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REET L2 · Science

Work, Energy and Power — REET Science Notes

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This page covers the 'Work, Energy and Power' topic for REET Level 2 (Maths-Science), following the Class 9 NCERT syllabus — the formulas and units for work and energy, and the law of conservation of energy. The exam asks both formula-based and definition-based questions on this topic, so every concept here comes with an example.

  • 0 questions in REET 2024
  • Class 6-8 level
  • 7 min read
Verified 17 September 2026 · Source: NCERT Class 9 Science — Chapter 7: Work, Energy, and Simple Machines

How this topic is asked in REET

In REET Level 2 Maths-Science, this topic is based on the Class 9 NCERT chapter 'Work and Energy'. No question from this topic appeared directly in the REET 2024 paper, so the question pattern described here is based on the syllabus and NCERT's language, not on any actual past question. This topic usually draws both formula-based questions (work W = Fs, conservation of energy, the unit of power) and definition-based ones, and a direct question based on an everyday example is also possible.

The definition and formula for work

In physics, work is done only when a force is applied to an object and the object is displaced in the direction of that force. Work (W) is defined as the product of force (F) and displacement (s) — W = F × s. If there's an angle θ between the force and the displacement, the full formula is W = F s cosθ. The SI unit of work is the joule (J) — 1 joule is the work done when a force of 1 newton causes a displacement of 1 metre (1 J = 1 N × 1 m = 1 kg m²/s²).

It's worth remembering the three situations where work is zero: (1) when the force is zero, (2) when the displacement is zero — like a person standing still while holding a heavy bag; there's a force acting, but since there's no displacement, the work done is also zero, and (3) when the force is perpendicular to the displacement (θ = 90°) — like the work done by gravity while walking on a horizontal surface, which is zero because this force acts perpendicular (vertically) to the direction of displacement (horizontal).

Energy: definition and units

Definition
The capacity of an object to do work is called energy
SI unit
Joule (J) — same as the unit of work
Other unit
Calorie (cal); 1 cal ≈ 4.186 J
Commercial unit of electrical energy
Kilowatt-hour (kWh); 1 kWh = 3.6 × 10⁶ J
Main forms of energy
Mechanical (kinetic + potential), heat, electrical, chemical, light, sound, and nuclear energy

Kinetic energy vs potential energy

Kinetic Energy

DefinitionEnergy an object has because of its motion

FormulaKE = ½ m v²

ExampleA moving car, flowing water, a running fan

Depends onMass and the square of velocity

Potential Energy

DefinitionEnergy an object has because of its position or height

FormulaPE = m g h (gravitational potential energy)

ExampleAn object placed at a height, a drawn bow, a compressed spring

Depends onMass, height, and g

m = mass (kg), v = velocity (m/s), g = acceleration due to gravity (9.8 m/s²), h = height (m)

The law of conservation of energy

The law of conservation of energy states that energy can neither be created nor destroyed — it can only be converted from one form to another, and the total energy of an isolated system always stays constant. This is a fundamental law of physics, and it's best explained using the example of a freely falling object.

When an object starts falling freely from a height h, its potential energy is at a maximum at the start (PE = mgh) and its kinetic energy is zero. As the object falls, its height decreases, so its potential energy decreases, and its velocity increases, so its kinetic energy increases. Just before it reaches the ground, its potential energy is zero and its kinetic energy is at a maximum (equal to mgh). Ignoring air resistance, the total mechanical energy (KE + PE) stays constant at every point during the fall — that's exactly what conservation of energy means.

Power

Definition
The rate of doing work is called power
Formula
P = W / t (work ÷ time)
SI unit
Watt (W); 1 watt = 1 joule/second
Larger unit
1 kilowatt (kW) = 1000 watts
Other common unit
1 horsepower (hp) ≈ 746 watts

Everyday examples of energy conversion

Electric fan
Electrical energy converts to kinetic energy (the fan's rotational motion)
Electric bulb
Electrical energy converts to light energy and some heat energy
Photosynthesis (in plants)
Solar (light) energy converts to chemical energy, i.e. food/glucose, which stays stored in plants
Hydroelectric plant
The potential energy of water falling from a dam converts first to kinetic energy, then to electrical energy through a turbine
Battery-powered torch
The battery's chemical energy converts to electrical energy, and then to light energy in the bulb
A drawn bow (archery)
Pulling the string stores the muscular energy of the arm as potential energy in the bow, which converts to kinetic energy the instant the arrow is released

How to teach this to students

Classroom activity

Have students lift a heavy book in different ways in class — lifting it straight up (work is done) versus holding it still (work is zero) — this makes the concept of 'when is work zero' practically clear. Follow this up by discussing that carrying the same bag horizontally for some distance also means zero work is done by the weight.

Teaching aids

A demonstration with a pendulum or a ball rolling down a ramp is a simple, effective way to show the conversion of energy (potential to kinetic and back) right in front of students. Ask students to pause at the highest and lowest points of the pendulum's swing and think about which energy is at its maximum at each point.

Common misconception

Students often equate everyday 'work' (like studying or thinking) with 'work' in physics. Teachers should make clear that in physics, work is only done when a force causes actual displacement; mental tiredness isn't work in the physics sense.

Assessment idea

Have students list 5 everyday activities and ask which ones involve work in the physics sense and which don't, asking them to give a reason in each case — this assesses a deeper understanding of the concept.

Memory trick

Remember zero work with 'F-s-θ' — zero force, zero displacement, or a 90° angle — any one of these three conditions makes work zero. To avoid mixing up the formulas for energy and power, remember: energy = the 'capacity' to do work (unit: joule), power = the 'speed' of doing work (unit: watt, i.e. joule/second).

