Rotational Motion – Phase 2 (Page 9)

Ultra-Tough Mixed Problems + Examiner Traps

This page completes Phase 2. Problems here are designed to break partial understanding and reward true IIT-level clarity.

TP-16 (IIT-JEE Advanced)

A solid sphere rolls down an incline and then enters a horizontal rough surface. Eventually it comes to rest. Which statement is correct?

Thinking:

• On incline → rolling with friction
• On rough horizontal → friction does negative work

Both translational and rotational KE decrease.

Answer: Both translational and rotational motion stop due to friction.

Examiner Trap: Assuming rolling continues forever.

TP-17 (IIT-JEE Advanced)

A disc and a sphere of same mass and radius rotate with same angular speed. Which has greater rotational kinetic energy?

Method:

Rotational KE = ½ Iω²

Disc: I = ½MR²
Sphere: I = 2/5MR²

Answer: Disc (larger moment of inertia)

TP-18 (IIT-JEE Advanced)

A rotating disc has angular momentum L. Its rotational kinetic energy is proportional to:

Logic:

K = L² / (2I)

Answer: Inversely proportional to moment of inertia.

Trap: Many assume larger I ⇒ larger energy (WRONG).

TP-19 (IIT-JEE Advanced)

A man stands at the edge of a rotating platform holding dumbbells. He pulls them inward. What changes?

Correct Analysis:

No external torque ⇒ angular momentum conserved

Angular velocity increases
Rotational kinetic energy increases

Answer: KE increases (work done by man)

TP-20 (IIT-JEE Advanced – Conceptual)

Why is it easier to balance a bicycle when it is moving?

Reason:

Rotating wheels have angular momentum
They resist change in orientation (gyroscopic effect)

Answer: Due to angular momentum stability.

PHASE-2 MASTER RULES (Very Important)

✔ Angular momentum > Linear momentum (in sudden events)
✔ Check where friction acts (work or torque?)
✔ Compare I/MR² quickly
✔ Energy may increase if internal work is done
✔ Never assume rolling unless stated

PHASE 2 COMPLETED ✅
Next: Phase 3 – Exam Mastery

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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.

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