy due to position or condition; the gravitational potential energy of a raised object is PE = m g h, where h is the height. A raised hammer has potential energy that becomes kinetic energy as it falls.

Energy

Formula

Depends On

Kinetic

KE = ½ m v²

mass and speed

Gravitational potential

PE = m g h

mass and height

4. Conservation of Energy

The law of conservation of energy states that energy cannot be created or destroyed, only changed from one form to another. The total energy stays the same. As a ball falls, its potential energy steadily becomes kinetic energy, but the sum of the two stays constant if we ignore air resistance.

Watch out

Holding a heavy bag still feels tiring, but in the scientific sense no work is done on the bag, because it does not move in the direction of the force.

5. Power

Power is the rate of doing work, or the rate at which energy is transferred, given by P = work ÷ time. Its SI unit is the watt (W), where one watt is one joule per second. Two machines may do the same work, but the more powerful one does it in less time.

6. Simple Machines

A simple machine makes a task easier by changing the size or direction of the force needed. A lever turns about a fulcrum so that a small effort can lift a large load; a pulley changes the direction of the effort, letting you pull down to lift up; and an inclined plane lets a load be raised gradually with a smaller force. The benefit is measured by the mechanical advantage, the load divided by the effort.

Diagram 1 – The Inclined Plane

A block on an inclined plane with weight pulling down and effort pushing up the slope

Fig 1. A gentle slope lets a smaller effort raise a heavy load, although the load must move a longer distance along the slope.

Diagram 2 – Lever and Pulley

A lever balanced on a fulcrum with effort and load, and a pulley with a rope lifting a load

Fig 2. A lever turns about a fulcrum so a small effort lifts a large load; a pulley changes the direction of the effort.

7. Key Reasoning (Principles)

Principle 1: No movement means no work

Work needs a force and movement in the direction of that force. A force that causes no movement, or acts at right angles to the motion, does no work.

Principle 2: Energy is conserved

Energy only changes form; the total stays the same. As an object falls, lost potential energy reappears as kinetic energy.

Principle 3: Machines trade force for distance

A machine that reduces the effort needed makes the load move a shorter distance than the effort. The machine does not reduce the total work, it just spreads it out.

8. Worked Examples

Example 1

Q: A force of 10 N moves a box 5 m in its own direction. Find the work done.

▶ Show Solution

W = F × d = 10 × 5.

= 50 J.

Answer: 50 J.

Example 2

Q: Find the kinetic energy of a 2 kg ball moving at 3 m/s.

▶ Show Solution

KE = ½ m v² = ½ × 2 × 9.

= 9 J.

Answer: 9 J.

Example 3

Q: Find the potential energy of a 2 kg object raised 5 m (take g = 10 m/s²).

▶ Show Solution

PE = m g h = 2 × 10 × 5.

= 100 J.

Answer: 100 J.

Example 4

Q: A machine does 200 J of work in 10 s. Find its power.

▶ Show Solution

P = work ÷ time = 200 ÷ 10.

= 20 W.

Answer: 20 W.

Example 5

Q: A force of 25 N drags a load 4 m. Find the work done.

▶ Show Solution

W = 25 × 4.

= 100 J.

Answer: 100 J.

Example 6

Q: Find the kinetic energy of a 4 kg object moving at 5 m/s.

▶ Show Solution

KE = ½ × 4 × 25.

= 50 J.

Answer: 50 J.

Example 7

Q: A 5 kg box is lifted 2 m (take g = 10 m/s²). Find its gain in potential energy.

▶ Show Solution

PE = m g h = 5 × 10 × 2.

= 100 J.

Answer: 100 J.

Example 8

Q: A crane does 600 J of work in 30 s. Find its power.

▶ Show Solution

P = 600 ÷ 30.

= 20 W.

Answer: 20 W.

Example 9

Q: A lever lifts a 200 N load with an effort of 50 N. Find the mechanical advantage.

▶ Show Solution

Mechanical advantage = load ÷ effort = 200 ÷ 50.

= 4.

Answer: 4.

Example 10

Q: Why is no work done while holding a heavy bag still?

▶ Show Solution

Work needs movement in the direction of the force; the still bag does not move.

