Work, Energy and Power
Work
Work is done when a force moves an object in the direction of the force.
W = F x d x cos(theta) or simply W = F x s
- Unit: joule (J). 1 J = work done when 1 N moves an object 1 m. CGS unit: erg; 1 J = 10^7 erg.
- No work is done if displacement is zero (pushing a wall) or the force is perpendicular to displacement (carrying a bag horizontally).
- Work is positive when force and displacement are in the same direction; negative when opposite (friction).
Energy
Energy is the capacity to do work. Unit: joule. It exists in many forms: mechanical, heat, light, sound, chemical, electrical, nuclear.
| Type | Formula | Meaning |
|---|---|---|
| Kinetic energy (KE) | KE = (1/2) m v^2 | Energy due to motion |
| Potential energy (PE) | PE = m g h | Energy due to position or condition (height, stretched spring) |
- KE becomes four times if velocity is doubled.
- Law of conservation of energy: energy can neither be created nor destroyed, only changed from one form to another. The total stays constant. In a falling object, PE decreases while KE increases.
- Mass-energy equivalence (Einstein):
E = m c^2; the basis of nuclear energy.
| Device | Energy change |
|---|---|
| Electric bulb | Electrical to light and heat |
| Electric motor | Electrical to mechanical |
| Generator (dynamo) | Mechanical to electrical |
| Microphone | Sound to electrical |
| Loudspeaker | Electrical to sound |
| Solar cell | Light to electrical |
| Battery (cell) | Chemical to electrical |
| Photosynthesis | Light to chemical |
| Candle | Chemical to heat and light |
| Hydroelectric plant | Water PE to electrical |
Power
Power is the rate of doing work: P = W / t. Unit: watt (W); 1 W = 1 J/s.
- 1 kilowatt (kW) = 1000 W. 1 horsepower (HP) = 746 W.
- Commercial unit of energy: kilowatt-hour (kWh) = 1 unit of electricity. 1 kWh = 3.6 x 10^6 J.
- Electricity bill example: a 100 W bulb used 10 hours a day for 30 days uses 100 x 10 x 30 / 1000 = 30 units.
Keep practising
Notes are only half the work. Test yourself with questions and discuss doubts with other aspirants.
Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.