Lesson 6 – Centre of Mass & System of Particles
Objective Questions (Previous IIT–JEE Oriented)
MCQs & Integer Type with Solutions
These objective questions are designed in the exact thinking style of IIT-JEE papers. Focus on concept clarity, not memorization.
Q1. (IIT-JEE Main | MCQ)
A system of particles is isolated. Which of the following quantities remains conserved?
A) Velocity of each particle
B) Momentum of the system
C) Kinetic energy of each particle
D) Acceleration of centre of mass
Correct Answer: B
Explanation:
No external force ⇒ total momentum conserved.
Individual velocities and energies may change.
Q2. (IIT-JEE Main | Integer Type)
Two particles of masses 2 kg and 4 kg move along x-axis with velocities 6 m/s and −2 m/s. Find the velocity of the centre of mass (m/s).
vcm = (2×6 + 4×(−2)) / 6 = (12 − 8) / 6
vcm = 2/3
Answer: 2/3
Q3. (IIT-JEE Advanced | MCQ)
A projectile explodes at the highest point of its trajectory. Which of the following statements is correct?
A) Centre of mass falls vertically downward
B) Centre of mass remains at rest
C) Centre of mass follows the same parabolic path
D) Centre of mass accelerates upward
Correct Answer: C
Explanation:
Explosion is internal.
Gravity is the only external force.
Hence COM follows the original parabolic path.
Q4. (IIT-JEE Main | MCQ)
The centre of mass of a system lies outside the body when:
A) The body is solid
B) The body has uniform density
C) The body is hollow or ring-shaped
D) The body is symmetric
Correct Answer: C
Explanation:
COM depends on mass distribution, not geometry alone.
Q5. (IIT-JEE Advanced | Integer Type)
A system of total mass 10 kg is acted upon by an external force of 20 N. Find the acceleration of the centre of mass (m/s²).
acm = F / M = 20 / 10 = 2
Answer: 2
Q6. (IIT-JEE Advanced | MCQ)
Two equal and opposite forces act on a rigid body at different points. Which of the following is correct?
A) COM accelerates
B) COM remains at rest but body rotates
C) Linear momentum changes
D) Net external force is non-zero
Correct Answer: B
Explanation:
Net force = 0 ⇒ no translation.
Torque ≠ 0 ⇒ rotation possible.
Q7. (IIT-JEE Main | MCQ)
The acceleration of centre of mass of a system depends on:
A) Internal forces only
B) External forces only
C) Both internal and external forces
D) Velocities of particles
Correct Answer: B
Q8. (IIT-JEE Advanced | Conceptual MCQ)
Which statement is always true?
A) Zero momentum ⇒ zero acceleration of COM
B) Zero acceleration of COM ⇒ zero momentum
C) Zero external force ⇒ constant velocity of COM
D) Zero velocity of COM ⇒ zero external force
Correct Answer: C
Q9. (IIT-JEE Main | Integer Type)
Three particles of equal mass move with velocities 2 m/s, −2 m/s, and 0 m/s. Find the velocity of centre of mass (m/s).
vcm = (2 − 2 + 0) / 3 = 0
Answer: 0
Q10. (IIT-JEE Advanced | One-Line MCQ)
Which single equation governs the motion of centre of mass?
A) F = ma
B) p = mv
C) Fexternal = M acm
D) Work–Energy theorem
Correct Answer: C
OBJECTIVE QUESTION SOLVING TIPS (IIT-JEE)
✔ First check: is the system isolated?
✔ Ask: external force present or not?
✔ MCQs → eliminate wrong options calmly
✔ Integer type → sign and direction matter
✔ Explosion / collision → think COM immediately
📘 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
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