Work & Energy
Physics · कार्य और ऊर्जा
📋Quick Overview
Work, Energy and Power are closely connected. Work is done when a force moves an object. Energy is the ability to do work. Power is how fast work is done. These concepts are among the MOST asked topics in physics for competitive exams.
📖Work (W = F × d)
Work = Force × Displacement (in direction of force). W = F × d × cos θ. If force and displacement are in the same direction, W = F × d. SI unit of work = Joule (J). 1 Joule = 1 Newton × 1 meter. Work is a SCALAR quantity.
- •Work is ZERO when: (1) No displacement (pushing a wall), (2) Force is perpendicular to displacement (coolie carrying load on head — he walks horizontally but gravity acts vertically)
- •Work is POSITIVE when force and displacement in same direction (pushing a box forward)
- •Work is NEGATIVE when force and displacement in opposite direction (friction does negative work)
📖Energy
Energy is the capacity to do work. SI unit = Joule (J). Energy can neither be created nor destroyed — it can only be converted from one form to another. This is the Law of Conservation of Energy.
| Type of Energy | Formula | Depends On | Example |
|---|---|---|---|
| Kinetic Energy (KE) | ½mv² | Mass & Velocity (speed matters more — v is squared!) | Moving car, running person, flying bullet |
| Potential Energy (PE) | mgh | Mass, gravity & height | Water in dam, book on shelf, stretched bow |
If velocity doubles, KE becomes 4 times (because v²). If velocity triples, KE becomes 9 times. This is why high-speed accidents are so dangerous!
📝Forms of Energy & Conversions
- •Solar Cell: Light energy → Electrical energy
- •Electric motor: Electrical → Mechanical
- •Generator/Dynamo: Mechanical → Electrical
- •Electric heater: Electrical → Heat
- •Microphone: Sound → Electrical
- •Speaker: Electrical → Sound
- •Photosynthesis: Light → Chemical
- •Battery: Chemical → Electrical
- •Candle/Fire: Chemical → Heat + Light
- •Nuclear reactor: Nuclear → Heat → Electrical
📖Power (P = W/t)
Power = Rate of doing work = Work / Time. P = W/t. SI unit = Watt (W). 1 Watt = 1 Joule per second. Named after James Watt. 1 Horse Power (HP) = 746 Watts. 1 kilowatt (kW) = 1000 watts. Electricity bill is measured in kWh (kilowatt-hour). 1 kWh = 1 'unit' = 3.6 × 10⁶ Joules.
Two people do the same work. Person A takes 5 minutes, Person B takes 10 minutes. Who has more power? Person A — because they did the same work in less time. Power = Work/Time.
📖Conservation of Energy — Falling Ball Example
When a ball is at height h: PE = mgh, KE = 0 (not moving). As it falls: PE decreases, KE increases. Just before hitting ground: PE = 0, KE = maximum. At all points: PE + KE = constant (Total energy is always the same). Energy just changes form — from potential to kinetic.