Lesson 6 – Centre of Mass & System of Particles
Objective Questions – Set 2
(Previous IIT–JEE Oriented)
This set strengthens conceptual accuracy. Many questions look simple but hide classic IIT traps.
Q1. (IIT-JEE Main | MCQ)
If the net external force on a system is zero, then:
A) Centre of mass is always at rest
B) Centre of mass moves with constant velocity
C) All particles move with constant velocity
D) Total kinetic energy is conserved
Correct Answer: B
Explanation:
Zero external force ⇒ zero acceleration of COM.
Velocity may be zero or non-zero but remains constant.
Q2. (IIT-JEE Main | Integer Type)
Three particles of masses 1 kg, 2 kg and 3 kg move with velocities 4 m/s, −2 m/s and 0 m/s respectively. Find the velocity of the centre of mass (m/s).
vcm = (1×4 + 2×(−2) + 3×0) / 6
vcm = (4 − 4) / 6 = 0
Answer: 0
Q3. (IIT-JEE Advanced | MCQ)
A system of particles is subjected to an external force. Which quantity is directly affected?
A) Relative motion of particles
B) Internal energy of system
C) Acceleration of centre of mass
D) Internal forces
Correct Answer: C
Explanation:
acm = Fexternal/M.
External force controls only COM motion.
Q4. (IIT-JEE Advanced | MCQ)
A bomb explodes in mid-air into several fragments. Which statement is always correct?
A) Fragments fall vertically downward
B) Centre of mass accelerates suddenly
C) Centre of mass continues original motion
D) Total kinetic energy remains constant
Correct Answer: C
Q5. (IIT-JEE Main | Integer Type)
A system of mass 5 kg experiences a net external force of 15 N. Find the acceleration of centre of mass (m/s²).
acm = 15 / 5 = 3
Answer: 3
Q6. (IIT-JEE Advanced | Conceptual MCQ)
Which of the following statements is correct?
A) Zero momentum implies zero external force
B) Zero external force implies zero momentum
C) Zero external force implies constant momentum
D) Constant momentum implies zero velocity
Correct Answer: C
Q7. (IIT-JEE Main | MCQ)
Two equal masses move with equal speed in opposite directions. The centre of mass is:
A) Moving with same speed
B) Moving with double speed
C) At rest
D) Accelerating
Correct Answer: C
Q8. (IIT-JEE Advanced | Integer Type)
A system initially at rest experiences an external force of 10 N for 2 s. Total mass of system is 5 kg. Find the velocity of centre of mass after 2 s (m/s).
acm = 10 / 5 = 2
v = a t = 2 × 2 = 4
Answer: 4
Q9. (IIT-JEE Advanced | MCQ)
Which situation definitely ensures that centre of mass remains stationary?
A) Zero velocity of particles
B) Zero external force and zero initial momentum
C) Equal masses in motion
D) Zero internal forces
Correct Answer: B
Q10. (IIT-JEE Advanced | One-Line MCQ)
Motion of centre of mass of a system is governed by:
A) Internal forces only
B) External forces only
C) Both internal and external forces
D) Relative motion of particles
Correct Answer: B
OBJECTIVE SOLVING TIPS (SET-2)
✔ Always think in terms of the system
✔ Zero external force ⇒ constant COM velocity
✔ Do not confuse momentum with force
✔ Integer type → write formula before calculating
✔ Explosions & collisions → COM never jumps
📘 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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