Rotational Motion – Stage 2 (Page 9)

Objective Questions – Solution Methods & Tricks

This page focuses on HOW to solve objective questions. Each problem includes: method → shortcut → examiner’s trap.

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Q1.

A disc and a ring roll down an incline without slipping. Which reaches the bottom first?

Method:

Use acceleration for rolling body:
a = g sinθ / (1 + I / MR²)

Disc: I = ½MR²
Ring: I = MR²

Conclusion: Disc has smaller denominator → larger acceleration → reaches first.

Examiner Trap: Students compare mass, not moment of inertia.

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Q2.

A rotating disc suddenly increases its radius. What happens to angular velocity?

Method:

Check external torque → zero
Apply conservation of angular momentum:
L = Iω = constant

Result: If I increases → ω decreases.

Shortcut: No torque → never use τ = Iα.

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Q3.

In pure rolling, work done by friction is:

Method:

Check type of friction → static
Check displacement of contact point → zero

Answer: Work done = 0

Common Mistake: Assuming friction always does negative work.

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Q4.

Which quantity remains conserved when net external torque is zero?

Method:

Recall fundamental law:
τ = dL/dt

Conclusion: τ = 0 ⇒ L = constant

Examiner Trick: Options include angular velocity to confuse you.

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Q5.

A wheel rolls without slipping. Velocity of topmost point is?

Method:

Total velocity = translational + rotational

v_top = v + v = 2v

Answer: 2v

Visualization Trick: Always draw velocity vectors.

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Q6.

Moment of inertia of same body differs because of:

Method:

Check formula:
I = Σmr²

Reason: Depends on distance from axis.

Examiner Trap: Ignoring axis of rotation.

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Golden Rules for Objective Questions (Rotational Motion)

✔ First identify axis of rotation
✔ Check if torque is zero or not
✔ Prefer conservation laws
✔ Compare moment of inertia, not mass
✔ Draw rough diagram mentally

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Next: Stage 2 – Page 10 (Assertion–Reason + Mixed Concept MCQs)

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🔄 Rotational Motion – Phase 3 (Exam Mastery Series)

Advanced exam-oriented thinking series focusing on traps, execution, pressure handling, and topper strategies.

• Phase 3 – Page 1
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