Rotational Motion – Ultra-Extreme Objective Questions (Page 8)

Top-Rank Filter • IIT-JEE Advanced Thinking Level

This page contains very rare, dangerous questions. These are designed to test thinking depth, not formula memory.

Q1. (IIT-JEE Advanced)

A disc is rotating freely with angular speed ω. A particle is dropped vertically onto the disc at distance r from center. Assume particle sticks. What is conserved?

Thinking Path:

• External torque about center = 0
• Linear momentum NOT conserved
• Angular momentum about center conserved

Answer: Angular momentum about center

Trap: Many students choose total momentum.

Q2. (JEE Advanced)

A uniform rod is rotating in horizontal plane about one end. Which quantity is same for all points on rod?

A) Linear speed
B) Linear acceleration
C) Angular velocity
D) Linear momentum

Key Insight:

In pure rotation, angular velocity is same for all points.

Answer: C) Angular velocity

Q3. (IIT-JEE Advanced)

A body rolls down an incline and reaches bottom with speed v. Same body slides without friction from same height. Which is faster?

Reasoning:

Rolling → energy shared between translation + rotation
Sliding → entire energy → translation

Answer: Sliding body

Examiner Trap: Assuming rolling is faster due to friction.

Q4. (IIT-JEE)

A disc rotates with constant angular momentum. Moment of inertia is doubled suddenly. What happens to rotational kinetic energy?

Method:

K = L² / (2I)

If I doubles → K becomes half

Answer: Rotational KE decreases to half

Q5. (JEE Advanced – Conceptual)

Why does a spinning top not fall immediately?

Correct Physics:

Change in direction of angular momentum
Gyroscopic stability

Answer: Due to gyroscopic effect

Q6. (IIT-JEE Advanced)

A ring and disc roll up an incline with same initial speed. Which reaches greater height?

Method:

Total energy same initially
Energy lost depends on I/MR²

Answer: Disc

Q7. (IIT-JEE Advanced)

In a rolling motion, friction is present but work done by friction is zero. Why?

Deep Insight:

Point of contact has zero displacement.

Answer: Because displacement at contact point is zero

Q8. (JEE Advanced)

A rotating body suddenly breaks into two equal parts. Which quantity remains conserved?

Key Rule:

If no external torque → angular momentum conserved

Answer: Angular momentum

HOW TOP 100 RANKERS THINK (PAGE-8 LEVEL)

✔ Always check external torque first
✔ Fix reference axis before writing equations
✔ Prefer angular momentum in sudden events
✔ Use energy only if friction does no work
✔ Never mix linear and angular ideas blindly

MOST DANGEROUS IIT TRAPS (PAGE-8)

❌ Conserving wrong quantity
❌ Forgetting reference dependence
❌ Using KE instead of L in collisions
❌ Assuming friction always dissipates energy

Next: Page 9 – Objective Questions + “How to Tackle Any Problem” (Tips-1)

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