Rotational Motion – Phase 1 (Page 4)

IIT-JEE / JEE Main – Numerical Traps & Variations

This page focuses on how IIT examiners twist simple concepts. Every problem below is inspired by previous year papers.

PYQ-18 (IIT-JEE – Trap Question)

Two bodies of same mass and radius roll down an incline. One is a solid disc, the other is a ring. Which statement is correct?

Correct Method:

Acceleration for rolling:
a = g sinθ / (1 + I / MR²)

Disc: I = ½MR² → denominator = 1.5
Ring: I = MR² → denominator = 2

Conclusion: Disc reaches first

Examiner Trap: Many students compare mass instead of I/MR².

PYQ-19 (JEE Main – Axis Trap)

Moment of inertia of a rod is different when rotated about:

Correct Reasoning:

Moment of inertia depends on axis of rotation.

About centre: I = ML²/12
About end: I = ML²/3

Key Point: Same body → different I due to axis

Common Mistake: Students memorize only one formula.

PYQ-20 (IIT-JEE – Energy vs Torque)

A disc rolls without slipping. Which method is best to find its speed?

Correct Method:

✔ Use energy conservation, not F = ma

Mgh = ½Mv² + ½Iω²

Why? Energy method avoids friction force calculation.

PYQ-21 (JEE Main – Angular Momentum Trap)

A rotating platform slows down when a man walks towards the centre. Why?

Correct Logic:

No external torque → angular momentum conserved

L = Iω = constant
As I decreases → ω increases

Result: Platform speeds up

Trap: Students wrongly apply torque equation.

PYQ-22 (IIT-JEE – Rolling + Friction)

In pure rolling, friction:

Correct Understanding:

• Friction provides torque
• But point of contact has zero displacement

Conclusion: Work done by friction = 0

EXAMINER’S FAVORITE TRAPS (Must Remember)

✔ Forgetting axis of rotation
✔ Confusing angular velocity with angular momentum
✔ Using force equations instead of energy
✔ Ignoring rolling condition v = ωR
✔ Memorizing without understanding I/MR² ratio

SMART SHORTCUT (JEE Rank Booster)

For rolling problems, compare I/MR² only. Smaller value → faster motion.

Next: Phase 1 – Page 5 (Mixed Numericals + Speed Tricks)

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

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