Lesson 6 – Centre of Mass & System of Particles
IIT–JEE Solved Questions & Answers
Phase 1 – Page 8 (PYQ-Style & Constraints)
This page trains students to solve previous IIT-style questions where constraints, symmetry, and external-force logic must be applied simultaneously.
Problem 34 (IIT–Advanced Style – Constraint Motion)
Question:
Two blocks of masses m and 2m are connected by a light rigid rod
and move on a smooth horizontal surface.
A horizontal force F is applied at the centre of the rod.
Find the acceleration of the centre of mass.
Solution:
Total mass of the system = 3m
Net external force acting on the system = F
Acceleration of centre of mass:
acm = F / (3m)
Answer: Acceleration of centre of mass = F / 3m
PYQ Insight: Location of force inside the system does not affect COM acceleration.
Problem 35 (IIT–Advanced Style – Rope & Pulley)
Question:
Two masses m and m are connected by a light string passing over
a smooth pulley.
Find the acceleration of the centre of mass of the system.
Solution:
Net external force on the system = difference of weights = 0
Hence, acceleration of centre of mass = 0
Answer: Centre of mass moves with constant velocity.
Examiner Trap: Individual masses accelerate, but COM does not.
Problem 36 (IIT–Advanced Style – Projectile Explosion)
Question:
A projectile explodes at some point in its trajectory.
Neglecting air resistance, what happens to the motion
of the centre of mass after explosion?
Solution:
Explosion is due to internal forces only.
The only external force is gravity.
Hence, the centre of mass continues to follow the same parabolic trajectory.
Answer: Centre of mass follows the original projectile path.
Problem 37 (IIT–Advanced Style – Variable Mass Trap)
Question:
A block of ice slides on a smooth surface while melting uniformly.
Does the melting affect the motion of the centre of mass?
Solution:
Melting is an internal process.
No external horizontal force acts on the system.
Hence, the centre of mass continues to move with constant velocity.
Answer: Motion of COM remains unchanged.
Problem 38 (IIT–Advanced Style – Combined Reasoning)
Question:
A system has zero net external force.
Can its centre of mass have acceleration in one direction
and constant velocity in another? Explain.
Solution:
Zero net external force implies zero acceleration of COM in all directions.
Acceleration is a vector quantity.
Answer: No, centre of mass cannot accelerate in any direction.
PYQ Master Strategy (Page 8)
✔ Identify system clearly (what is included?)
✔ List only external forces on the system
✔ Use F = M acm immediately
✔ Ignore internal forces without hesitation
✔ Check symmetry before calculations
Transition Note
After this page, students are fully ready for actual previous IIT-JEE PYQs and Advanced multi-correct questions.
📘 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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