Lesson 6 – Centre of Mass & System of Particles
Objective Questions – Set 3
(Advanced Bias · Trap Heavy)
This set tests recognition + elimination. Read each option carefully — many are almost correct.
Q1. (IIT–Advanced | MCQ)
A system has zero net external force. Which of the following quantities must remain constant?
A) Velocity of each particle
B) Momentum of the system
C) Kinetic energy of the system
D) Velocity of centre of mass
Correct Answers: B, D
Explanation:
Zero external force ⇒ momentum conserved ⇒ COM velocity constant.
Individual velocities and total KE may change.
Q2. (IIT–Advanced | Integer Type)
Two particles of masses 3 kg and 1 kg move along x-axis with velocities 2 m/s and −4 m/s. Find the velocity of centre of mass (m/s).
vcm = (3×2 + 1×(−4)) / 4 = (6 − 4)/4 = 0.5
Answer: 0.5
Q3. (IIT–Advanced | MCQ)
A projectile explodes at the highest point. Immediately after explosion, which quantities are unchanged?
A) Velocity of centre of mass
B) Acceleration of centre of mass
C) Total momentum
D) Total kinetic energy
Correct Answers: A, B, C
Explanation:
Only internal forces act during explosion.
Gravity unchanged ⇒ acceleration same.
KE changes due to internal energy release.
Q4. (IIT–Advanced | Conceptual MCQ)
A system has zero momentum at an instant. Which statements are correct?
A) Centre of mass velocity is zero
B) Centre of mass acceleration is zero
C) Net external force is zero
D) System must remain at rest
Correct Answer: A
Trap:
Momentum value does NOT determine force or acceleration.
Q5. (IIT–Advanced | Integer Type)
A system of mass 8 kg is subjected to an external force that produces an acceleration of 1.5 m/s². Find the magnitude of the external force (N).
F = M a = 8 × 1.5 = 12
Answer: 12
Q6. (IIT–Advanced | MCQ)
Two equal and opposite forces act on a system. Which outcomes are possible?
A) Translation of COM
B) Rotation of system
C) Change in linear momentum
D) Zero acceleration of COM
Correct Answers: B, D
Explanation:
Net force zero ⇒ no translation.
Torque may be non-zero ⇒ rotation possible.
Q7. (IIT–Advanced | MCQ)
A system is observed from a non-inertial frame. Which modification is required to apply COM laws?
A) Ignore internal forces
B) Add pseudo forces as external forces
C) Change mass of system
D) Use energy conservation only
Correct Answer: B
Q8. (IIT–Advanced | Integer Type)
An isolated system initially at rest explodes. Total mass = 10 kg. One fragment of mass 2 kg moves with velocity 5 m/s. Find the velocity of centre of mass (m/s).
Isolated system ⇒ COM velocity remains zero.
Answer: 0
Q9. (IIT–Advanced | MCQ)
Which statement is always true for any system?
A) COM lies inside the body
B) COM coincides with geometric centre
C) COM depends on mass distribution
D) COM is always fixed in space
Correct Answer: C
Q10. (IIT–Advanced | One-Line MCQ)
Which equation correctly describes COM motion in an inertial frame?
A) ΣFinternal = 0
B) Σp = constant
C) ΣFexternal = M acm
D) Work = ΔKE
Correct Answer: C
ADVANCED OBJECTIVE TIPS (SET–3)
✔ Distinguish value vs rate of change
✔ Frame matters — inertial vs non-inertial
✔ COM laws ignore internal chaos
✔ Multiple-correct MCQs: eliminate impossibles first
✔ Integer type: sign & units are critical
📘 Centre of Mass & System of Particles – Complete Physics Library
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It includes theory, solved examples, objective questions, IIT-level problems, revision strategies, tricks, and exam-oriented guidance.
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📂 Stage 3: Objective Questions, Tough Problems & Revision
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