Lesson 6 – Centre of Mass & System of Particles

Objective Type Questions & Answers (Intermediate + IIT-JEE Foundation)

These objective questions are prepared based on previous board patterns and IIT-JEE thinking. Students are advised to understand the explanation after each question.

Multiple Choice Questions (MCQs)

1. The centre of mass of a uniform circular ring is located at:

A) On the circumference
B) At the centre of the ring
C) Outside the ring
D) Depends on orientation

Answer: B

Explanation: Due to symmetry, mass is evenly distributed around the centre.

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2. The centre of mass of a system depends on:

A) Shape only
B) Size only
C) Mass distribution
D) External forces

Answer: C

Explanation: COM depends only on how mass is distributed.

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3. If two equal masses are placed at the ends of a rod, the COM lies:

A) At one end
B) At the midpoint
C) Outside the rod
D) At heavier mass

Answer: B

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4. Which of the following is true?

A) COM always lies inside the body
B) COM depends on gravitational field
C) COM may lie outside the body
D) COM is always a physical particle

Answer: C

Numerical Objective Questions

5. Two particles of masses 2 kg and 4 kg are placed at x = 0 m and x = 6 m. The centre of mass is at:

A) 2 m
B) 3 m
C) 4 m
D) 5 m

Answer: C

Explanation: x_cm = (2×0 + 4×6) / 6 = 24/6 = 4 m

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6. If all external forces acting on a system are zero, the centre of mass:

A) Remains at rest only
B) Moves with constant velocity
C) Moves with acceleration
D) Disappears

Answer: B

Assertion – Reason Questions

7. Assertion (A): Internal forces do not affect the motion of centre of mass.
Reason (R): Internal forces occur in equal and opposite pairs.

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

Answer: A

Very Short Objective Questions

• Centre of mass of a uniform rod – Midpoint
• COM of a triangular lamina lies at – Intersection of medians
• COM depends on – Mass distribution
• Internal forces affect COM motion – No

Exam Tips (Objective Questions)

✔ Always visualize symmetry first
✔ Use weighted average idea for numericals
✔ Ignore internal forces in COM motion questions
✔ Read assertion–reason carefully