Work Energy and Power Class 10 ICSE — Selina Chapter 2

Work energy and power class 10 ICSE forms the core of how physics measures motion and its effects. This chapter from the Selina Concise Physics book follows the official CISCE syllabus for Class 10. It explains when a force truly does work, how to calculate it, and how energy changes form without ever being lost. You will also learn how power measures the rate at which work happens.

Download Work Energy and Power Class 10 ICSE Physics Chapter 2 PDF

Download the Work Energy and Power Class 10 ICSE Physics chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 28 pages, 18 sections, 116 questions.

What is inside Chapter 2

28 pages · 18 sections · 3 worked examples · 116 exercise questions

• EXERCISE-2(A): 30 questions
• MULTIPLE CHOICE TYPE: 17 questions
• EXERCISE-2(B): 27 questions
• MULTIPLE CHOICE TYPE: 23 questions
• EXERCISE-2(C): 19 questions

2.1 WORK

Physics uses the word “work” more strictly than daily speech does. Writing, reading and eating involve effort, but none count as work in physics. Work happens only when a force makes a body move.

The condition is simple: work is done only when the force applied on a body makes the body move. If you push a wall and it does not move, you have done zero work. A coolie standing still with a load on his head does no work, however tired he feels, because displacement is zero.

This rule governs every problem that follows. Always look for displacement before reaching for a formula. You can explore the rest of the Class 10 ICSE Physics book collection to see how later chapters build on this foundation.

2.2 MEASUREMENT OF WORK

Work depends on two factors: the magnitude of the force and the displacement it produces. A larger force over the same distance does more work. The same force over a longer distance does more work too.

When force and displacement are parallel, work is their product:

W = F \times S

When the force acts at an angle \theta to the displacement, only the component of force along the displacement does work:

W = F S \cos \theta

Three special cases follow from the cosine value. At \theta = 0^\circ, \cos 0^\circ = 1, so W = FS and work is positive. At \theta = 90^\circ, \cos 90^\circ = 0, so W = 0 — a perpendicular force does no work. At \theta = 180^\circ, \cos 180^\circ = -1, so W = -FS, meaning negative work when force opposes motion. The sign of work matters; many students forget the minus sign when friction or gravity opposes motion.

Worked pattern: A force of 30 N acts at 60° to a displacement of 4 m.

W = 30 \times 4 \times \cos 60^\circ = 30 \times 4 \times \frac{1}{2} = 60 \text{ J}

For a variable force, plot force against displacement. The area under the graph equals the work done. Work is a scalar quantity, expressed as the dot product \vec{F} \cdot \vec{S}. The idea of force as a vector builds directly on Chapter 1 on force.

2.3 WORK DONE BY THE FORCE OF GRAVITY W = mgh

When a body of mass m falls through a vertical height h, gravity does work on it. The force of gravity is mg and the vertical displacement is h, so:

W = mg \times h = mgh

The path does not matter. Coming down stairs, a slope or a lift gives the same work, because only the vertical height counts. Going up, the work done by gravity is -mgh. The work done by the person against gravity is +mgh.

Worked pattern: A 2 kg box falls 5 m. Taking g = 10 \text{ m/s}^2:

W = 2 \times 10 \times 5 = 100 \text{ J}

Gravity does 100 J of positive work on the box as it falls.

2.4 UNITS OF WORK

The S.I. unit of work is the joule (J). One joule of work is done when a force of 1 newton displaces a body through 1 metre in its own direction.

Bigger units are the kilojoule (\text{kJ} = 10^3 \text{ J}), megajoule (\text{MJ} = 10^6 \text{ J}) and gigajoule (\text{GJ} = 10^9 \text{ J}). The C.G.S. unit is the erg, defined by a force of 1 dyne acting over 1 cm.

Since 1 \text{ N} = 10^5 \text{ dyne} and 1 \text{ m} = 10^2 \text{ cm}:

1 \text{ J} = 10^5 \text{ dyne} \times 10^2 \text{ cm} = 10^7 \text{ erg}

Worked pattern: Convert 2.5 kJ to erg.

2.5 \text{ kJ} = 2.5 \times 10^3 \text{ J} = 2.5 \times 10^3 \times 10^7 = 2.5 \times 10^{10} \text{ erg}

A common unit-conversion error is mixing up joule and watt. Joule measures work; watt measures power.

2.5 POWER (P = W/t)

Power is the rate of doing work. If work W is done in time t, then:

P = \frac{W}{t}

Power depends on two factors: the amount of work done and the time taken. A machine that does the same work in less time has more power.

