Lesson 6 – Centre of Mass & System of Particles
PHASE–2 : IIT–JEE Advanced Core Practice
Part 5 – Extreme Traps & Examiner Psychology
This part contains very few questions, but each one tests a critical IIT-Advanced concept. Answering these correctly means you truly understand Centre of Mass.
Q1. (Multi-Correct | Deep Trap)
A system of particles has zero acceleration of its centre of mass. Which of the following statements are necessarily true?
A) Net external force on the system is zero
B) Linear momentum of the system is conserved
C) All particles move with constant velocity
D) Internal forces may still act
Correct Options: A, B, D
Explanation:
• acm = 0 ⇒ Fext = 0
• Momentum conserved
• Individual particles can accelerate due to internal forces
Q2. (Integer Type | Combined Force Trap)
A system of total mass 8 kg is acted upon by two perpendicular external forces of 6 N and 8 N. Find the magnitude of acceleration of the centre of mass (m/s²).
Net force = √(6² + 8²) = 10 N
acm = 10 / 8 = 1.25
Answer: 1.25
Trap: Never add forces algebraically if directions differ.
Q3. (Assertion–Reason | Very Subtle)
Assertion (A):
Centre of mass of a system can move in a straight line
even when particles follow curved paths.
Reason (R):
Centre of mass motion depends only on external forces.
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
Q4. (Multi-Correct | Frame + Pseudo Force)
In a non-inertial frame accelerating with acceleration a, which of the following statements are correct?
A) A pseudo force acts on each particle
B) Net pseudo force acts at the centre of mass
C) Pseudo force cancels internal forces
D) COM acceleration equation changes
Correct Options: A, B, D
Explanation:
• Pseudo force behaves like external force in that frame
• Internal forces still cancel among themselves
Q5. (Multi-Correct | Ultimate Concept Test)
Which statements correctly describe the centre of mass?
A) It is always inside the body
B) It obeys Newton’s laws like a single particle
C) It depends on distribution of mass
D) It can exist in empty space
Correct Options: B, C, D
Key Insight:
COM is a mathematical point, not a material object.
PHASE–2 MASTER SOLVER MINDSET (Part–5)
✔ Never mix particle motion with COM motion
✔ Ask first: what are the external forces?
✔ Zero acm ⇒ zero net external force (always)
✔ Rotation does not affect COM translation
✔ Frame of reference can change observed COM motion
✔ If confused, redraw system boundary
End of PHASE–2 (Lesson 6)
With Part–5, PHASE–2 is complete. A student who masters Phase–2 will not fear any Centre of Mass question in IIT-JEE 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