Practice questions

Question 1

1 joule of work is equal to

  1. A1 न्यूटन बल से 1 सेमी विस्थापन
  2. B1 न्यूटन बल से 1 मीटर विस्थापन
  3. C1 किग्रा बल से 1 मीटर विस्थापन
  4. D1 डाइन बल से 1 सेमी विस्थापन

Explanation

Work W = F × s. 1 joule is the work done when a force of 1 newton causes a displacement of 1 metre in the direction of the force.

Question 2

The formula for the kinetic energy of an object is

  1. Amgh
  2. BF × s
  3. CW / t
  4. D½ m v²

Explanation

Kinetic energy is due to an object's motion and is found using KE = ½ m v², where m is mass and v is velocity.

Question 3

A person carrying a bag on their head walks straight along a horizontal road. The work done by the weight of the bag will be

  1. Aअधिकतम
  2. Bशून्य
  3. Cऋणात्मक
  4. Dअनंत

Explanation

The bag's weight acts vertically while the displacement is horizontal, so the angle is 90°, making cosθ = 0 and the work done zero.

Question 4

The SI unit of power is

  1. Aजूल
  2. Bवाट
  3. Cन्यूटन
  4. Dकैलोरी

Explanation

Power = work/time, and its SI unit is the watt (W), where 1 watt = 1 joule/second.

Question 5

1 kilowatt-hour (kWh) is equal to

  1. A3.6 × 10⁶ जूल
  2. B3600 जूल
  3. C3.6 × 10³ जूल
  4. D36 × 10⁶ जूल

Explanation

1 kWh = 1000 watts × 3600 seconds = 3.6 × 10⁶ joules. This is the 'unit' on an electricity bill.

Question 6

A ball is dropped from a height. Assuming air resistance is negligible, at any point during the fall

  1. Aकुल यांत्रिक ऊर्जा (KE + PE) स्थिर रहती है
  2. Bकेवल गतिज ऊर्जा स्थिर रहती है
  3. Cकेवल स्थितिज ऊर्जा स्थिर रहती है
  4. Dकुल ऊर्जा घटती जाती है

Explanation

By the law of conservation of energy, with negligible air resistance, the sum of potential and kinetic energy stays constant at every point during the fall, even as each one changes individually.

Question 7

1 horsepower is approximately equal to how many watts?

  1. A100 वाट
  2. B1000 वाट
  3. C746 वाट
  4. D550 वाट

Explanation

1 horsepower (hp) is roughly equal to 746 watts, an older but still commonly used unit of power.

Question 8

A person applies a force of 20 newtons and displaces an object by 5 metres in the direction of the force. The work done will be

  1. A4 जूल
  2. B25 जूल
  3. C15 जूल
  4. D100 जूल

Explanation

W = F × s = 20 N × 5 m = 100 joules, since the force and displacement are in the same direction (θ = 0°, cosθ = 1).

Question 9

While teaching the concept of 'work' in Class 9, the most suitable activity for a teacher would be

  1. Aबच्चों से भारी वस्तु उठाने व स्थिर पकड़े रहने के बीच अंतर करवाना
  2. Bकेवल सूत्र रटवाना
  3. Cकेवल पाठ्यपुस्तक पढ़ाना
  4. Dपरीक्षा में पूछे गए प्रश्न याद करवाना

Explanation

A hands-on demonstration lets students experience for themselves that applying force without displacement means zero work is done in physics, which makes the concept stick permanently.

Question 10

If a student says that 'getting tired from pushing a wall' counts as work in physics, the teacher should explain that

  1. Aयह सही है क्योंकि बल लगाया गया
  2. Bयह गलत है क्योंकि बल शून्य है
  3. Cयह गलत है क्योंकि दीवार का विस्थापन शून्य है, इसलिए भौतिक कार्य शून्य है
  4. Dयह कार्य तभी होगा जब छात्र थके नहीं

Explanation

Work in physics needs both force and displacement. Since the wall stays still (zero displacement), physical work is considered zero, even though a force was applied.

Frequently asked questions

How many REET questions come from the Work, Energy and Power topic?+

In the REET Level 2 Science section, each physics sub-topic usually has up to a 1-question chance. No direct question from this topic appeared in the REET 2024 paper, but since it's part of the syllabus, there's a good chance it will appear in upcoming exams.

What is the SI unit of work, and what quantities does it depend on?+

The SI unit of work is the joule (J). It depends on both force (in newtons) and displacement (in metres) — W = F × s.

How many joules is 1 kilowatt-hour (kWh) equal to?+

1 kWh = 3.6 × 10⁶ joules (3.6 megajoules). This is the 'unit' used on electricity bills.

What's the main difference between kinetic energy and potential energy?+

Kinetic energy is due to an object's motion (KE = ½mv²), while potential energy is due to an object's position or height (PE = mgh).

What's the difference between power and energy?+

Energy is the total capacity to do work, while power is the rate at which that work is done — power = energy/time. Their units are different too: joule for energy, watt for power. Two people can do the same amount of work, but the one who does it in less time is said to have more power.

Read next

Pressure

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Heat and Temperature

Next topic — REET Science notes

REET Syllabus

See the full REET Level 1 and Level 2 syllabus

REET 2024 Solved Paper

All REET 2024 solved questions here

Official Sources

  1. NCERT Class 9 Science — Chapter 7: Work, Energy, and Simple Machines

Change Log

  • 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked