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Work, Power and Energy Revision Notes

Comprehensive exam revision covering work, power, and energy definitions, conditions, formulas, units, types, the law of conservation of energy, renewable versus non-renewable sources, and key device transformations.

Teaching
Physics
English · 6 topics
Checked against the video on 25 Oct 2026

Made by AI from the YouTube video UP SUPERTET SCIENCE Class-6 | कार्य, सामर्थ्य और ऊर्जा | उ.प्र. प्राथमिक शिक्षक भर्ती 2026 (opens YouTube in a new tab) by Balaji Study Adda, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Work, Power and Energy Revision Notes

English6 topics · 8 MCQs · 13 flashcardsWatch the video (opens YouTube in a new tab)

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In short

Comprehensive exam revision covering work, power, and energy definitions, conditions, formulas, units, types, the law of conservation of energy, renewable versus non-renewable sources, and key device transformations.

Work: Definition, Conditions, and Formula

Work is defined as the force applied to an object that causes it to travel a certain distance in the direction of the force.
Must knowThe essential conditions for work to take place are that force must be applied, distance must be covered in the direction of force, and force and displacement must not be perpendicular.
When force is applied but displacement is zero, or when force and displacement are mutually perpendicular (90 degrees), the work done is zero.
The standard formula to calculate work done is W = F × s, where W is work, F is force, and s is displacement.
In the general case where an angle theta exists between force and displacement, the formula is W = F × s × cos(theta).
Must knowWork is a scalar quantity (adhish rashi) whose value depends on both force and displacement.

Units and Types of Work

Must knowIn the SI system, the unit of work is Joule, whereas in the CGS system, the unit of work is Erg.
One Joule is defined as the work done when a force of 1 Newton displaces an object by 1 meter in its own direction.
One Erg is the work done when a force of 1 Dyne displaces an object by 1 centimeter in its own direction.
Must knowOne Joule equals 10⁷ Erg, and correspondingly 1 Erg equals 10⁻⁷ Joule.
Positive work (+W) occurs when the directions of force and displacement are identical (angle of 0 degrees).
Negative work (-W) occurs when the direction of displacement is opposite to the direction of force (angle of 180 degrees).

Type of Work	Angle (θ)	Condition
Positive (+W)	0°	Force and displacement in same direction
Negative (-W)	180°	Force and displacement in opposite directions
Zero (W = 0)	90° or s = 0	Perpendicular force or zero displacement

Energy: Kinetic, Potential, and Mechanical

Energy is defined as the capacity to do work; its SI unit is Joule (J) and its commercial unit is kilowatt-hour (kWh).
Kinetic energy (KE) is the energy possessed by an object due to its motion and is given by KE = 1/2 × m × v².
Must knowKinetic energy depends on the mass (m) and the square of velocity (v); doubling the velocity increases the kinetic energy fourfold.
Potential energy (PE) is the stored energy in an object due to its position or state, with gravitational potential energy calculated as PE = m × g × h.
Must knowMechanical energy (ME) is the total energy due to both motion and position, calculated as ME = KE + PE.
The acceleration due to gravity (g) is standardly taken as 9.8 m/s² (or approximated as 10 m/s² in problems).

Power: Definition, Units, and Conversions

Must knowPower (samarth) is defined as the rate of doing work, represented by the formula P = W / t.
The SI unit of power is Watt (W), where 1 Watt is equal to 1 Joule per second (1 J/s).
An object or machine with greater power performs the same amount of work in less time.
Must knowOne kilowatt (kW) equals 1000 Watts (10³ W), and one Megawatt (MW) equals 10⁶ Watts.
Must knowOne horsepower (HP), commonly used to rate motors and engines, is approximately equal to 746 Watts.
The commercial unit of electrical energy is 1 kilowatt-hour (1 kWh), also called 1 unit, which equals 3.6 × 10⁶ Joules (3,600,000 J).

Law of Conservation of Energy and Transformations

Must knowAccording to the law of conservation of energy, energy can neither be created nor destroyed; it only transforms from one form to another, keeping total energy constant.
In a waterfall, potential energy of stored water transforms into kinetic energy as it falls, then into sound and thermal energy upon impact.
Friction converts mechanical energy into thermal (heat) energy.
When an arrow is released from a bow, elastic potential energy stored in the stretched string is converted into kinetic energy.
A dynamo (generator) converts mechanical energy into electrical energy.
An electric motor converts electrical energy into mechanical energy.

Device / Event	Input Energy	Output Energy
Dynamo / Generator	Mechanical Energy	Electrical Energy
Electric Motor	Electrical Energy	Mechanical Energy
Electric Fan	Electrical Energy	Kinetic / Mechanical Energy
Microphone	Sound Energy	Electrical Energy
Loudspeaker	Electrical Energy	Sound Energy
Battery (Discharge)	Chemical Energy	Electrical Energy
Solar Cell	Solar Energy	Electrical Energy

Forms of Potential Energy and Energy Sources

Gravitational potential energy arises from raising a body above the Earth's surface against gravity.
Elastic potential energy is stored when flexible materials like springs or rubber bands are stretched or compressed.
Electric potential energy exists when an electric charge is moved within an electric field, and magnetic potential energy exists when magnetic materials are moved in a magnetic field.
Must knowRenewable (alternative) energy sources are naturally replenished and inexhaustible, including solar, wind, hydro, biomass, biogas, geothermal, tidal, and hydrogen energy.
Must knowNon-renewable (conventional) energy sources are limited and exhaustible, including coal, petroleum, natural gas, nuclear fuels (uranium, thorium), diesel, LPG, shale gas, and tar sand.

Energy Source Class	Exhaustibility	Examples
Renewable	Inexhaustible	Solar, Wind, Hydro, Biogas, Geothermal, Tidal
Non-renewable	Exhaustible / Limited	Coal, Petroleum, Natural Gas, Nuclear (Uranium, Thorium), LPG

Key terms

Work: The product of force and displacement in the direction of force (W = F × s)
Joule: The SI unit of work and energy, equal to 1 Newton-meter
Erg: The CGS unit of work, equal to 1 Dyne-centimeter
Kinetic Energy: Energy possessed by a body by virtue of its motion (1/2 × m × v²)
Potential Energy: Energy stored in a body due to its position, state, or configuration (m × g × h)
Mechanical Energy: The sum of kinetic energy and potential energy (KE + PE)
Power: The rate of doing work (P = W / t)
Watt: The SI unit of power, equivalent to 1 Joule per second
Horsepower: A practical unit of power equal to approximately 746 Watts
1 Unit (Commercial Energy): One kilowatt-hour (1 kWh), equal to 3.6 × 10⁶ Joules

Formulas

W = F × s: Standard formula for work done
W = F × s × cos(θ): Work done when force is at an angle theta to displacement
KE = 1/2 × m × v²: Kinetic energy of a moving object
PE = m × g × h: Gravitational potential energy
ME = KE + PE: Total mechanical energy
P = W / t: Rate of doing work (Power)
1 kWh = P(kW) × t(h) = 3.6 × 10⁶ J: Commercial electrical energy conversion

Exam tips and traps

If force and displacement are perpendicular (theta = 90 degrees), no work is done even if a heavy load is carried across horizontal ground.
Remember that doubling the velocity quadruples kinetic energy because velocity is squared in the formula.

Quick revision

Read these the night before the exam.

Work and energy are both scalar quantities having Joule as their SI unit.
1 Joule equals exactly 10⁷ Erg.
Work is zero at a 90-degree angle and negative at a 180-degree angle.
Doubling the velocity of an object increases its kinetic energy four times.
Mechanical energy is the sum of kinetic energy and potential energy.
1 Horsepower (HP) equals 746 Watts.
1 kilowatt-hour (1 commercial unit) equals 3.6 × 10⁶ Joules.
A dynamo converts mechanical energy into electrical energy.
An electric motor converts electrical energy into mechanical energy.
Geothermal and tidal energy are renewable, while nuclear energy (uranium, thorium) is non-renewable.

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