Answer: Because the bag does not move, no work is done.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. The SI unit of work is the: (a) newton (b) joule (c) watt (d) pascal

2. The SI unit of power is the: (a) joule (b) newton (c) watt (d) metre

3. Kinetic energy is the energy of a body due to its: (a) position (b) motion (c) height (d) shape

4. Which is a simple machine? (a) battery (b) lever (c) magnet (d) thermometer

5. Energy can be: (a) created (b) destroyed (c) changed in form (d) made from nothing

▶ Reveal Answers

1. (b) joule.

2. (c) watt.

3. (b) motion.

4. (b) lever.

5. (c) changed in form.

Set B – Short Answer (Understanding)

1. When is work said to be done in science?

2. State the law of conservation of energy.

3. Define power and give its SI unit.

4. How does a simple machine make work easier?

5. Give one example of energy changing form.

▶ Reveal Answers

1. When a force moves an object in the direction of the force.

2. Energy cannot be created or destroyed, only changed from one form to another.

3. Power is the rate of doing work; its SI unit is the watt (W).

4. By changing the size or direction of the force needed to move a load.

5. A torch changes chemical energy into electrical energy and then into light and heat.

Set C – Application and Reasoning

1. A force of 15 N moves a load 6 m. Find the work done.

2. Find the kinetic energy of a 3 kg object moving at 2 m/s.

3. A 4 kg object is raised 3 m (g = 10 m/s²). Find its potential energy.

4. A motor does 900 J of work in 45 s. Find its power.

5. A pulley lifts a 300 N load with an effort of 100 N. Find the mechanical advantage.

▶ Reveal Answers

1. W = 15 × 6 = 90 J.

2. KE = ½ × 3 × 4 = 6 J.

3. PE = 4 × 10 × 3 = 120 J.

4. P = 900 ÷ 45 = 20 W.

5. Mechanical advantage = 300 ÷ 100 = 3.

Set D – Higher Order (Challenge)

1. A ball of mass 1 kg is dropped from 5 m (g = 10 m/s²). Find its potential energy at the top, and its kinetic energy just before landing.

2. Explain why a machine that reduces effort makes the load travel a shorter distance than the effort.

3. A pump raises 50 kg of water through 4 m in 20 s (g = 10 m/s²). Find the work done and the power.

4. Two cranes do the same work, but crane A is more powerful. What does this tell you?

5. Why does kinetic energy increase fourfold when the speed of an object doubles?

▶ Reveal Answers

1. PE at top = 1 × 10 × 5 = 50 J; by conservation of energy, KE just before landing = 50 J.

2. A machine does not reduce the total work, so if the effort is smaller, it must act over a longer distance, meaning the load moves a shorter distance.

3. Work = m g h = 50 × 10 × 4 = 2000 J; power = 2000 ÷ 20 = 100 W.

4. Crane A does the same work in less time, because power is the rate of doing work.

5. Because KE = ½ m v² depends on the square of the speed, so doubling the speed multiplies the energy by 2 squared, that is four times.

Chapter Summary

Work

Force times distance moved in its direction, W = F × d, in joules.

Energy

The capacity to do work, in joules, in many forms.

Kinetic and Potential

KE = ½ m v² for motion; PE = m g h for height.

Conservation of Energy

Energy only changes form; the total stays the same.

Power

Rate of doing work, P = work ÷ time, in watts.

Simple Machines

Lever, pulley and inclined plane trade force for distance to ease a task.

Quantity

Unit

Symbol

Work and energy

joule

Power

watt

Force

newton

8-Point Exam Quick-Check

Work = force times distance moved in the direction of the force, in joules.

Energy is the capacity to do work and comes in many forms.

Kinetic energy = half m v squared; potential energy = m g h.

Energy is conserved: it only changes form, never created or destroyed.

Power is the rate of doing work, measured in watts.

Simple machines change the size or direction of the force needed.

Mechanical advantage = load divided by effort.

No movement in the direction of the force means no work is done.

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Class 9 Science Chapter 7: Work, Energy and Simple Machines, Complete Notes and Practice

This revision guide follows the NCERT 2026 to 27 Exploration syllabus and connects force and motion to work and energy, covering the meaning of work, kinetic and potential energy, the conservation of energy, power, and the simple machines lever, pulley and inclined plane, with two labelled diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.