Lesson 6 – Centre of Mass & System of Particles
IIT–JEE Solved Questions & Answers
Phase 1 – Page 7 (Elite Reasoning)
This page is designed to train students in examiner-level thinking. Problems here may look simple, but they test deep conceptual clarity.
Problem 29 (IIT–Advanced – Thought Experiment)
Question:
A system of particles is isolated.
Suddenly, strong internal forces start acting between particles.
Will the centre of mass start accelerating? Explain.
Solution:
Internal forces act in equal and opposite pairs.
Hence, internal forces cancel when considering the system as a whole.
Since no external force acts, acceleration of centre of mass remains zero.
Answer:
Centre of mass will not accelerate.
IIT Philosophy: COM “does not know” what happens inside the system.
Problem 30 (IIT–Advanced – Observer Frame Trap)
Question:
A system of particles is observed from a non-inertial frame.
Can the centre of mass appear to accelerate even if no
external force acts? Explain.
Solution:
In a non-inertial frame, pseudo forces appear.
These pseudo forces act as external forces in that frame.
Hence, the centre of mass may appear to accelerate.
Answer:
Yes, in a non-inertial frame, COM can appear to accelerate.
Problem 31 (IIT–Advanced – COM vs Geometrical Centre)
Question:
For which type of bodies does the centre of mass coincide with
the geometrical centre? Give examples.
Solution:
When mass distribution is uniform and symmetric, centre of mass coincides with geometrical centre.
Examples include uniform sphere, uniform cube, and uniform disc.
Answer:
COM coincides with geometrical centre for uniformly symmetric bodies.
Problem 32 (IIT–Advanced – Combined Forces)
Question:
A system is acted upon by multiple external forces.
How do you determine the direction of acceleration
of the centre of mass?
Solution:
Vector sum of all external forces gives the net force.
Acceleration of centre of mass is along the direction of the net external force.
Answer:
COM accelerates in the direction of net external force.
Problem 33 (IIT–Advanced – Extreme Concept)
Question:
Is it possible that centre of mass of a system lies
at a point where there is no mass at all?
Explain with an example.
Solution:
Centre of mass is a weighted average position of mass distribution.
In bodies like rings or hollow spheres, mass is distributed around empty space.
Answer:
Yes, COM can lie in empty space (e.g., centre of a ring).
IIT Examiner’s Final Mental Model (Page 7)
✔ Think of COM as a single particle of mass M
✔ Only external forces matter for COM motion
✔ Internal complexity is irrelevant at COM level
✔ Frame of reference matters in interpretation
✔ If confused, step back and apply F = M acm
End of Phase 1 (Lesson 6)
With this page, Phase 1 of IIT/JEE solved problems for Centre of Mass is complete. You are now fully prepared to handle any COM problem in IIT Main or Advanced.
📘 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
No comments:
Post a Comment