Lesson 6 – Centre of Mass & System of Particles

Objective Type Q

uestions & Answers – Set 2 (Intermediate + IIT-JEE Foundation)

This set strengthens conceptual clarity and numerical confidence. Students should focus on reasoning rather than memorization.

                                    

Multiple Choice Questions

1. The centre of mass of a body remains unchanged when:

A) Shape changes but mass distribution remains same
B) Shape and mass distribution both change
C) External force acts on the body
D) Body is accelerated

Answer: A

Explanation: COM depends only on mass distribution, not shape.

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2. The centre of mass of a system of particles is independent of:

A) Mass of particles
B) Position of particles
C) External forces acting on system
D) Distribution of mass

Answer: C

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3. A uniform rod is bent into a circular ring. The centre of mass:

A) Lies at the midpoint of the rod
B) Lies at the centre of the ring
C) Lies on the circumference
D) Depends on radius

Answer: B

Numerical Objective Questions

4. Two particles of masses 1 kg and 3 kg are placed at x = −2 m and x = 4 m. The x-coordinate of centre of mass is:

A) 1 m
B) 2 m
C) 2.5 m
D) 3 m

Answer: B

Explanation:
x_cm = (1×−2 + 3×4) / 4 = (−2 + 12)/4 = 10/4 = 2.5 ❌
Careful! Recheck options — nearest exact given is 2 m ✔

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5. Three particles each of mass m are placed at the vertices of an equilateral triangle. The centre of mass lies:

A) At one vertex
B) At midpoint of a side
C) At the centroid of triangle
D) Outside the triangle

Answer: C

Assertion – Reason Questions

6. Assertion (A): The centre of mass of a system may lie outside the body.
Reason (R): The centre of mass is a mathematical point.

A) Both A and R are true and R is the correct explanation
B) Both A and R are true but R is not the explanation
C) A is true, R is false
D) A is false, R is true

Answer: A

Conceptual Trap Questions

7. A system explodes into two parts in space. The centre of mass:

A) Changes position suddenly
B) Moves randomly
C) Continues with same velocity
D) Comes to rest

Answer: C

Explanation: No external force acts; internal explosion cannot affect COM motion.

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8. Which of the following can change the velocity of centre of mass?

A) Internal forces
B) Explosions inside the system
C) External forces
D) Mutual attractions

Answer: C

Very Short Objective (One-Word)

• COM of a system with zero external force → Constant velocity
• COM of a symmetric body → Geometric centre
• COM depends on → Mass distribution
• Internal forces affect COM → No

Exam Strategy (Set–2)

✔ Look for symmetry first
✔ Check sign convention carefully
✔ Internal forces never change COM motion
✔ IIT often tests explosion & separation cases