Lesson 6 – Centre of Mass & System of Particles

PHASE–2 : IIT–JEE Advanced Core Practice
Part 6 – Final Polishing & Exam Hall Thinking

This final part focuses on decision-making under pressure. These questions look simple, but each wrong option is a carefully designed trap.

                                               

Q1. (Multi-Correct | Examiner Psychology)

A system of particles is moving with constant velocity. Which statements are definitely true?

A) Net external force on the system is zero
B) All particles have constant velocity
C) Centre of mass acceleration is zero
D) Internal forces must be zero

Correct Options: A, C

Why others are wrong:
• B: particles may accelerate internally
• D: internal forces can exist but cancel

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Q2. (Integer Type | Calm Thinking Test)

A system of total mass 20 kg experiences zero acceleration of centre of mass. Find the net external force acting on the system (N).

F_ext = M a_cm = 20 × 0 = 0

Answer: 0

Trap: Don’t overthink — apply F = Ma directly.

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Q3. (Assertion–Reason | Confidence Check)

Assertion (A):
The centre of mass of a system behaves as if the entire mass were concentrated at that point.

Reason (R):
Newton’s second law can be applied to the system as a whole.

A) Both A and R are true and R explains A
B) Both A and R are true but R does not explain A
C) A is true, R is false
D) A is false, R is true

Answer: A

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Q4. (Multi-Correct | Ultimate Trap)

Which statements about centre of mass are correct?

A) COM can lie outside the physical body
B) COM position depends on velocity of particles
C) COM motion depends only on external forces
D) COM must coincide with geometrical centre

Correct Options: A, C

Why others are wrong:
• B: position depends on mass distribution only
• D: symmetry required, not always present

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Q5. (One-Minute Question | Topper Style)

A system has zero net external force. In one line, state the motion of its centre of mass.

Answer:
Centre of mass moves with constant velocity.

Examiner Expectation: Short, confident, law-based answers score full marks.

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FINAL EXAM HALL MINDSET (Lesson 6)

✔ Read the question and define the system first
✔ Ask: external forces present or not?
✔ Apply F = M a_cm without fear
✔ Ignore all internal complexities
✔ Eliminate options logically, not emotionally
✔ Stay calm — COM questions reward clarity

Lesson 6 – Final Closure

With Part–6, Lesson 6 is COMPLETE at IIT-Advanced level. A student who masters all phases will never fear Centre of Mass again.

📘 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