Lesson 6 – Centre of Mass & System of Particles
Objective Type Questions & Answers – Set 4 (Advanced Intermediate + IIT-JEE)
This set emphasizes momentum-based reasoning, motion of centre of mass, and IIT-style traps.
Advanced Conceptual MCQs
1. The centre of mass of a system of particles moves with constant velocity when:
A) Internal forces are zero
B) Net external force is zero
C) Particles are identical
D) The system is symmetric
Answer: B
Explanation: Only external forces affect COM motion.
2. A shell explodes into two fragments in mid-air. The centre of mass of the fragments:
A) Falls vertically downward
B) Moves in a straight line
C) Moves in a parabolic path
D) Comes to rest
Answer: C
Explanation: Gravity is the only external force.
Numerical Objective Questions (Accurate)
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 the centre of mass is:
A) −1 m/s
B) −0.5 m/s
C) 0 m/s
D) 1 m/s
Answer: B
Explanation:
vcm = (2×4 + 6×(−2)) / (2+6)
= (8 − 12) / 8 = −4 / 8 = −0.5 m/s
4. Three particles of masses 1 kg, 2 kg, and 3 kg are placed at x = 0 m, x = 2 m, and x = 6 m respectively. The centre of mass is located at:
A) 2 m
B) 3 m
C) 4 m
D) 5 m
Answer: C
Explanation:
xcm = (1×0 + 2×2 + 3×6) / 6 = (0 + 4 + 18)/6 = 22/6 ≈ 3.67 m
Closest correct option → 4 m
Assertion – Reason (IIT Style)
5. Assertion (A): Motion of centre of mass of a system is unaffected by internal forces.
Reason (R): Internal forces obey Newton’s third law.
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
IIT Trap Questions
6. A man walks from one end of a boat to the other in still water. The centre of mass of the man–boat system:
A) Moves in the direction of motion
B) Moves opposite to motion
C) Remains at same position (w.r.t water)
D) Oscillates
Answer: C
7. Two particles attract each other due to mutual gravitational force. The centre of mass of the system:
A) Accelerates toward heavier particle
B) Remains stationary or moves uniformly
C) Moves randomly
D) Oscillates
Answer: B
One-Line Objective (Fast Revision)
- External force on COM = Total mass × acceleration of COM
- Explosion effect on COM velocity → None
- COM motion depends on → External forces only
- COM of isolated system → Constant velocity
IIT Solver’s Strategy (Set 4)
✔ First question: external force present or not?
✔ For velocity of COM, use momentum conservation directly
✔ Ignore internal forces immediately
✔ In options, check nearest physical value carefully
✔ Explosion ≠ change in COM motion
📘 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.
📂 Stage 1: Intermediate Complete Notes (Concept Foundation)
📂 Stage 2: Advanced Concepts & Applications
📂 Stage 3: Objective Questions, Tough Problems & Revision
🔗 Learning Hub & Community
Prepared by: Shaktimatha Learning 🌱
Vision: Concept clarity • Logical thinking • Exam excellence
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