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Chapter 4 — Chapter 4 — Work, Energy and Power
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📘Easy summary
Work is said to be done when a force acts on a body and the body moves through some distance in the direction of the force. Work is the scalar product of force and displacement: W = F·d = Fd cos θ. SI unit joule (1 J = 1 N m); C.G.S. unit erg (1 J = 10⁷ erg). Work is positive when 0° ≤ θ ＜ 90° (force helps motion), zero when θ = 90° (e.g. centripetal force, carrying a bag horizontally), and negative when 90° ＜ θ ≤ 180° (friction, gravity on a rising body).
⭐Key points
- W = Fd cos θ.
- 1 J = 1 N m.
- Sign depends on θ.
🔬Diagram
🌿Real-life example
200 N at 45° drags a suitcase 50 m: W = 200 × 50 × cos 45° = 7.07 kJ.
💡Exam tip
- Use the component of force along displacement.
✅Quick check
1. Work done by centripetal force is:
2. 1 joule equals:
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