Lesson 6 – Centre of Mass & System of Particles

PHASE–3 · PART–2
Real Question Handling & Error Control

Part–2 teaches how to react when a question looks confusing, lengthy, or unfamiliar. This is where marks are actually saved.

1️⃣ The 3-Step Decision Flow (Must Memorize)

Step–1: Identify SYSTEM or PARTICLE question
Step–2: Check EXTERNAL FORCE present or not
Step–3: Apply COM law or conservation immediately

If Step–1 is wrong, entire solution collapses.

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2️⃣ Typical Student Mistake vs Topper Move

Question: A body explodes into fragments.

❌ Average Student:
Tries to calculate fragment velocities

✅ Topper:
Writes: “Explosion = internal forces → COM unchanged”

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3️⃣ When You Feel “I Know This But I’m Stuck”

This is the most dangerous exam moment.

Immediately write mentally:
ΣF_external = M a_cm

80% of confusion disappears after this step.

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4️⃣ Handling Lengthy Questions (Psychological Trick)

IIT often writes long stories to distract you.

✔ Ignore numbers initially
✔ Ignore geometry initially
✔ Focus only on: forces acting on system

Numbers come later — laws come first.

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5️⃣ Internal vs External Force Confusion (Rank Killer)

Ask this exact question:

“Does this force come from outside the system boundary?”

If NO → internal → ignore for COM motion.

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6️⃣ Frame-Based Questions: Safe Handling

Whenever lift, wedge, train, accelerating frame appears:

✔ First decide inertial / non-inertial
✔ If non-inertial → add pseudo force
✔ Then apply COM law normally

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7️⃣ How Toppers Recover from a Wrong Start

Even toppers start wrong sometimes.

✔ They stop early (within 20 seconds)
✔ They re-check: system or particle?
✔ They restart with COM law

Never continue blindly.

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8️⃣ Final Safety Checklist (Before Marking Answer)

✔ Direction sign correct?
✔ Frame assumption correct?
✔ Internal force ignored properly?
✔ Answer independent of internal chaos?

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🏁 PHASE–3 · PART–2 CLOSURE

Part–2 converts knowledge into safe, repeatable exam performance.

After this, COM questions feel predictable and controllable.

📘 Centre of Mass & System of Particles – Complete Physics Library

This library is a complete learning package for Intermediate + IIT JEE Physics covering Centre of Mass and System of Particles.

It includes theory, solved examples, objective questions, IIT-level problems, revision strategies, tricks, and exam-oriented guidance.

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📂 Stage 1: Intermediate Complete Notes (Concept Foundation)

1. Part 1
2. Part 2
3. Part 3
4. Part 4
5. Part 5
6. Part 6
7. Part 7
8. Part 8
9. Part 9
10. Part 10

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📂 Stage 2: Advanced Concepts & Applications

11. Part 11
12. Part 12
13. Part 13
14. Part 14
15. Part 15
16. Part 16
17. Part 17
18. Part 18
19. Part 19
20. Part 20

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📂 Stage 3: Objective Questions, Tough Problems & Revision

21. Part 21
22. Part 22
23. Part 23
24. Part 24
25. Part 25
26. Part 26
27. Part 27
28. Part 28
29. Part 29
30. Part 30
31. Part 31
32. Part 32
33. Part 33
34. Part 34
35. Part 35
36. Part 36
37. Part 37
38. Part 38
39. Part 39
40. Part 40
41. Part 41
42. Part 42
43. Part 43
44. Part 44
45. Part 45
46. Part 46
47. Part 47
48. Part 48
49. Part 49
50. Part 50
51. Part 51
52. Part 52
53. Part 53
54. Part 54
55. Part 55
56. Part 56
57. Part 57

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Prepared by: Shaktimatha Learning 🌱
Vision: Concept clarity • Logical thinking • Exam excellence