Lesson 6 – Centre of Mass & System of Particles
PHASE–2 : IIT–JEE Advanced Core Practice
Part 4 – Quality-Focused Examiner Logic
This part emphasizes quality over quantity. Each question is chosen to test a specific IIT-Advanced thinking skill.
Q1. (Multi-Correct | Non-Inertial Frame)
A system of particles is observed from a frame accelerating with constant acceleration a. Which statements are correct?
A) Pseudo force acts on each particle
B) Pseudo force acts on the centre of mass
C) COM may appear to accelerate even without real external force
D) Internal forces cancel pseudo forces
Correct Options: A, B, C
Explanation:
• In non-inertial frame, pseudo force = m a acts on each mass
• Net pseudo force acts at COM
• Internal forces do NOT cancel pseudo forces
Q2. (Multi-Correct | Constraint + COM)
Two blocks are connected by a rigid rod and placed on a smooth surface. A horizontal force is applied at one end of the rod. Which statements are correct?
A) Acceleration of COM depends on total mass
B) Acceleration of COM depends on point of application of force
C) Internal forces affect COM acceleration
D) COM acceleration = F / (total mass)
Correct Options: A, D
Examiner Insight:
Point of application matters for rotation,
never for COM translation.
Q3. (Integer Type | Direction Trap)
A system of total mass 5 kg experiences two external forces: 10 N east and 6 N west. Find the acceleration of the centre of mass (m/s²).
Net force = 10 − 6 = 4 N (east)
acm = 4 / 5 = 0.8
Answer: 0.8
Trap: Always use vector sum, not magnitudes.
Q4. (Assertion–Reason | Examiner Favourite)
Assertion (A):
Centre of mass of a system can move uniformly
even when individual particles accelerate.
Reason (R):
Internal forces may change particle motion
without affecting net external force.
A) Both A and R are true and R explains A
B) Both A and R are true but R does not explain A
C) A is true, R is false
D) A is false, R is true
Answer: A
Q5. (Multi-Correct | Deep Concept)
Which of the following statements best describe the centre of mass?
A) It is a physical particle
B) It obeys Newton’s laws like a single particle
C) It depends on mass distribution only
D) It always lies inside the body
Correct Options: B, C
Concept Clarity:
COM is a mathematical point,
not a material object.
PHASE–2 QUALITY SOLVER RULES (Part–4)
✔ First identify the frame of reference
✔ Decide: inertial or non-inertial
✔ External force decides COM acceleration
✔ Internal forces never enter COM equation
✔ Rotation and translation are independent ideas
✔ Never mix particle motion with COM motion
Why This Part Matters
IIT-Advanced rarely asks direct formulas. It tests whether you can separate physics ideas cleanly. Mastery of this part guarantees confidence in the hardest questions.
📘 Centre of Mass & System of Particles – Complete Physics Library
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It includes theory, solved examples, objective questions, IIT-level problems, revision strategies, tricks, and exam-oriented guidance.
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📂 Stage 2: Advanced Concepts & Applications
📂 Stage 3: Objective Questions, Tough Problems & Revision
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Vision: Concept clarity • Logical thinking • Exam excellence
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