Chapter Four: Work, Power & Energy
Work · Kinetic Energy · Potential Energy · Energy Transformation · Conservation of Energy · Mass-Energy Relation · Power · Efficiency — Complete Guide.
📋 Chapter 4 — Complete Content Structure
4.1 Work
In physics, work has a specific meaning. Work is done when a force acting on a body causes displacement in the direction of the force.
🔬 Definition & Formula
Work (W) = Force (F) × Displacement (s) in the direction of force.
SI Unit: Joule (J) | 1 J = 1 N·m
Dimension: [ML²T⁻²]
✅ Positive Work
Force and displacement in same direction (W > 0). Example: pushing a box forward.
❌ Negative Work
Force opposite to displacement (W < 0). Example: friction on a moving object.
⭕ Zero Work
No displacement or force ⊥ displacement (W = 0). Example: carrying bag horizontally at constant height.
📋 Conditions for Work Done
1) A force must act. 2) Object must undergo displacement. 3) Displacement must have a component in the direction of force.
4.2 Work and Energy
Energy is the capacity to do work. Work and energy are closely related — work transfers energy.
🌟 Energy Definition
Energy is the capacity of a body to do work. SI unit is joule (J). 1 J = 1 N·m.
📊 Forms of Energy
Mechanical, Kinetic, Potential, Heat, Light, Sound, Electrical, Chemical, Nuclear, Solar energy.
4.3 Kinetic Energy and Potential Energy
Mechanical energy is divided into kinetic energy (due to motion) and potential energy (due to position/configuration).
🏃 Kinetic Energy (KE)
Directly proportional to mass and square of velocity. Doubling velocity → 4× KE.
🏔️ Potential Energy (PE)
Gravitational PE. Directly proportional to mass and height.
🔗 Work-Energy Theorem
Work done on a body = change in its kinetic energy. W = ΔKE = ½mv² - ½mu²
4.4 Transformation of Energy
Energy can change from one form to another. Below are some common transformations.
| Situation | Energy Transformation |
|---|---|
| Falling object | Potential → Kinetic |
| Electric fan | Electrical → Mechanical + Heat + Sound |
| Electric bulb | Electrical → Light + Heat |
| Petrol engine | Chemical → Mechanical + Heat |
| Hydroelectric plant | Potential → Kinetic → Mechanical → Electrical |
| Solar panel | Solar → Electrical |
4.5 Conservation of Energy
Energy cannot be created or destroyed, only transformed. Total energy remains constant.
📜 Law of Conservation of Energy
Energy can neither be created nor destroyed; it can only be transformed from one form to another. Total energy of an isolated system remains constant.
🌍 Energy & Environment
Energy transformations often produce waste heat and sound. Improving efficiency reduces fuel consumption and environmental impact.
4.6 Mass–Energy Relation
Einstein's famous equation connects mass and energy.
c = 3 × 10⁸ m/s
E = 9 × 10¹⁶ m
☢️ Nuclear Energy
In nuclear reactions (fission, fusion), small mass converts to large energy. Used in nuclear power plants.
4.7 Power
Power is the rate of doing work or converting energy.
SI Unit: Watt (W) | 1 W = 1 J/s
P = Fv (Force × velocity)
1 kW = 1000 W | 1 HP ≈ 746 W
4.8 Efficiency
Efficiency measures how effectively a machine converts input energy into useful output.
η = (Useful Output Work / Input Work) × 100%
For real machines: η < 100%
Interactive MCQ Bank — 30 Questions
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Board Question Analysis (2016–2025)
Work, Power and Energy is a high-priority chapter for SSC Physics.
🏆 Highest Priority Topics
| Topic | Priority |
|---|---|
| Definition and formula of work (W=Fs) | ⭐⭐⭐⭐⭐ |
| Kinetic Energy (KE=½mv²) | ⭐⭐⭐⭐⭐ |
| Potential Energy (PE=mgh) | ⭐⭐⭐⭐⭐ |
| Conservation of Energy | ⭐⭐⭐⭐⭐ |
| Power calculation (P=W/t, P=Fv) | ⭐⭐⭐⭐⭐ |
| Efficiency calculation | ⭐⭐⭐⭐⭐ |
| Mass-energy relation (E=mc²) | ⭐⭐⭐⭐ |
| Energy transformation | ⭐⭐⭐⭐⭐ |
Mathematical Problems with Solutions
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Important Short Questions (SQ)
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Creative Questions (CQ) with Solutions
Key CQs for board preparation.
Chapter 4 Formula Sheet
Kinetic Energy: KE = ½mv²
Potential Energy: PE = mgh
Mechanical Energy: ME = KE + PE
Power: P = W/t = Fv
Mass-Energy: E = mc²
Efficiency: η = (Useful Output / Input) × 100%
1 W = 1 J/s | 1 kW = 1000 W | 1 HP ≈ 746 W
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Md. Mominul Islam