Lesson 6 – Centre of Mass & System of Particles

PHASE–3 · PART–3
Real IIT Question Walkthrough (Topper Thinking)

In this part, we do live walkthroughs. Each question shows:
• what to think first
• what to ignore
• how to avoid traps

Walkthrough–1 (IIT–Advanced Style)

Question:
A projectile explodes at the highest point into two fragments. One fragment falls vertically downward. Describe the motion of the other fragment.

Topper’s First 10 Seconds:

✔ Explosion → internal forces ✔ External force = gravity only ✔ COM motion unchanged

Key Logic:

Since one fragment falls vertically, the other must move such that the centre of mass follows the original parabolic path.

Final Conclusion:

The other fragment moves forward horizontally to maintain COM trajectory.

❌ Common Mistake: Trying to calculate fragment speeds

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Walkthrough–2 (System Push Problem)

Question:
Two persons standing on frictionless ice push each other. Describe the motion of the system.

Topper’s First 10 Seconds:

✔ Push = internal force ✔ No external horizontal force ✔ COM velocity constant

Final Answer:

Persons move in opposite directions, but centre of mass remains at rest.

✔ Rank-saving recognition question

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Walkthrough–3 (Frame Trap)

Question:
A system is at rest in a lift accelerating upward. What is the acceleration of its centre of mass as seen from the ground?

Topper’s First 10 Seconds:

✔ Lift frame is non-inertial ✔ Ground frame is inertial ✔ COM acceleration = lift acceleration

Final Answer:

Centre of mass accelerates upward with the same acceleration as the lift.

❌ Common Mistake: Saying “COM is at rest”

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Walkthrough–4 (Zero Momentum Trap)

Question:
A system has zero total momentum at an instant. Can its centre of mass accelerate at that instant?

Topper’s Thinking:

✔ Momentum value ≠ force ✔ Acceleration depends on force

Final Answer:

Yes. If external force is present, COM can accelerate even when momentum is zero.

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Walkthrough–5 (Lengthy Story Trap)

Question:
A long story involving springs, strings, masses, and relative motions is given. The question finally asks about system motion.

Topper’s Shortcut:

✔ Ignore internal details ✔ Ask only: external force present or not?

Result:

COM motion determined in one line, while others waste time.

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📘 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