Stage 1 : Centre of Mass
(Intermediate Physics – Page 3)
In this final page of Stage 1, we consolidate the concept of centre of mass using diagram-based understanding, short answer questions, and exam-oriented presentation tips.
Diagram-Based Understanding of Centre of Mass
Consider a uniform rod of length L placed horizontally. Since the mass is uniformly distributed along its length, the centre of mass lies exactly at the midpoint of the rod.
If two unequal masses are placed at the ends of a rod, the centre of mass shifts towards the heavier mass. This idea should always be visualized while solving problems.
Diagram Idea for Exam:
Draw a straight line, mark masses at ends,
and show COM closer to heavier mass.
Centre of Mass During Motion
When a body moves, different particles of the body may have different velocities. However, the centre of mass moves with a definite velocity.
For example, when a person jumps forward, his arms and legs move in different directions, but the centre of mass follows a smooth curved path.
This property makes the centre of mass extremely useful in studying motion of complex bodies.
Centre of Mass and External Forces
The motion of the centre of mass of a system depends only on the external forces acting on the system.
Internal forces between particles cancel each other and do not affect the motion of the centre of mass.
Important Result:
Internal forces cannot change the motion of centre of mass.
Short Answer Questions (Intermediate)
1. What is centre of mass?
Centre of mass is the point that represents the average position of the total mass of a system.
2. Can centre of mass lie outside the body?
Yes. In bodies like a ring or hollow sphere, the centre of mass lies in the empty region.
3. On what factors does centre of mass depend?
It depends on the mass distribution of the system.
4. Does centre of mass depend on shape?
Only indirectly. Shape matters only because it affects mass distribution.
Very Short Answer Questions
- Centre of mass of a uniform rod – at midpoint
- Centre of mass of a uniform disc – at centre
- Centre of mass of a circular ring – at centre
- Centre of mass of a solid sphere – at centre
How to Write Answers in Exam
While writing answers related to centre of mass:
- Start with a clear definition
- Use simple language
- Draw a neat diagram wherever possible
- Highlight key points like mass distribution
Stage 1 – Final Recap
✔ Centre of mass simplifies motion of complex systems
✔ It depends only on mass distribution
✔ It may lie inside or outside the body
✔ Motion of COM depends only on external forces
✔ Strong conceptual clarity helps in higher chapters
Transition to Next Stage
In the next stage, we will develop the mathematical definition of centre of mass for two particles and many particles, which is essential for numerical problems.
📘 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
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