Lesson 6 – Centre of Mass & System of Particles

FINAL REVISION · PART–3
15 Must-Do Questions (Logic-Based)

These 15 questions represent the entire Centre of Mass chapter. Solve them with logic — not speed.

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Q1️⃣

A system is isolated. What can you say about the motion of its centre of mass?

Logic: No external force → a_cm = 0

Answer: COM moves with constant velocity (may be zero).

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Q2️⃣

A stationary system explodes into fragments. One fragment is observed moving east. What about COM motion?

Logic: Explosion = internal forces

Answer: COM remains at rest.

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Q3️⃣

Can centre of mass accelerate when total momentum is zero?

Logic: Momentum value ≠ force

Answer: Yes, if external force exists.

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Q4️⃣

A person jumps inside a frictionless boat. What happens to the COM of the system?

Logic: All forces internal horizontally

Answer: COM remains at same position horizontally.

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Q5️⃣

Where does COM lie for a uniform circular ring?

Logic: Symmetry

Answer: At geometrical centre (outside material).

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Q6️⃣

Two particles of unequal masses move with equal and opposite velocities. Is COM at rest?

Logic: Momentum may not cancel

Answer: Not necessarily.

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Q7️⃣

A system is inside an accelerating lift. From ground frame, how does COM move?

Logic: Ground frame inertial

Answer: COM accelerates with lift.

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Q8️⃣

Does internal friction change COM motion?

Logic: Internal force only

Answer: No.

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Q9️⃣

A body breaks into two pieces while falling. What happens to COM path?

Logic: Gravity is only external force

Answer: COM continues same parabolic path.

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Q🔟

Can COM lie outside a solid body?

Logic: Depends on mass distribution

Answer: Yes.

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Q1️⃣1️⃣

What decides acceleration of centre of mass?

Logic: Newton’s 2nd law for system

Answer: Net external force only.

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Q1️⃣2️⃣

In which frame is COM acceleration same for all observers?

Logic: Inertial frame property

Answer: All inertial frames.

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Q1️⃣3️⃣

Does rotation of a system affect COM motion?

Logic: Translation and rotation independent

Answer: No.

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Q1️⃣4️⃣

A system has constant COM velocity. What can you say about external force?

Logic: a_cm = 0

Answer: Net external force is zero.

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Q1️⃣5️⃣

Why is centre of mass concept powerful?

Logic: Simplifies multi-particle systems

Answer: Converts complex motion into single-body motion.

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✅ REVISION PART–3 CLOSURE

If a student can answer these 15 questions calmly, Centre of Mass is fully under control.

This completes the FINAL REVISION SERIES for Lesson 6.

📘 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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11. Part 11
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📂 Stage 3: Objective Questions, Tough Problems & Revision

21. Part 21
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31. Part 31
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Vision: Concept clarity • Logical thinking • Exam excellence