For a constant force F moving a body a distance S in time t:

P = \frac{F \times S}{t} = F \times v

where v = S/t is the average speed. So power also equals force times average speed.

Worked pattern: Machine A does 600 J in 3 s; machine B does the same 600 J in 6 s.

P_A = \frac{600}{3} = 200 \text{ W}, \quad P_B = \frac{600}{6} = 100 \text{ W}

Machine A has twice the power because it works at double the rate.

2.6 UNITS OF POWER

The S.I. unit of power is the watt (W). One watt is the power when 1 joule of work is done in 1 second:

1 \text{ W} = 1 \text{ J/s}

Bigger units are kilowatt (\text{kW} = 10^3 \text{ W}), megawatt (\text{MW} = 10^6 \text{ W}) and gigawatt (\text{GW} = 10^9 \text{ W}). Smaller units are milliwatt (\text{mW} = 10^{-3} \text{ W}) and microwatt (\mu\text{W} = 10^{-6} \text{ W}).

The C.G.S. unit is erg per second. Since 1 \text{ J} = 10^7 \text{ erg}:

1 \text{ W} = 10^7 \text{ erg/s}

Horsepower is another unit used in mechanical engineering:

1 \text{ H.P.} = 746 \text{ W}

The table compares S.I. and C.G.S. units for work and power:

Work does not depend on time, while power always does.

2.7 ENERGY (Energy as work done)

A body capable of doing work possesses energy. The energy of a body is its capacity to do work, measured by the amount of work the body can perform.

Energy and work are closely linked. When a body does work, its energy decreases. When work is done on a body, its energy increases. Every act of doing work involves a transfer of energy.

Like work, energy is a scalar quantity. This short section sets up the detailed energy forms that follow in sections 2.9 through 2.15.

2.8 UNITS OF ENERGY

Energy uses the same units as work: the joule (J) in S.I. and the erg in C.G.S., with 1 \text{ J} = 10^7 \text{ erg}.

Two bigger units measure practical energy use. One watt hour (Wh) is the energy spent by a 1 W source in 1 hour:

1 \text{ Wh} = 1 \text{ W} \times 3600 \text{ s} = 3600 \text{ J}

One kilowatt hour (kWh) is the energy spent by a 1 kW source in 1 hour:

1 \text{ kWh} = 1000 \text{ W} \times 3600 \text{ s} = 3.6 \times 10^6 \text{ J}

Heat energy uses the calorie, defined as the heat needed to raise 1 g of water through 1°C:

1 \text{ cal} = 4.18 \text{ J}

Atomic particles use the electron volt, the energy gained by an electron accelerated through 1 volt:

1 \text{ eV} = 1.6 \times 10^{-19} \text{ J}

Watt and kilowatt measure power. Watt hour and kilowatt hour measure energy, since power \times time = energy.

2.9 MECHANICAL ENERGY AND ITS DIFFERENT FORMS

Mechanical energy is the energy a body possesses due to its state of rest or of motion. It exists in exactly two forms: potential energy and kinetic energy.

The total mechanical energy of a body equals the sum of its potential energy and kinetic energy. This definition is the foundation for the conservation principle covered later in section 2.16.

2.10 POTENTIAL ENERGY (U)

Potential energy is the energy a body possesses by virtue of its changed position or configuration. The symbol U denotes it.

Examples include a body at a height, a wound watch spring, a compressed spring, a bent bow and a stretched rubber string.

In mechanics, potential energy comes in two forms:

• Gravitational potential energy: energy due to the earth’s force of attraction on the body. At infinity, it is zero; at a finite distance, it is negative.
• Elastic potential energy: energy stored when a body’s configuration changes, such as a deformed spring. It equals the work done in deforming the body.

Elasticity is the property by which a body regains its original shape when the external force is removed.

2.11 GRAVITATIONAL POTENTIAL ENERGY AT A HEIGHT (U = mgh)

Gravitational potential energy at a height is measured by the work done in lifting the body from the ground to that height. The ground level is taken as zero potential energy.

To lift a body of mass m to height h without acceleration, the least upward force equals the force of gravity, mg. The work done stores as potential energy:

U = mgh

When a body rises from h_1 to h_2, the gain is:

\text{Gain} = mg(h_2 - h_1)

When it falls from h_1 to h_2 (with h_2 below h_1), the loss is:

\text{Loss} = mg(h_1 - h_2)

This is why a ball thrown up gains potential energy as it rises, and loses it as it falls back to the ground.

2.12 KINETIC ENERGY

Kinetic energy is the energy a body possesses by virtue of its motion, denoted K. A fast stone can break a window; a moving car, a bullet or a rolling ball all carry kinetic energy.

The expression is derived from the work done by a retarding force to stop the body:

K = \frac{1}{2} m v^2

Momentum p relates to kinetic energy:

p = \sqrt{2mK} \quad \text{or} \quad K = \frac{p^2}{2m}

The work-energy theorem states that the work done by a force on a moving body equals the increase in its kinetic energy:

W = K_f - K_i = \frac{1}{2} m v^2 - \frac{1}{2} m u^2

Kinetic energy has three forms: translational (straight-line motion, like a car on a road), rotational (spinning top, rotating wheel) and vibrational (struck wire, vibrating atoms in a solid).

Worked pattern: A 0.5 kg ball moves at 6 m/s.

K = \frac{1}{2} \times 0.5 \times 6^2 = \frac{1}{2} \times 0.5 \times 36 = 9 \text{ J}

Potential energy does not depend on speed; kinetic energy does.

2.13 CONVERSION OF POTENTIAL ENERGY INTO THE KINETIC ENERGY

Potential energy changes into kinetic energy whenever it is put to use. Four classic examples show the pattern:

• Falling stone on a nail: the stone’s gravitational potential energy becomes kinetic energy as it falls, driving the nail into wood.
• Wound watch spring: elastic potential energy in the wound spring changes into kinetic energy as it unwinds, moving the hands.
• Bow and arrow: the bent bow stores elastic potential energy, which becomes the arrow’s kinetic energy on release.
• Compressed spring: the stored elastic potential energy changes into the kinetic energy of a ball placed on it, sending the ball flying.

In every case, stored potential energy converts to the kinetic energy that does visible work.

2.14 DIFFERENT FORMS OF ENERGY

The chapter lists twelve forms of energy found in nature. A mnemonic for them is: “So Happy Little Children Have Energy — Nature’s Grand Work Sounds Marvelous, Magically!”

• Solar: energy radiated by the sun, captured by solar panels, furnaces and cells.
• Heat: released on burning coal, oil, wood or gas; steam carries it.
• Light: the form in which objects are seen; the sun is its natural source.
• Chemical: stored in fuels, food and batteries.
• Hydro: energy of fast-moving water, used in hydroelectric stations.
• Electrical: from charged bodies, cells and generators.
• Nuclear: released by fission and fusion, from loss in mass via E = mc^2.
• Geothermal: stored deep inside the earth from nuclear disintegration, producing natural steam.
• Wind: energy of fast-moving air, driving windmills and turbines.
• Sound: carried by vibrating bodies reaching the ear.
• Magnetic: energy of a magnet attracting iron filings.
• Mechanical: energy due to state of rest or motion.

Each form converts into others, which the next section explains.

2.15 CONVERSION OF ONE FORM OF ENERGY INTO THE OTHER FORM

One form of energy converts into another in countless daily devices. The chapter walks through seventeen conversions, including these key ones:

• Mechanical to electrical: a generator or dynamo.
• Electrical to mechanical: an electric motor in fans, washing machines and mixers.
• Electrical to heat: heaters, ovens, geysers and toasters.
• Chemical to electrical: an electric cell when current is drawn.
• Electrical to sound: a loudspeaker and electric bell.
• Light to electrical: a photoelectric or solar cell.
• Chemical to light: a burning candle or kerosene lamp.
• Heat to mechanical: a steam engine.

A key note: when mechanical energy converts to other forms, it always does so through kinetic energy first. Potential energy first becomes kinetic, then converts onward.

Not all energy converts usefully. A part changes into undesirable forms, usually heat from friction, or is lost to surroundings. This is called the dissipation of energy, and the lost part is degraded energy.

2.16 PRINCIPLE OF CONSERVATION OF ENERGY

According to the principle of conservation of energy, energy can neither be created nor destroyed. It only changes from one form to another.

The sum of all forms of energy in the universe stays constant. When energy transforms, the total remains the same. When only potential and kinetic energy interchange, the total mechanical energy stays constant:

K + U = \text{constant}

This holds when no frictional forces act. The principle is one of the fundamental laws of nature.

2.17 THEORETICAL VERIFICATION OF K + U = CONSTANT FOR A FREELY FALLING BODY

Let a body of mass m fall freely from height h. The chapter verifies K + U = \text{constant} at three points: A (top), B (middle, after falling distance x) and C (ground).

Worked pattern: m = 1 \text{ kg}, h = 20 \text{ m}, g = 10 \text{ m/s}^2.

At the top (A): K = 0, U = mgh = 1 \times 10 \times 20 = 200 \text{ J}. Total = 200 J.

At the middle (B, after falling 10 m): velocity comes from v^2 = 2gx = 2 \times 10 \times 10 = 200, so K = \frac{1}{2} \times 1 \times 200 = 100 \text{ J}. Height above ground = 10 m, so U = 1 \times 10 \times 10 = 100 \text{ J}. Total = 200 J.

At the ground (C): K = mgh = 200 \text{ J}, U = 0. Total = 200 J.

The table pattern shows the energy split at each height:

As the body falls, potential energy decreases while kinetic energy increases by the same amount. This holds strictly only when air friction is ignored; with friction, some kinetic energy dissipates as heat.

2.17 APPLICATION OF PRINCIPLE OF CONSERVATION OF ENERGY TO A SIMPLE PENDULUM

Consider a simple pendulum with resting position A. When displaced to B and released, it swings through A to C, reaching equal heights on both sides.

At the extreme positions B and C, the bob is momentarily at rest, so it has only potential energy (mgh). At the mean position A, its height is zero, so it has only kinetic energy. At any intermediate point, it has both, but the sum stays constant:

\frac{1}{2} m v^2 = mgh

at the mean position, where v = \sqrt{2gh}.

This holds strictly only in vacuum. Air friction gradually removes energy, so the swing dies down in practice. The chapter treats the ideal case first, then accounts for friction.

What to practise from Work Energy and Power Class 10 ICSE

Work energy and power class 10 ICSE gives you five blocks to practise, spread across the chapter:

• EXERCISE-2(A) — 30 questions on work, its measurement, special cases, units and power basics.
• MULTIPLE CHOICE TYPE — 17 questions testing concepts from sections 2.1 through 2.8.
• EXERCISE-2(B) — 27 questions on potential energy, kinetic energy, energy forms and conversions.
• MULTIPLE CHOICE TYPE — 23 questions on mechanical energy and energy changes.
• EXERCISE-2(C) — 19 questions on conservation of energy, free fall and the pendulum.

The numerical types map cleanly to the blocks. Work and power numericals concentrate in EXERCISE-2(A). Potential and kinetic energy numericals sit in EXERCISE-2(B). Conservation and free-fall problems appear in EXERCISE-2(C).

Work Energy and Power Class 10 ICSE frequently asked questions

What does work energy and power class 10 ICSE cover?

The chapter covers the definition of work, its measurement as W = FS \cos \theta, power as the rate of doing work, units of work and power, potential and kinetic energy, energy conversions and the conservation of mechanical energy.

Why is work done zero when the force is perpendicular to displacement in Class 10 ICSE Physics?

When \theta = 90^\circ, \cos 90^\circ = 0, so W = F S \cos \theta = 0. The centripetal force in circular motion and gravity on a coolie walking horizontally are classic examples.

What is the difference between work and power in this ICSE chapter?

Work is force times displacement in the direction of force, and it does not depend on time. Power is the rate of doing work, so it depends on time. Their units differ too: joule for work, watt for power.

How is 1 kWh related to joule in the Class 10 ICSE syllabus?

1 kWh is the energy spent by a 1 kW source in 1 hour. Since 1 \text{ kW} = 1000 \text{ J/s} and 1 hour = 3600 s, 1 \text{ kWh} = 3.6 \times 10^6 \text{ J}.

What is the conservation of mechanical energy for a freely falling body in Class 10 ICSE Physics?

For a freely falling body, K + U stays constant at every point. At the top, energy is all potential (mgh). At the ground, it is all kinetic (also mgh). At any middle height, the two share the total. This holds only when air friction is ignored.

Reference: ICSE Class 10 Physics textbook material, chapter 2: Work Energy and Power — CISCE-aligned.

More from this book

• Chapter 1: Force
• Chapter 3: Machines
• Chapter 4: Refraction of Light At Plane Surfaces
• Chapter 5: Refraction Through a Lens
• Chapter 6: Spectrum
• Chapter 7: Sound
• Chapter 8: Current Electricity
• Chapter 9: Household Circuits
• Chapter 10: Electro Magnetism
• Chapter 11: Calorimetry
• Chapter 12: Radioactivity