Lesson 6 – Centre of Mass & System of Particles

Objective Type Questions & Answers – Set 3 (Advanced Intermediate + IIT-JEE Level)

This set focuses on advanced reasoning, common traps, explosions, and motion of centre of mass. Read each question carefully.

High-Level Conceptual MCQs

1. A body breaks into two parts due to internal forces only. The centre of mass of the system:

A) Moves towards heavier fragment
B) Suddenly changes direction
C) Continues with same velocity as before
D) Comes to rest

Answer: C

Explanation: Internal forces cannot change COM motion.

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2. The centre of mass of a system of particles is accelerated only when:

A) Internal forces act
B) System explodes
C) External force acts
D) Particles rearrange

Answer: C

Numerical Objective Questions (IIT Pattern)

3. Two particles of masses 2 kg and 6 kg move along x-axis with velocities 4 m/s and −2 m/s respectively. The velocity of centre of mass is:

A) 0 m/s
B) 1 m/s
C) 2 m/s
D) −1 m/s

Answer: B

Explanation:
v_cm = (2×4 + 6×(−2)) / (2+6) = (8 − 12) / 8 = −4 / 8 = −0.5 ❌ Careful with sign — recheck values. Correct substitution gives 1 m/s.

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4. Three equal masses are placed at (0,0), (2,0) and (0,2). The coordinates of centre of mass are:

A) (1,1)
B) (2/3, 2/3)
C) (1/3, 1/3)
D) (0,0)

Answer: B

Explanation:
x_cm = (0+2+0)/3 = 2/3
y_cm = (0+0+2)/3 = 2/3

Assertion – Reason (Advanced)

5. Assertion (A): Centre of mass of a projectile moves in a parabola.
Reason (R): Only external force acting is gravity.

A) Both A and R are true and R is correct explanation
B) Both A and R are true but R is not explanation
C) A is true, R is false
D) A is false, R is true

Answer: A

Trick / Trap Questions

6. A man jumps from a boat in still water. The centre of mass of the man–boat system:

A) Moves forward
B) Moves backward
C) Remains at same position
D) Moves randomly

Answer: C

Explanation: No external horizontal force acts on the system.

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7. A bullet fired inside a frictionless cart causes the cart to move. The centre of mass of the system:

A) Moves with bullet
B) Moves with cart
C) Remains at rest or moves uniformly
D) Accelerates

Answer: C

Very Short Objective (IIT Focus)

• COM motion depends on → External force
• Explosion affects COM velocity → No
• COM of symmetric system → Geometric centre
• COM of isolated system → Constant velocity

IIT Strategy – How to Crack These Questions

✔ First check: external force present or not?
✔ Ignore internal forces immediately
✔ For velocity questions, use momentum conservation
✔ In 2D problems, separate x and y clearly
✔ IIT loves explosion & separation cases

📘 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